线程池的使用(ThreadPoolExecutor详解)

为什么要使用线程池?

线程是一个操作系统概念。操作系统负责这个线程的创建、挂起、运行、阻塞和终结操作。而操作系统创建线程、切换线程状态、终结线程都要进行CPU调度——这是一个耗费时间和系统资源的事情。

另一方面,大多数实际场景中是这样的:处理某一次请求的时间是非常短暂的,但是请求数量是巨大的。这种技术背景下,如果我们为每一个请求都单独创建一个线程,那么物理机的所有资源基本上都被操作系统创建线程、切换线程状态、销毁线程这些操作所占用,用于业务请求处理的资源反而减少了。所以最理想的处理方式是,将处理请求的线程数量控制在一个范围,既保证后续的请求不会等待太长时间,又保证物理机将足够的资源用于请求处理本身。

另外,一些操作系统是有最大线程数量限制的。当运行的线程数量逼近这个值的时候,操作系统会变得不稳定。这也是我们要限制线程数量的原因。

线程池的基本使用方式

Java语言为我们提供了两种基础线程池的选择:ScheduledThreadPoolExecutor和ThreadPoolExecutor。它们都实现了ExecutorService接口(注意,ExecutorService接口本身和“线程池”并没有直接关系,它的定义更接近“执行器”,而“使用线程管理的方式进行实现”只是其中的一种实现方式)。这篇文章中,我们主要围绕ThreadPoolExecutor类进行讲解。

ThreadPoolExecutor类的使用方式:

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">PoolThreadSimple</span> {</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">main</span>(String[] args) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throws</span> Throwable { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* * corePoolSize:核心大小,线程池初始化的时候,就会有这么大 * maximumPoolSize:线程池最大线程数 * keepAliveTime:如果当前线程池中线程数大于corePoolSize。 * 多余的线程,在等待keepAliveTime时间后如果还没有新的线程任务指派给它,它就会被回收 * * unit:等待时间keepAliveTime的单位 * * workQueue:等待队列。这个对象的设置是本文将重点介绍的内容 * */</span> ThreadPoolExecutor poolExecutor = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ThreadPoolExecutor(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, TimeUnit.MINUTES, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> SynchronousQueue<Runnable>()); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> index = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> ; index < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span> ; index ++) { poolExecutor.submit(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> PoolThreadSimple.TestRunnable(index)); } } <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * 这个就是测试用的线程 */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">TestRunnable</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Runnable</span> {</span> <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * 日志 */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> Log LOGGER = LogFactory.getLog(TestRunnable.class); <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * 记录任务的唯一编号,这样在日志中好做识别 */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> Integer index; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">TestRunnable</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> index) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.index = index; } <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> the index */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Integer <span class="hljs-title" style="box-sizing: border-box;">getIndex</span>() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> index; } <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* * 线程中,就只做一件事情: * 等待60秒钟的事件,以便模拟业务操作过程 * */</span> Thread currentThread = Thread.currentThread(); TestRunnable.LOGGER.info(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"线程:"</span> + currentThread.getId() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" 中的任务("</span> + <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.getIndex() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">")开始执行==="</span>); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">synchronized</span> (currentThread) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> { currentThread.wait(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">60000</span>); } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (InterruptedException e) { TestRunnable.LOGGER.error(e.getMessage(), e); } } TestRunnable.LOGGER.info(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"线程:"</span> + currentThread.getId() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" 中的任务("</span> + <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.getIndex() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">")执行完成"</span>); } } }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; 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下文中,将对线程池中的corePoolSize、maximumPoolSize、keepAliveTime、timeUnit、workQueue、threadFactory、handler参数和一些常用/不常用的设置项进行逐一讲解。

ThreadPoolExecutor逻辑结构和工作方式

在上面的代码中,我们创建线程池的时候使用了ThreadPoolExecutor中最简单的一个构造函数:

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ThreadPoolExecutor</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> corePoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maximumPoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

构造函数中需要传入的参数包括corePoolSize、maximumPoolSize、keepAliveTime、timeUnit和workQueue。要明确理解这些参数(和后续将要介绍的参数)的含义,就首先要搞清楚ThreadPoolExecutor线程池的逻辑结构。

