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itemprop="name headline">深入学习Java线程池</h2><div class="post-meta"> <time title="创建于" itemprop="dateCreated datePublished" datetime="2019-03-20T20:35:35+08:00">2019-03-20 </time> |   Visit count </div></header><div class="post-body" itemprop="articleBody">在前面的例子中，我们都是通过<code>new Thread</code>来创建一个线程，由于线程的创建和销毁都需要消耗一定的CPU资源，所以在高并发下这种创建线程的方式将严重影响代码执行效率。而线程池的作用就是让一个线程执行结束后不马上销毁，继续执行新的任务，这样就节省了不断创建线程和销毁线程的开销。<a id="more"></a><h2 id="ThreadPoolExecutor"><a href="#ThreadPoolExecutor" class="headerlink" title="ThreadPoolExecutor"></a>ThreadPoolExecutor</h2>创建Java线程池最为核心的类为<code>ThreadPoolExecutor</code>：<img src="img/QQ截图20190630215357.png" alt="QQ截图20190630215357.png">它提供了四种构造函数来创建线程池，其中最为核心的构造函数如下所示：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 </pre></td><td class="code"><pre>public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) </pre></td></tr></table></figure>这7个参数的含义如下：<ol><li>corePoolSize 线程池核心线程数。即线程池中保留的线程个数，即使这些线程是空闲的，也不会被销毁，除非通过ThreadPoolExecutor的<code>allowCoreThreadTimeOut(true)</code>方法开启了核心线程的超时策略；</li><li>maximumPoolSize 线程池中允许的最大线程个数；</li><li>keepAliveTime 用于设置那些超出核心线程数量的线程的最大等待时间，超过这个时间还没有新任务的话，超出的线程将被销毁；</li><li>unit 超时时间单位；</li><li>workQueue 线程队列。用于保存通过execute方法提交的，等待被执行的任务；</li><li>threadFactory 线程创建工程，即指定怎样创建线程；</li><li>handler 拒绝策略。即指定当线程提交的数量超出了maximumPoolSize后，该使用什么策略处理超出的线程。</li></ol>在通过这个构造方法创建线程池的时候，这几个参数必须满足以下条件，否则将抛出<code>IllegalArgumentException</code>异常：<ol><li>corePoolSize不能小于0；</li><li>keepAliveTime不能小于0；</li><li>maximumPoolSize 不能小于等于0；</li><li>maximumPoolSize不能小于corePoolSize；</li></ol>此外，workQueue、threadFactory和handler不能为null，否则将抛出空指针异常。下面举些例子来深入理解这几个参数的含义。使用上面的构造方法创建一个线程池：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 </pre></td><td class="code"><pre>ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 1, 2, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy()); System.out.println("线程池创建完毕"); int activeCount = -1; int queueSize = -1; while (true) { if (activeCount != threadPoolExecutor.getActiveCount() || queueSize != threadPoolExecutor.getQueue().size()) { System.out.println("活跃线程个数 " + threadPoolExecutor.getActiveCount()); System.out.println("核心线程个数 " + threadPoolExecutor.getCorePoolSize()); System.out.println("队列线程个数 " + threadPoolExecutor.getQueue().size()); System.out.println("最大线程数 " + threadPoolExecutor.getMaximumPoolSize()); System.out.println("------------------------------------"); activeCount = threadPoolExecutor.getActiveCount(); queueSize = threadPoolExecutor.getQueue().size(); } } </pre></td></tr></table></figure>上面的代码创建了一个核心线程数量为1，允许最大线程数量为2，最大活跃时间为10秒，线程队列长度为1的线程池。假如我们通过execute方法向线程池提交1个任务，看看结果如何：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 </pre></td><td class="code"><pre>ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 1, 2, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy()); System.out.println("线程池创建完毕"); threadPoolExecutor.execute(() -> sleep(100)); int activeCount = -1; int queueSize = -1; while (true) { if (activeCount != threadPoolExecutor.getActiveCount() || queueSize != threadPoolExecutor.getQueue().size()) { System.out.println("活跃线程个数 " + threadPoolExecutor.getActiveCount()); System.out.println("核心线程个数 " + threadPoolExecutor.getCorePoolSize()); System.out.println("队列线程个数 " + threadPoolExecutor.getQueue().size()); System.out.println("最大线程数 " + threadPoolExecutor.getMaximumPoolSize()); System.out.println("------------------------------------"); activeCount = threadPoolExecutor.getActiveCount(); queueSize = threadPoolExecutor.getQueue().size(); } } </pre></td></tr></table></figure><div class="note info">ThreadPoolExecutor的execute和submit方法都可以向线程池提交任务，区别是，submit方法能够返回执行结果，返回值类型为Future</div>sleep方法代码：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 </pre></td><td class="code"><pre>private static void sleep(long value) { try { System.out.println(Thread.currentThread().getName() + "线程执行sleep方法"); TimeUnit.SECONDS.sleep(value); } catch (InterruptedException e) { e.printStackTrace(); } } </pre></td></tr></table></figure>启动程序，控制台输出如下： <img src="img/QQ截图20190630222238.png" alt="QQ截图20190630222238.png">线程池核心线程数量为1，通过execute提交了一个任务后，由于核心线程是空闲的，所以任务被执行了。由于这个任务的逻辑是休眠100秒，所以在这100秒内，线程池的活跃线程数量为1。此外，因为提交的任务被核心线程执行了，所以并没有线程需要被放到线程队列里等待，线程队列长度为0。假如我们通过execute方法向线程池提交2个任务，看看结果如何：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 </pre></td><td class="code"><pre>threadPoolExecutor.execute(() -> sleep(100)); threadPoolExecutor.execute(() -> sleep(100)); </pre></td></tr></table></figure><img src="img/QQ截图20190701183457.png" alt="QQ截图20190701183457.png">线程池核心线程数量为1，通过execute提交了2个任务后，一开始核心线程是空闲的，Thread-0被执行。由于这个任务的逻辑是休眠100秒，所以在这100秒内，线程池的活跃线程数量为1。因为核心线程数量为1，所以另外一个任务在这100秒内不能被执行，于是被放到线程队列里等待，线程队列长度为1。假如我们通过execute方法向线程池提交3个任务，看看结果如何：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 </pre></td><td class="code"><pre>threadPoolExecutor.execute(() -> sleep(100)); threadPoolExecutor.execute(() -> sleep(100)); threadPoolExecutor.execute(() -> sleep(100)); </pre></td></tr></table></figure><img src="img/QQ截图20190701184303.png" alt="QQ截图20190701184303.png">这三个任务都是休眠100秒，所以核心线程池中第一个任务正在被执行，第二个任务被放入到了线程队列。而当第三个任务被提交进来时，线程队列满了（我们定义的长度为1），由于该线程池允许的最大线程数量为2，所以线程池还可以再创建一个线程来执行另外一个任务，于是乎之前在线程队列里的线程被取出执行（FIFO），第三个任务被放入到了线程队列。改变第二个和第三个任务的睡眠时间，观察输出：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 </pre></td><td class="code"><pre>threadPoolExecutor.execute(() -> sleep(100)); threadPoolExecutor.execute(() -> sleep(5)); threadPoolExecutor.execute(() -> sleep(5)); </pre></td></tr></table></figure><img src="img/QQ截图20190701185215.png" alt="QQ截图20190701185215.png">第二个任务提交5秒后，任务执行完毕，所以线程队列里的任务被执行，于是队列线程个数为0，活跃线程数量为2（第一个和第三个任务）。再过5秒后，第三个任务执行完毕，于是活跃线程数量为1（第一个100秒还没执行完毕）。在第三个任务结束的瞬间，我们观察线程快照:<img src="img/QQ截图20190701185617.png" alt="QQ截图20190701185617.png">可以看到，线程池中有两个线程，Thread-0在执行第一个任务（休眠100秒，还没结束），Thread-1执行完第三个任务后并没有马上被销毁。过段时间后（10秒钟后）再观察线程快照:<img src="img/QQ截图20190701190444.png" alt="QQ截图20190701190444.png">可以看到，Thread-1这个线程被销毁了，因为我们在创建线程池的时候，指定keepAliveTime 为10秒，10秒后，超出核心线程池线程外的那些线程将被销毁。假如一次性提交4个任务，看看会怎样：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 </pre></td><td class="code"><pre>threadPoolExecutor.execute(() -> sleep(100)); threadPoolExecutor.execute(() -> sleep(100)); threadPoolExecutor.execute(() -> sleep(100)); threadPoolExecutor.execute(() -> sleep(100)); </pre></td></tr></table></figure><img src="img/QQ截图20190701190808.png" alt="QQ截图20190701190808.png">因为我们设置的拒绝策略为AbortPolicy，所以最后提交的那个任务直接被拒绝了。更多拒绝策略下面会介绍到。<h2 id="关闭线程池"><a href="#关闭线程池" class="headerlink" title="关闭线程池"></a>关闭线程池</h2>线程池包含以下几个状态：<img src="img/QQ截图20190702100110.png" alt="QQ截图20190702100110.png">当线程池中所有任务都处理完毕后，线程并不会自己关闭。我们可以通过调用<code>shutdown</code>和<code>shutdownNow</code>方法来关闭线程池。