ExecutorService线程池的使用

我就是我 2022-08-09 12:42 225阅读 0赞

# ExecutorService线程池讲解 #

**ExecutorService 建立多线程的步骤：**

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   <td>1。定义线程类</td> 
   <td>class Handler implements Runnable{ <br> }</td> 
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   <td>2。建立ExecutorService线程池</td> 
   <td>ExecutorService executorService = Executors.newCachedThreadPool(); <p>或者</p> <p>int cpuNums = Runtime.getRuntime().availableProcessors();<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//获取当前系统的CPU 数目<br> ExecutorService executorService =Executors.newFixedThreadPool(cpuNums * POOL_SIZE);<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//ExecutorService通常根据系统资源情况灵活定义线程池大小</p> </td> 
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   <td>3。调用线程池操作</td> 
   <td>循环操作，成为daemon,把新实例放入Executor池中<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;while(true){ <br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;executorService.execute(new Handler(<a href="http://cpro.baidu.com/cpro/ui/uijs.php?adclass=0&amp;app_id=0&amp;c=news&amp;cf=1001&amp;ch=0&amp;di=128&amp;fv=15&amp;is_app=0&amp;jk=86cd8d57758b1b43&amp;k=socket&amp;k0=socket&amp;kdi0=0&amp;luki=4&amp;n=10&amp;p=baidu&amp;q=34005130_cpr&amp;rb=0&amp;rs=1&amp;seller_id=1&amp;sid=431b8b75578dcd86&amp;ssp2=1&amp;stid=0&amp;t=tpclicked3_hc&amp;td=1862747&amp;tu=u1862747&amp;u=http%3A%2F%2Fwww.cnphp6.com%2Farchives%2F61093&amp;urlid=0" rel="nofollow"><span style="color:#0000ff;">socket</span></a>));&nbsp;<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// class Handler implements Runnable{ <br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;或者<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;executorService.execute(createTask(i));<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//private static Runnable createTask(final int taskID)<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} <p>execute(Runnable对象)方法<br> 其实就是对Runnable对象调用start()方法<br> （当然还有一些其他后台动作，比如队列，优先级，IDLE timeout，active激活等）</p> </td> 
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**几种不同的ExecutorService线程池对象**

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   <td>1.newCachedThreadPool()&nbsp;</td> 
   <td>-缓存型池子，先查看池中有没有以前建立的线程，如果有，就reuse.如果没有，就建一个新的线程加入池中<br> -缓存型池子通常用于执行一些<strong>生存期很短的异步型任务</strong><br> &nbsp;因此在一些面向连接的daemon型SERVER中用得不多。<br> -能reuse的线程，必须是timeout IDLE内的池中线程，<strong>缺省timeout是60s</strong>,超过这个IDLE时长，线程实例将被终止及移出池。<br> &nbsp;注意，放入CachedThreadPool的线程不必担心其结束，<strong>超过TIMEOUT不活动，其会自动被终止。</strong></td> 
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   <td>2. newFixedThreadPool</td> 
   <td>-newFixedThreadPool与cacheThreadPool差不多，也是能reuse就用，但不能随时建新的线程<br> -其独特之处:任意时间点，最多只能有固定数目的活动线程存在，<strong>此时如果有新的线程要建立，只能放在另外的队列中等待</strong>，<strong>直到当前的线程中某个线程终止直接被移出池子</strong><br> -和cacheThreadPool不同，FixedThreadPool没有IDLE机制（可能也有，但既然文档没提，肯定非常长，类似依赖上层的TCP或UDP IDLE机制之类的），所以<strong>FixedThreadPool多数针对一些很稳定很固定的正规并发线程，多用于服务器</strong><br> -从方法的源代码看，cache池和fixed 池调用的是同一个<a href="http://cpro.baidu.com/cpro/ui/uijs.php?adclass=0&amp;app_id=0&amp;c=news&amp;cf=1001&amp;ch=0&amp;di=128&amp;fv=15&amp;is_app=0&amp;jk=86cd8d57758b1b43&amp;k=%B5%D7%B2%E3&amp;k0=%B5%D7%B2%E3&amp;kdi0=0&amp;luki=3&amp;n=10&amp;p=baidu&amp;q=34005130_cpr&amp;rb=0&amp;rs=1&amp;seller_id=1&amp;sid=431b8b75578dcd86&amp;ssp2=1&amp;stid=0&amp;t=tpclicked3_hc&amp;td=1862747&amp;tu=u1862747&amp;u=http%3A%2F%2Fwww.cnphp6.com%2Farchives%2F61093&amp;urlid=0" rel="nofollow"><span style="color:#0000ff;">底层</span></a>池，只不过参数不同:<br><strong>fixed池线程数固定，并且是0秒IDLE（无IDLE）</strong><br><strong>cache池线程数支持0-Integer.MAX_VALUE(显然完全没考虑主机的资源承受能力），60秒IDLE&nbsp;</strong>&nbsp;</td> 
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   <td>3.ScheduledThreadPool</td> 
   <td>-调度型线程池<br> -这个池子里的线程可以按schedule依次delay执行，或周期执行</td> 
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   <td>4.SingleThreadExecutor</td> 
   <td>-单例线程，任意时间池中只能有一个线程<br> -<strong>用的是和cache池和fixed池相同的底层池，但线程数目是1-1,0秒IDLE（无IDLE）</strong></td> 
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**上面四种线程池，都使用Executor的缺省线程工厂建立线程，也可单独定义自己的线程工厂**  
下面是缺省线程工厂代码:

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   <td>&nbsp;&nbsp;&nbsp;static class DefaultThreadFactory implements ThreadFactory { <br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;static final AtomicInteger poolNumber = new AtomicInteger(1);<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;final ThreadGroup <a href="http://cpro.baidu.com/cpro/ui/uijs.php?adclass=0&amp;app_id=0&amp;c=news&amp;cf=1001&amp;ch=0&amp;di=128&amp;fv=15&amp;is_app=0&amp;jk=86cd8d57758b1b43&amp;k=group&amp;k0=group&amp;kdi0=0&amp;luki=6&amp;n=10&amp;p=baidu&amp;q=34005130_cpr&amp;rb=0&amp;rs=1&amp;seller_id=1&amp;sid=431b8b75578dcd86&amp;ssp2=1&amp;stid=0&amp;t=tpclicked3_hc&amp;td=1862747&amp;tu=u1862747&amp;u=http%3A%2F%2Fwww.cnphp6.com%2Farchives%2F61093&amp;urlid=0" rel="nofollow"><span style="color:#0000ff;">group</span></a>;<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;final AtomicInteger threadNumber = new AtomicInteger(1);<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;final String namePrefix; <p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;DefaultThreadFactory() { <br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;SecurityManager s = System.getSecurityManager();<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;group = (s != null)? s.getThreadGroup() :Thread.currentThread().getThreadGroup();<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;namePrefix = “pool-” + poolNumber.getAndIncrement() + “-thread-“;<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}</p> <p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;public Thread newThread(Runnable r) { <br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Thread t = new Thread(<a href="http://cpro.baidu.com/cpro/ui/uijs.php?adclass=0&amp;app_id=0&amp;c=news&amp;cf=1001&amp;ch=0&amp;di=128&amp;fv=15&amp;is_app=0&amp;jk=86cd8d57758b1b43&amp;k=group&amp;k0=group&amp;kdi0=0&amp;luki=6&amp;n=10&amp;p=baidu&amp;q=34005130_cpr&amp;rb=0&amp;rs=1&amp;seller_id=1&amp;sid=431b8b75578dcd86&amp;ssp2=1&amp;stid=0&amp;t=tpclicked3_hc&amp;td=1862747&amp;tu=u1862747&amp;u=http%3A%2F%2Fwww.cnphp6.com%2Farchives%2F61093&amp;urlid=0" rel="nofollow"><span style="color:#0000ff;">group</span></a>, r,namePrefix + threadNumber.getAndIncrement(),0);<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (t.isDaemon())<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;t.setDaemon(false);<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (t.getPriority() != Thread.NORM_PRIORITY)<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;t.setPriority(Thread.NORM_PRIORITY);<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return t;<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<br> &nbsp;&nbsp;&nbsp;}</p> </td> 
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也可自己定义ThreadFactory，加入建立池的参数中

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   <td>&nbsp;public static ExecutorService newCachedThreadPool<strong>(ThreadFactory threadFactory</strong>) { </td> 
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**Executor的execute()方法**  
execute() 方法将Runnable实例加入pool中,并进行一些pool size计算和优先级处理  
execute() 方法本身在Executor接口中定义,有多个实现类都定义了不同的execute()方法  
如ThreadPoolExecutor类（cache,fiexed,single三种池子都是调用它）的execute方法如下：

public void execute(Runnable command) \{  
       if (command == null)  
           throw new NullPointerException();  
       if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) \{  
           if (runState == RUNNING && workQueue.offer(command)) \{  
               if (runState != RUNNING || poolSize == 0)  
                   ensureQueuedTaskHandled(command);  
           \}  
           else if (!addIfUnderMaximumPoolSize(command))  
               reject(command); // is shutdown or saturated  
       \}  
   \}