一定要注意一个概念,即存在于线程池中容器的一定是Thread对象,而不是你要求运行的任务(所以叫线程池而不叫任务池也不叫对象池);你要求运行的任务将被线程池分配给某一个空闲的Thread运行。

从上图中,我们可以看到构成线程池的几个重要元素:

● 等待队列:顾名思义,就是你调用线程池对象的submit()方法或者execute()方法,要求线程池运行的任务(这些任务必须实现Runnable接口或者Callable接口)。但是出于某些原因线程池并没有马上运行这些任务,而是送入一个队列等待执行。

● 核心线程:线程池主要用于执行任务的是“核心线程”,“核心线程”的数量是你创建线程时所设置的corePoolSize参数决定的。如果不进行特别的设定,线程池中始终会保持corePoolSize数量的线程数(不包括创建阶段)。

● 非核心线程:一旦任务数量过多(由等待队列的特性决定),线程池将创建“非核心线程”临时帮助运行任务。你设置的大于corePoolSize参数小于maximumPoolSize参数的部分,就是线程池可以临时创建的“非核心线程”的最大数量。这种情况下如果某个线程没有运行任何任务,在等待keepAliveTime时间后,这个线程将会被销毁,直到线程池的线程数量重新达到corePoolSize。

● maximumPoolSize参数也是当前线程池允许创建的最大线程数量。那么如果设置的corePoolSize参数和设置的maximumPoolSize参数一致时,线程池在任何情况下都不会回收空闲线程。keepAliveTime和timeUnit也就失去了意义。

● keepAliveTime参数和timeUnit参数也是配合使用的。keepAliveTime参数指明等待时间的量化值,timeUnit指明量化值单位。例如keepAliveTime=1,timeUnit为TimeUnit.MINUTES,代表空闲线程的回收阀值为1分钟。

说完了线程池的逻辑结构,下面我们讨论一下线程池是怎样处理某一个运行任务的。

1、首先可以通过线程池提供的submit()方法或者execute()方法,要求线程池执行某个任务。线程池收到这个要求执行的任务后,会有几种处理情况:

1.1、如果当前线程池中运行的线程数量还没有达到corePoolSize大小时,线程池会创建一个新的线程运行你的任务,无论之前已经创建的线程是否处于空闲状态。

1.2、如果当前线程池中运行的线程数量已经达到设置的corePoolSize大小,线程池会把你的这个任务加入到等待队列中。直到某一个的线程空闲了,线程池会根据设置的等待队列规则,从队列中取出一个新的任务执行。

1.3、如果根据队列规则,这个任务无法加入等待队列。这时线程池就会创建一个“非核心线程”直接运行这个任务。注意,如果这种情况下任务执行成功,那么当前线程池中的线程数量一定大于corePoolSize。

1.4、如果这个任务,无法被“核心线程”直接执行,又无法加入等待队列,又无法创建“非核心线程”直接执行,且你没有为线程池设置RejectedExecutionHandler。这时线程池会抛出RejectedExecutionException异常,即线程池拒绝接受这个任务。(实际上抛出RejectedExecutionException异常的操作,是ThreadPoolExecutor线程池中一个默认的RejectedExecutionHandler实现:AbortPolicy,这在后文会提到)

2、一旦线程池中某个线程完成了任务的执行,它就会试图到任务等待队列中拿去下一个等待任务(所有的等待任务都实现了BlockingQueue接口,按照接口字面上的理解,这是一个可阻塞的队列接口),它会调用等待队列的poll()方法,并停留在哪里。

3、当线程池中的线程超过你设置的corePoolSize参数,说明当前线程池中有所谓的“非核心线程”。那么当某个线程处理完任务后,如果等待keepAliveTime时间后仍然没有新的任务分配给它,那么这个线程将会被回收。线程池回收线程时,对所谓的“核心线程”和“非核心线程”是一视同仁的,直到线程池中线程的数量等于你设置的corePoolSize参数时,回收过程才会停止。

不常用的设置

在ThreadPoolExecutor线程池中,有一些不常用的甚至不需要的设置

allowCoreThreadTimeOut:

线程池回收线程只会发生在当前线程池中线程数量大于corePoolSize参数的时候;当线程池中线程数量小于等于corePoolSize参数的时候,回收过程就会停止。

allowCoreThreadTimeOut设置项可以要求线程池:将包括“核心线程”在内的,没有任务分配的任何线程,在等待keepAliveTime时间后全部进行回收:

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">ThreadPoolExecutor poolExecutor = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ThreadPoolExecutor(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, TimeUnit.MINUTES, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ArrayBlockingQueue<Runnable>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)); poolExecutor.allowCoreThreadTimeOut(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

以下是设置前的效果:

以下是设置后的效果:

prestartAllCoreThreads

前文我们还讨论到,当线程池中的线程还没有达到你设置的corePoolSize参数值的时候,如果有新的任务到来,线程池将创建新的线程运行这个任务,无论之前已经创建的线程是否处于空闲状态。这个描述可以用下面的示意图表示出来:

prestartAllCoreThreads设置项,可以在线程池创建,但还没有接收到任何任务的情况下,先行创建符合corePoolSize参数值的线程数:

ThreadPoolExecutor poolExecutor =new
ThreadPoolExecutor(5,10,1,
TimeUnit.MINUTES, new
ArrayBlockingQueue<Runnable>(1));

poolExecutor.prestartAllCoreThreads();

我们继续讨论ThreadPoolExecutor线程池。上面给出的最简单的ThreadPoolExecutor线程池的使用方式中,我们只采用了ThreadPoolExecutor最简单的一个构造函数:

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ThreadPoolExecutor</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> corePoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maximumPoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

实际上ThreadPoolExecutor线程池有很多种构造函数,其中最复杂的一种构造函数是:

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ThreadPoolExecutor</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> corePoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maximumPoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

在上文中我们还没有介绍的workQueue、threadFactory和handler参数,将是本文讲解的重点。

一:ThreadFactory的使用

线程池最主要的一项工作,就是在满足某些条件的情况下创建线程。而在ThreadPoolExecutor线程池中,创建线程的工作交给ThreadFactory来完成。要使用线程池,就必须要指定ThreadFactory。

类似于上文中,如果我们使用的构造函数时并没有指定使用的ThreadFactory,这个时候ThreadPoolExecutor会使用一个默认的ThreadFactory:DefaultThreadFactory。(这个类在Executors工具类中)

当然,在某些特殊业务场景下,还可以使用一个自定义的ThreadFactory线程工厂,如下代码片段:

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> java.util.concurrent.ThreadFactory; <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * 测试自定义的一个线程工厂 */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">TestThreadFactory</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ThreadFactory</span> {</span> <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Thread <span class="hljs-title" style="box-sizing: border-box;">newThread</span>(Runnable r) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Thread(r); } }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

二:线程池的等待队列

在使用ThreadPoolExecutor线程池的时候,需要指定一个实现了BlockingQueue接口的任务等待队列。在ThreadPoolExecutor线程池的API文档中,一共推荐了三种等待队列,它们是:SynchronousQueue、LinkedBlockingQueue和ArrayBlockingQueue;

队列和栈

● 队列:是一种特殊的线性结构,允许在线性结构的前端进行删除/读取操作;允许在线性结构的后端进行插入操作;这种线性结构具有“先进先出”的操作特点:

但是在实际应用中,队列中的元素有可能不是以“进入的顺序”为排序依据的。例如我们将要讲到的PriorityBlockingQueue队列。

● 栈:栈也是一种线性结构,但是栈和队列相比只允许在线性结构的一端进行操作,入栈和出栈都是在一端完成。

2.1有限队列

● SynchronousQueue:

“是这样 一种阻塞队列,其中每个 put 必须等待一个 take,反之亦然。同步队列没有任何内部容量。翻译一下:这是一个内部没有任何容量的阻塞队列,任何一次插入操作的元素都要等待相对的删除/读取操作,否则进行插入操作的线程就要一直等待,反之亦然。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">SynchronousQueue<<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>> queue = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> SynchronousQueue<<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>>(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 不要使用add,因为这个队列内部没有任何容量,所以会抛出异常“IllegalStateException”</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// queue.add(new Object());</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 操作线程会在这里被阻塞,直到有其他操作线程取走这个对象</span> queue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>());</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