两者的区别在于：<ol><li><code>shutdown</code>方法将线程池置为shutdown状态，拒绝新的任务提交，但线程池并不会马上关闭，而是等待所有正在执行的和线程队列里的任务都执行完毕后，线程池才会被关闭。所以这个方法是平滑的关闭线程池。</li><li><code>shutdownNow</code>方法将线程池置为stop状态，拒绝新的任务提交，中断正在执行的那些任务，并且清除线程队列里的任务并返回。所以这个方法是比较“暴力”的。</li></ol>举两个例子观察下两者的区别：<code>shutdown</code>例子:<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 </pre></td><td class="code"><pre>public static void main(String[] args) { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy()); threadPoolExecutor.execute(new shortTask()); threadPoolExecutor.execute(new longTask()); threadPoolExecutor.execute(new longTask()); threadPoolExecutor.execute(new shortTask()); threadPoolExecutor.shutdown(); System.out.println("已经执行了线程池shutdown方法"); } static class shortTask implements Runnable { @Override public void run() { try { TimeUnit.SECONDS.sleep(1); System.out.println(Thread.currentThread().getName() + "执行shortTask完毕"); } catch (InterruptedException e) { System.err.println("shortTask执行过程中被打断" + e.getMessage()); } } } static class longTask implements Runnable { @Override public void run() { try { TimeUnit.SECONDS.sleep(5); System.out.println(Thread.currentThread().getName() + "执行longTask完毕"); } catch (InterruptedException e) { System.err.println("longTask执行过程中被打断" + e.getMessage()); } } } </pre></td></tr></table></figure>启动程序，控制台输出如下：<img src="img/QQ截图20190702101041.png" alt="QQ截图20190702101041.png">可以看到，虽然在任务都被提交后马上执行了<code>shutdown</code>方法，但是并不会马上关闭线程池，而是等待所有被提交的任务都执行完了才关闭。<code>shutdownNow</code>例子：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 </pre></td><td class="code"><pre>public static void main(String[] args) { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy()); threadPoolExecutor.execute(new shortTask()); threadPoolExecutor.execute(new longTask()); threadPoolExecutor.execute(new longTask()); threadPoolExecutor.execute(new shortTask()); List<Runnable> runnables = threadPoolExecutor.shutdownNow(); // 马上关闭，并返回还未被执行的任务 System.out.println(runnables); System.out.println("已经执行了线程池shutdownNow方法"); } static class shortTask implements Runnable { @Override public void run() { try { TimeUnit.SECONDS.sleep(1); System.out.println(Thread.currentThread().getName() + "执行shortTask完毕"); } catch (InterruptedException e) { System.err.println("shortTask执行过程中被打断" + e.getMessage()); } } } static class longTask implements Runnable { @Override public void run() { try { TimeUnit.SECONDS.sleep(5); System.out.println(Thread.currentThread().getName() + "执行longTask完毕"); } catch (InterruptedException e) { System.err.println("longTask执行过程中被打断" + e.getMessage()); } } } </pre></td></tr></table></figure>启动程序，控制台输出如下： <img src="img/QQ截图20190702101355.png" alt="QQ截图20190702101355.png">可以看到，在执行<code>shutdownNow</code>方法后，线程池马上就被关闭了，正在执行中的两个任务被打断，并且返回了线程队列中等待被执行的两个任务。通过上面两个例子我们还可以看到<code>shutdown</code>和<code>shutdownNow</code>方法都不是阻塞的。常与<code>shutdown</code>搭配的方法有<code>awaitTermination</code>。<code>awaitTermination</code>方法接收timeout和TimeUnit两个参数，用于设定超时时间及单位。当等待超过设定时间时，会监测ExecutorService是否已经关闭，若关闭则返回true，否则返回false。