● ArrayBlockingQueue:

一个由数组支持的有界阻塞队列。此队列按 FIFO(先进先出)原则对元素进行排序。新元素插入到队列的尾部,队列获取操作则是从队列头部开始获得元素。这是一个典型的“有界缓存区”,固定大小的数组在其中保持生产者插入的元素和使用者提取的元素。一旦创建了这样的缓存区,就不能再增加其容量。试图向已满队列中放入元素会导致操作受阻塞;试图从空队列中提取元素将导致类似阻塞。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 我们创建了一个ArrayBlockingQueue,并且设置队列空间为2</span> ArrayBlockingQueue<<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>> arrayQueue = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ArrayBlockingQueue<<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 插入第一个对象</span> arrayQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 插入第二个对象</span> arrayQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 插入第三个对象时,这个操作线程就会被阻塞。</span> arrayQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 请不要使用add操作,和SynchronousQueue的add操作一样,它们都使用了AbstractQueue中的add实现</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

2.2无限队列

● LinkedBlockingQueue:

LinkedBlockingQueue是我们在ThreadPoolExecutor线程池中常用的等待队列。它可以指定容量也可以不指定容量。由于它具有“无限容量”的特性,所以我还是将它归入了无限队列的范畴(实际上任何无限容量的队列/栈都是有容量的,这个容量就是Integer.MAX_VALUE)。

LinkedBlockingQueue的实现是基于链表结构,而不是类似ArrayBlockingQueue那样的数组。但实际使用过程中,不需要关心它的内部实现,如果指定了LinkedBlockingQueue的容量大小,那么它反映出来的使用特性就和ArrayBlockingQueue类似了。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">LinkedBlockingQueue<<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>> linkedQueue = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> LinkedBlockingQueue<<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>); linkedQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 插入第二个对象</span> linkedQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 插入第三个对象时,这个操作线程就会被阻塞。</span> linkedQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>());</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

=======================================

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 或者如下使用:</span> LinkedBlockingQueue<<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>> linkedQueue = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> LinkedBlockingQueue<<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>>(); linkedQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 插入第二个对象</span> linkedQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 插入第N个对象时,都不会阻塞</span> linkedQueue.put(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span>());</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

● LinkedBlockingDeque

LinkedBlockingDeque是一个基于链表的双端队列。LinkedBlockingQueue的内部结构决定了它只能从队列尾部插入,从队列头部取出元素;但是LinkedBlockingDeque既可以从尾部插入/取出元素,还可以从头部插入元素/取出元素。

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">LinkedBlockingDeque<TempObject> linkedDeque = new LinkedBlockingDeque<TempObject>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">push</span> ,可以从队列的头部插入元素 linkedDeque<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.push</span>(new TempObject(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> linkedDeque<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.push</span>(new TempObject(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> linkedDeque<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.push</span>(new TempObject(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // poll , 可以从队列的头部取出元素 TempObject tempObject = linkedDeque<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.poll</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // 这里会打印 tempObject<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.index</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span> System<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.out</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.println</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"tempObject.index = "</span> + tempObject<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getIndex</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // put , 可以从队列的尾部插入元素 linkedDeque<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.put</span>(new TempObject(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> linkedDeque<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.put</span>(new TempObject(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // pollLast , 可以从队列尾部取出元素 tempObject = linkedDeque<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.pollLast</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // 这里会打印 tempObject<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.index</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span> System<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.out</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.println</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"tempObject.index = "</span> + tempObject<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getIndex</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

● PriorityBlockingQueue

PriorityBlockingQueue是一个按照优先级进行内部元素排序的无限队列。存放在PriorityBlockingQueue中的元素必须实现Comparable接口,这样才能通过实现compareTo()方法进行排序。优先级最高的元素将始终排在队列的头部;PriorityBlockingQueue不会保证优先级一样的元素的排序,也不保证当前队列中除了优先级最高的元素以外的元素,随时处于正确排序的位置。

这是什么意思呢?PriorityBlockingQueue并不保证除了队列头部以外的元素排序一定是正确的。请看下面的示例代码:

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">PriorityBlockingQueue<TempObject> priorityQueue = new PriorityBlockingQueue<TempObject>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.put</span>(new TempObject(-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.put</span>(new TempObject(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.put</span>(new TempObject(-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.put</span>(new TempObject(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // 第一个元素是<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span> // 实际上在还没有执行priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.poll</span>()语句的时候,队列中的第二个元素不一定是<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> TempObject targetTempObject = priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.poll</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> System<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.out</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.println</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"tempObject.index = "</span> + targetTempObject<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getIndex</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // 第二个元素是<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> targetTempObject = priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.poll</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> System<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.out</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.println</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"tempObject.index = "</span> + targetTempObject<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getIndex</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // 第三个元素是-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> targetTempObject = priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.poll</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> System<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.out</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.println</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"tempObject.index = "</span> + targetTempObject<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getIndex</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> // 第四个元素是-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span> targetTempObject = priorityQueue<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.poll</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span> System<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.out</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.println</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"tempObject.index = "</span> + targetTempObject<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getIndex</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>
<code class="hljs axapta has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 这个元素类,必须实现Comparable接口</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">TempObject</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Comparable</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">TempObject</span>> {</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">index</span>; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> TempObject(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">index</span>) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">index</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">index</span>; } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * @return the index */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> getIndex() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">index</span>; } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* (non-Javadoc) * @see java.lang.Comparable#compareTo(java.lang.Object) */</span> @Override <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> compareTo(TempObject o) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> o.getIndex() - <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">index</span>; } }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul>

● LinkedTransferQueue

LinkedTransferQueue也是一个无限队列,它除了具有一般队列的操作特性外(先进先出),还具有一个阻塞特性:LinkedTransferQueue可以由一对生产者/消费者线程进行操作,当消费者将一个新的元素插入队列后,消费者线程将会一直等待,直到某一个消费者线程将这个元素取走,反之亦然。

LinkedTransferQueue的操作特性可以由下面这段代码提现。在下面的代码片段中,有两中类型的线程:生产者和消费者,这两类线程互相等待对方的操作:

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * 生产者线程 */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ProducerRunnable</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Runnable</span> {</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> LinkedTransferQueue<TempObject> linkedQueue; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ProducerRunnable</span>(LinkedTransferQueue<TempObject> linkedQueue) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.linkedQueue = linkedQueue; } <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> index = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> ; ; index++) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 向LinkedTransferQueue队列插入一个新的元素</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 然后生产者线程就会等待,直到有一个消费者将这个元素从队列中取走</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.linkedQueue.transfer(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TempObject(index)); } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (InterruptedException e) { e.printStackTrace(System.out); } } } } <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * 消费者线程 */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ConsumerRunnable</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Runnable</span> {</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> LinkedTransferQueue<TempObject> linkedQueue; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ConsumerRunnable</span>(LinkedTransferQueue<TempObject> linkedQueue) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.linkedQueue = linkedQueue; } <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() { Thread currentThread = Thread.currentThread(); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span>(!currentThread.isInterrupted()) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 等待,直到从LinkedTransferQueue队列中得到一个元素</span> TempObject targetObject = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.linkedQueue.take(); System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"线程("</span> + currentThread.getId() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">")取得targetObject.index = "</span> + targetObject.getIndex()); } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (InterruptedException e) { e.printStackTrace(System.out); } } } }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li></ul>

=====以下是启动代码:

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">LinkedTransferQueue<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>TempObject<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> linkedQueue <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> LinkedTransferQueue<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>TempObject<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span>(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 这是一个生产者线程</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Thread</span> producerThread <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Thread</span>(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> ProducerRunnable(linkedQueue)); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 这里有两个消费者线程</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Thread</span> consumerRunnable1 <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Thread</span>(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> ConsumerRunnable(linkedQueue)); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Thread</span> consumerRunnable2 <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Thread</span>(<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> ConsumerRunnable(linkedQueue)); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 开始运行</span> producerThread<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>start(); consumerRunnable1<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>start(); consumerRunnable2<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>start(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 这里只是为了main不退出,没有任何演示含义</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Thread</span> currentThread <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">Thread</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>currentThread(); synchronized (currentThread) { currentThread<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>wait(); }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

三:拒绝任务(handler)