该方法是阻塞的：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy()); threadPoolExecutor.execute(new shortTask()); threadPoolExecutor.execute(new longTask()); threadPoolExecutor.execute(new longTask()); threadPoolExecutor.execute(new shortTask()); threadPoolExecutor.shutdown(); boolean isShutdown = threadPoolExecutor.awaitTermination(3, TimeUnit.SECONDS); if (isShutdown) { System.out.println("线程池在3秒内成功关闭"); } else { System.out.println("等了3秒还没关闭，不等了╰（‵□′）╯"); } System.out.println("------------"); } static class shortTask implements Runnable { @Override public void run() { try { TimeUnit.SECONDS.sleep(1); System.out.println(Thread.currentThread().getName() + "执行shortTask完毕"); } catch (InterruptedException e) { System.err.println("shortTask执行过程中被打断" + e.getMessage()); } } } static class longTask implements Runnable { @Override public void run() { try { TimeUnit.SECONDS.sleep(5); System.out.println(Thread.currentThread().getName() + "执行longTask完毕"); } catch (InterruptedException e) { System.err.println("longTask执行过程中被打断" + e.getMessage()); } } } </pre></td></tr></table></figure>启动程序输出如下：<img src="img/QQ截图20190702102156.png" alt="QQ截图20190702102156.png"><h2 id="4大拒绝策略"><a href="#4大拒绝策略" class="headerlink" title="4大拒绝策略"></a>4大拒绝策略</h2>当线程池无法再接收新的任务的时候，可采取如下四种策略： <img src="img/QQ截图20190302111014.png" alt="QQ截图20190302111014.png"><h3 id="CallerRunsPolicy"><a href="#CallerRunsPolicy" class="headerlink" title="CallerRunsPolicy"></a>CallerRunsPolicy</h3><code>CallerRunsPolicy</code>策略：由调用线程处理该任务：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 3, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.CallerRunsPolicy()); threadPoolExecutor.execute(new shortTask("任务1")); threadPoolExecutor.execute(new longTask("任务2")); threadPoolExecutor.execute(new longTask("任务3")); threadPoolExecutor.execute(new shortTask("任务4")); threadPoolExecutor.execute(new shortTask("任务5")); threadPoolExecutor.shutdown(); } static class shortTask implements Runnable { private String name; public shortTask(String name) { this.name = name; } @Override public void run() { try { TimeUnit.SECONDS.sleep(1); System.out.println(Thread.currentThread().getName() + "执行shortTask-name-" + name + "完毕"); } catch (InterruptedException e) { System.err.println("shortTask执行过程中被打断" + e.getMessage()); } } } static class longTask implements Runnable { private String name; public longTask(String name) { this.name = name; } @Override public void run() { try { TimeUnit.SECONDS.sleep(5); System.out.println(Thread.currentThread().getName() + "执行longTask-name-" + name + "完毕"); } catch (InterruptedException e) { System.err.println("longTask执行过程中被打断" + e.getMessage()); } } } </pre></td></tr></table></figure>上面的线程池最多只能一次性提交4个任务，第5个任务提交后会被拒绝策略处理。启动程序输出如下：<img src="img/QQ截图20190702103818.png" alt="QQ截图20190702103818.png">可以看到，第5个提交的任务由调用线程（即main线程）处理该任务。<h3 id="AbortPolicy"><a href="#AbortPolicy" class="headerlink" title="AbortPolicy"></a>AbortPolicy</h3><code>AbortPolicy</code>策略：丢弃任务，并抛出<code>RejectedExecutionException</code>异常。前面的例子就是使用该策略，所以不再演示。<h3 id="DiscardOldestPolicy"><a href="#DiscardOldestPolicy" class="headerlink" title="DiscardOldestPolicy"></a>DiscardOldestPolicy</h3><code>DiscardOldestPolicy</code>策略：丢弃最早被放入到线程队列的任务，将新提交的任务放入到线程队列末端：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 3, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.DiscardOldestPolicy()); threadPoolExecutor.execute(new shortTask("任务1")); threadPoolExecutor.execute(new longTask("任务2")); threadPoolExecutor.execute(new longTask("任务3")); threadPoolExecutor.execute(new shortTask("任务4")); threadPoolExecutor.execute(new shortTask("任务5")); threadPoolExecutor.shutdown(); } static class shortTask implements Runnable { private String name; public shortTask(String name) { this.name = name; } @Override public void run() { try { TimeUnit.SECONDS.sleep(1); System.out.println(Thread.currentThread().getName() + "执行shortTask-name-" + name + "完毕"); } catch (InterruptedException e) { System.err.println("shortTask执行过程中被打断" + e.getMessage()); } } } static class longTask implements Runnable { private String name; public longTask(String name) { this.name = name; } @Override public void run() { try { TimeUnit.SECONDS.sleep(5); System.out.println(Thread.currentThread().getName() + "执行longTask-name-" + name + "完毕"); } catch (InterruptedException e) { System.err.println("longTask执行过程中被打断" + e.getMessage()); } } } </pre></td></tr></table></figure>启动程序输出如下：<img src="img/QQ截图20190702105646.png" alt="QQ截图20190702105646.png">可以看到最后提交的任务被执行了，而第3个任务是第一个被放到线程队列的任务，被丢弃了。<h3 id="DiscardPolicy"><a href="#DiscardPolicy" class="headerlink" title="DiscardPolicy"></a>DiscardPolicy</h3><code>DiscardPolicy</code>策略：直接丢弃新的任务，不抛异常：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 3, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.DiscardPolicy()); threadPoolExecutor.execute(new shortTask("任务1")); threadPoolExecutor.execute(new longTask("任务2")); threadPoolExecutor.execute(new longTask("任务3")); threadPoolExecutor.execute(new shortTask("任务4")); threadPoolExecutor.execute(new shortTask("任务5")); threadPoolExecutor.shutdown(); } static class shortTask implements Runnable { private String name; public shortTask(String name) { this.name = name; } @Override public void run() { try { TimeUnit.SECONDS.sleep(1); System.out.println(Thread.currentThread().getName() + "执行shortTask-name-" + name + "完毕"); } catch (InterruptedException e) { System.err.println("shortTask执行过程中被打断" + e.getMessage()); } } } static class longTask implements Runnable { private String name; public longTask(String name) { this.name = name; } @Override public void run() { try { TimeUnit.SECONDS.sleep(5); System.out.println(Thread.currentThread().getName() + "执行longTask-name-" + name + "完毕"); } catch (InterruptedException e) { System.err.println("longTask执行过程中被打断" + e.getMessage()); } } } </pre></td></tr></table></figure>启动程序，输出如下： <img src="img/QQ截图20190702110022.png" alt="QQ截图20190702110022.png">第5个任务直接被拒绝丢弃了，而没有抛出任何异常。<h2 id="线程池工厂方法"><a href="#线程池工厂方法" class="headerlink" title="线程池工厂方法"></a>线程池工厂方法</h2>除了使用ThreadPoolExecutor的构造方法创建线程池外，我们也可以使用<code>Executors</code>提供的工厂方法来创建不同类型的线程池：<img src="img/QQ截图20190702110350.png" alt="QQ截图20190702110350.png"><h3 id="newFixedThreadPool"><a href="#newFixedThreadPool" class="headerlink" title="newFixedThreadPool"></a>newFixedThreadPool</h3>查看<code>newFixedThreadPool</code>方法源码：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 </pre></td><td class="code"><pre>public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); } </pre></td></tr></table></figure>可以看到，通过<code>newFixedThreadPool</code>创建的是一个固定大小的线程池，大小由<code>nThreads</code>参数指定，它具有如下几个特点:<ol><li>因为corePoolSize和maximumPoolSize的值都为nThreads，所以线程池中线程数量永远等于nThreads，不可能新建除了核心线程数的线程来处理任务，即keepAliveTime实际上在这里是无效的。</li><li>LinkedBlockingQueue是一个无界队列（最大长度为Integer.MAX_VALUE），所以这个线程池理论是可以无限的接收新的任务，这就是为什么上面没有指定拒绝策略的原因。