在ThreadPoolExecutor线程池中还有一个重要的接口:RejectedExecutionHandler。当提交给线程池的某一个新任务无法直接被线程池中“核心线程”直接处理,又无法加入等待队列,也无法创建新的线程执行;又或者线程池已经调用shutdown()方法停止了工作;又或者线程池不是处于正常的工作状态;这时候ThreadPoolExecutor线程池会拒绝处理这个任务,触发创建ThreadPoolExecutor线程池时定义的RejectedExecutionHandler接口的实现

在创建ThreadPoolExecutor线程池时,一定会指定RejectedExecutionHandler接口的实现。如果调用的是不需要指定RejectedExecutionHandler接口的构造函数,如:

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ThreadPoolExecutor</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> corePoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maximumPoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ThreadPoolExecutor</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> corePoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maximumPoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

那么ThreadPoolExecutor线程池在创建时,会使用一个默认的RejectedExecutionHandler接口实现,源代码片段如下:

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ThreadPoolExecutor</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">AbstractExecutorService</span> {</span> ...... <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * The default rejected execution handler */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> RejectedExecutionHandler defaultHandler = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> AbortPolicy(); ...... <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 可以看到,ThreadPoolExecutor中的两个没有指定RejectedExecutionHandler</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 接口的构造函数,都是使用了一个RejectedExecutionHandler接口的默认实现:AbortPolicy</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ThreadPoolExecutor</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> corePoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maximumPoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), defaultHandler); } ...... <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ThreadPoolExecutor</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> corePoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maximumPoolSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, defaultHandler); } ...... }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li></ul>

实际上,在ThreadPoolExecutor中已经提供了四种可以直接使用的RejectedExecutionHandler接口的实现:

● CallerRunsPolicy:

这个拒绝处理器,将直接运行这个任务的run方法。但是,请注意并不是在ThreadPoolExecutor线程池中的线程中运行,而是直接调用这个任务实现的run方法。源代码如下:

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CallerRunsPolicy</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">RejectedExecutionHandler</span> {</span> <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * Creates a {@code CallerRunsPolicy}. */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">CallerRunsPolicy</span>() { } <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * Executes task r in the caller‘s thread, unless the executor * has been shut down, in which case the task is discarded. * *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> r the runnable task requested to be executed *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> e the executor attempting to execute this task */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">rejectedExecution</span>(Runnable r, ThreadPoolExecutor e) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!e.isShutdown()) { r.run(); } } }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

● AbortPolicy:

这个处理器,在任务被拒绝后会创建一个RejectedExecutionException异常并抛出。这个处理过程也是ThreadPoolExecutor线程池默认的RejectedExecutionHandler实现。

● DiscardPolicy:

DiscardPolicy处理器,将会默默丢弃这个被拒绝的任务,不会抛出异常,也不会通过其他方式执行这个任务的任何一个方法,更不会出现任何的日志提示。

● DiscardOldestPolicy:

这个处理器很有意思。它会检查当前ThreadPoolExecutor线程池的等待队列。并调用队列的poll()方法,将当前处于等待队列列头的等待任务强行取出,然后再试图将当前被拒绝的任务提交到线程池执行:

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: ‘Source Code Pro‘, monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">DiscardOldestPolicy</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">RejectedExecutionHandler</span> {</span> ...... <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">rejectedExecution</span>(Runnable r, ThreadPoolExecutor e) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!e.isShutdown()) { e.getQueue().poll(); e.execute(r); } } ...... }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

实际上查阅这四种ThreadPoolExecutor线程池自带的拒绝处理器实现,您可以发现CallerRunsPolicy、DiscardPolicy、DiscardOldestPolicy处理器针对被拒绝的任务并不是一个很好的处理方式。

CallerRunsPolicy在非线程池以外直接调用任务的run方法,可能会造成线程安全上的问题;DiscardPolicy默默的忽略掉被拒绝任务,也没有输出日志或者提示,开发人员不会知道线程池的处理过程出现了错误;DiscardOldestPolicy中e.getQueue().poll()的方式好像是科学的,但是如果等待队列出现了容量问题,大多数情况下就是这个线程池的代码出现了BUG。最科学的的还是AbortPolicy提供的处理方式:抛出异常,由开发人员进行处理。

时间: 04-15

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