</li></ol><h3 id="newCachedThreadPool"><a href="#newCachedThreadPool" class="headerlink" title="newCachedThreadPool"></a>newCachedThreadPool</h3>查看<code>newCachedThreadPool</code>方法源码：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 </pre></td><td class="code"><pre>public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); } </pre></td></tr></table></figure>这是一个理论上无限大小的线程池：<ol><li>核心线程数为0，SynchronousQueue队列是没有长度的队列，所以当有新的任务提交，如果有空闲的还未超时的（最大空闲时间60秒）线程则执行该任务，否则新增一个线程来处理该任务。</li><li>因为线程数量没有限制，理论上可以接收无限个新任务，所以这里也没有指定拒绝策略。</li></ol><h3 id="newSingleThreadExecutor"><a href="#newSingleThreadExecutor" class="headerlink" title="newSingleThreadExecutor"></a>newSingleThreadExecutor</h3>查看<code>newSingleThreadExecutor</code>源码：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 </pre></td><td class="code"><pre>public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); } </pre></td></tr></table></figure><ol><li>核心线程数和最大线程数都为1，每次只能有一个线程处理任务。</li><li>LinkedBlockingQueue队列可以接收无限个新任务。</li></ol><h3 id="newScheduledThreadPool"><a href="#newScheduledThreadPool" class="headerlink" title="newScheduledThreadPool"></a>newScheduledThreadPool</h3>查看<code>newScheduledThreadPool</code>源码：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 </pre></td><td class="code"><pre>public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) { return new ScheduledThreadPoolExecutor(corePoolSize); } ...... public ScheduledThreadPoolExecutor(int corePoolSize) { super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS, new DelayedWorkQueue()); } </pre></td></tr></table></figure>所以<code>newScheduledThreadPool</code>理论是也是可以接收无限个任务，DelayedWorkQueue也是一个无界队列。使用newScheduledThreadPool创建的线程池除了可以处理普通的Runnable任务外，它还具有调度的功能：1.延迟指定时间后执行：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 </pre></td><td class="code"><pre>ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1); // 延迟5秒执行 executorService.schedule(() -> System.out.println("hello"), 5, TimeUnit.SECONDS); </pre></td></tr></table></figure>2.按指定的速率执行：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 </pre></td><td class="code"><pre>ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1); // 延迟1秒执行，然后每5秒执行一次 executorService.scheduleAtFixedRate( () -> System.out.println(LocalTime.now()), 1, 5, TimeUnit.SECONDS ); </pre></td></tr></table></figure><img src="img/QQ截图20190702152117.png" alt="QQ截图20190702152117.png">3.按指定的时延执行：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 </pre></td><td class="code"><pre>ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1); executorService.scheduleWithFixedDelay( () -> System.out.println(LocalTime.now()), 1, 5, TimeUnit.SECONDS ); </pre></td></tr></table></figure><img src="img/QQ截图20190702152440.png" alt="QQ截图20190702152440.png">乍一看，scheduleAtFixedRate和scheduleWithFixedDelay没啥区别，实际它们还是有区别的：<ul><li>scheduleAtFixedRate按照固定速率执行任务，比如每5秒执行一个任务，即使上一个任务没有结束，5秒后也会开始处理新的任务；</li><li>scheduleWithFixedDelay按照固定的时延处理任务，比如每延迟5秒执行一个任务，无论上一个任务处理了1秒，1分钟还是1小时，下一个任务总是在上一个任务执行完毕后5秒钟后开始执行。</li></ul>对于这些线程池工厂方法的使用，阿里巴巴编程规程指出：<img src="img/QQ截图20190702153306.png" alt="QQ截图20190702153306.png">因为这几个线程池理论是都可以接收无限个任务，所以这就有内存溢出的风险。实际上只要我们掌握了ThreadPoolExecutor构造函数7个参数的含义，我们就可以根据不同的业务来创建出符合需求的线程池。一般线程池的创建可以参考如下规则：<ul><li>IO密集型任务：IO密集型任务线程并不是一直在执行任务，应该配置尽可能多的线程，线程池线程数量推荐设置为2 * CPU核心数；对于IO密集型任务，网络上也有另一种线程池数量计算公式：CPU核心数/(1 - 阻塞系数)，阻塞系数取值0.8~0.9，至于这两种公式使用哪一个，可以根据实际环境测试比较得出；</li><li>计算密集型任务：此类型需要CPU的大量运算，所以尽可能的去压榨CPU资源，线程池线程数量推荐设置为CPU核心数 + 1。</li></ul>CPU核心数可以使用<code>Runtime</code>获得：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 </pre></td><td class="code"><pre>Runtime.getRuntime().availableProcessors() </pre></td></tr></table></figure><h2 id="一些API的用法"><a href="#一些API的用法" class="headerlink" title="一些API的用法"></a>一些API的用法</h2>ThreadPoolExecutor提供了几个判断线程池状态的方法：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 1, 2, 5, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy() ); threadPoolExecutor.execute(() -> { try { TimeUnit.SECONDS.sleep(5); } catch (InterruptedException e) { e.printStackTrace(); } }); threadPoolExecutor.shutdown(); System.out.println("线程池为shutdown状态：" + threadPoolExecutor.isShutdown()); System.out.println("线程池正在关闭：" + threadPoolExecutor.isTerminating()); System.out.println("线程池已经关闭：" + threadPoolExecutor.isTerminated()); threadPoolExecutor.awaitTermination(6, TimeUnit.SECONDS); System.out.println("线程池已经关闭" + threadPoolExecutor.isTerminated()); } </pre></td></tr></table></figure>程序输出如下：<img src="img/20190703205843.png" alt="20190703205843.png">前面我们提到，线程池核心线程即使是空闲状态也不会被销毁，除非使用<code>allowCoreThreadTimeOut</code>设置了允许核心线程超时：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 1, 2, 3, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy() ); threadPoolExecutor.allowCoreThreadTimeOut(true); threadPoolExecutor.execute(() -> { try { TimeUnit.SECONDS.sleep(5); System.out.println("任务执行完毕"); } catch (InterruptedException e) { e.printStackTrace(); } }); } </pre></td></tr></table></figure>程序输出如下所示：<img src="img/asdfasdfaaaaa.gif" alt="asdfasdfaaaaa.gif">5秒后任务执行完毕，核心线程处于空闲的状态。因为通过<code>allowCoreThreadTimeOut</code>方法设置了允许核心线程超时，所以3秒后（keepAliveTime设置为3秒），核心线程被销毁。核心线程被销毁后，线程池也就没有作用了，于是就自动关闭了。<div class="note danger">值得注意的是，如果一个线程池调用了<code>allowCoreThreadTimeOut(true)</code>方法，那么它的<code>keepAliveTime</code>不能为0。</div>ThreadPoolExecutor提供了一<code>remove</code>方法，查看其源码：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 </pre></td><td class="code"><pre>public boolean remove(Runnable task) { boolean removed = workQueue.remove(task); tryTerminate(); // In case SHUTDOWN and now empty return removed; } </pre></td></tr></table></figure>可看到，它删除的是线程队列中的任务，而非正在被执行的任务。举个例子：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 1, 2, 3, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy() ); threadPoolExecutor.execute(() -> { try { TimeUnit.SECONDS.sleep(5); System.out.println("任务执行完毕"); } catch (InterruptedException e) { e.printStackTrace(); } }); Runnable r = () -> System.out.println("看看我是否会被删除"); threadPoolExecutor.execute(r); threadPoolExecutor.remove(r); threadPoolExecutor.shutdown(); } </pre></td></tr></table></figure>执行程序，输出如下：<img src="img/QQ截图20190703211746.png" alt="QQ截图20190703211746.png">可看到任务并没有被执行，已经被删除，因为唯一一个核心线程已经在执行任务了，所以后提交的这个任务被放到了线程队列里，然后通过remove方法删除。默认情况下，只有当往线程池里提交了任务后，线程池才会启动核心线程处理任务。我们可以通过调用<code>prestartCoreThread</code>方法，让核心线程即使没有任务提交，也处于等待执行任务的活跃状态：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 2, 3, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy() ); System.out.println("活跃线程数: " + threadPoolExecutor.getActiveCount()); threadPoolExecutor.prestartCoreThread(); System.out.println("活跃线程数: " + threadPoolExecutor.getActiveCount()); threadPoolExecutor.prestartCoreThread(); System.out.println("活跃线程数: " + threadPoolExecutor.getActiveCount()); threadPoolExecutor.prestartCoreThread(); System.out.println("活跃线程数: " + threadPoolExecutor.getActiveCount()); } </pre></td></tr></table></figure>程序输出如下所示：<img src="img/QQ截图20190703213145.png" alt="QQ截图20190703213145.png">该方法返回boolean类型值，如果所以核心线程都启动了，返回false，反之返回true。还有一个和它类似的<code>prestartAllCoreThreads</code>方法，它的作用是一次性启动所有核心线程，让其处于活跃地等待执行任务的状态。ThreadPoolExecutor的invokeAny方法用于随机执行任务集合中的某个任务，并返回执行结果，该方法是同步方法：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException, ExecutionException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 5, 3, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy() ); // 任务集合 List<Callable<Integer>> tasks = IntStream.range(0, 4).boxed().map(i -> (Callable<Integer>) () -> { TimeUnit.SECONDS.sleep(ThreadLocalRandom.current().nextInt(5)); return i; }).collect(Collectors.toList()); // 随机执行结果 Integer result = threadPoolExecutor.invokeAny(tasks); System.out.println("-------------------"); System.out.println(result); threadPoolExecutor.shutdownNow(); } </pre></td></tr></table></figure>启动程序，输出如下：<img src="img/QQ截图20190704091530.png" alt="QQ截图20190704091530.png">ThreadPoolExecutor的invokeAll则是执行任务集合中的所有任务，返回Future集合：<figure class="highlight java"><table><tr><td class="gutter"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 </pre></td><td class="code"><pre>public static void main(String[] args) throws InterruptedException, ExecutionException { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( 2, 5, 3, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1), (ThreadFactory) Thread::new, new ThreadPoolExecutor.AbortPolicy() ); List<Callable<Integer>> tasks = IntStream.range(0, 4).boxed().map(i -> (Callable<Integer>) () -> { TimeUnit.SECONDS.sleep(ThreadLocalRandom.current().nextInt(5)); return i; }).collect(Collectors.toList()); List<Future<Integer>> futureList = threadPoolExecutor.invokeAll(tasks); futureList.stream().map(f->{ try { return f.get(); } catch (InterruptedException | ExecutionException e) { return null; } }).forEach(System.out::println); threadPoolExecutor.shutdownNow(); } </pre></td></tr></table></figure>输出如下： <img src="img/QQ截图20190704091836.png" alt="QQ截图20190704091836.png">总结下这些方法：<table><tr><th>方法</th><th>描述</th></tr><tr><td>allowCoreThreadTimeOut(boolean value)</td><td>是否允许核心线程空闲后超时，是的话超时后核心线程将销毁，线程池自动关闭</td></tr><tr><td>awaitTermination(long timeout, TimeUnit unit)</td><td>阻塞当前线程，等待线程池关闭，timeout用于指定等待时间。</td></tr><tr><td>execute(Runnable command)</td><td>向线程池提交任务，没有返回值</td></tr><tr><td>submit(Runnable task)</td><td>向线程池提交任务，返回Future</td></tr><tr><td>isShutdown()</td><td>判断线程池是否为shutdown状态</td></tr><tr><td>isTerminating()</td><td>判断线程池是否正在关闭</td></tr><tr><td>isTerminated()</td><td>判断线程池是否已经关闭</td></tr><tr><td>remove(Runnable task)</td><td>移除线程队列中的指定任务</td></tr><tr><td>prestartCoreThread()</td><td>提前让一个核心线程处于活跃状态，等待执行任务</td></tr><tr><td>prestartAllCoreThreads()</td><td>提前让所有核心线程处于活跃状态，等待执行任务</td></tr><tr><td>getActiveCount()</td><td>获取线程池活跃线程数</td></tr><tr><td>getCorePoolSize()</td><td>获取线程池核心线程数</td></tr><tr><td>threadPoolExecutor.getQueue()</td><td>获取线程池线程队列</td></tr><tr><td>getMaximumPoolSize()</td><td>获取线程池最大线程数</td></tr><tr><td>shutdown()</td><td>让线程池处于shutdown状态，不再接收任务，等待所有正在运行中的任务结束后，关闭线程池。</td></tr><tr><td>shutdownNow()</td><td>让线程池处于stop状态，不再接受任务，尝试打断正在运行中的任务，并关闭线程池，返回线程队列中的任务。</td></tr></table><script>$(".post-body 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