Java线程池任务执行完毕后回收线程

线程池中的所有任务执行完毕后，线程并没有停止,导致JVM出现OOM问题。后来查找了下面链接的资料，解决问题。
ref: http://www.cnblogs.com/pengineer/p/5011965.html

问题及现象：

public static void main(String[] args) {
   	BlockingQueue queue = new LinkedBlockingQueue();
   	ThreadPoolExecutor executor = new ThreadPoolExecutor(3, 6, 10, TimeUnit.SECONDS, queue);
   	for (int i = 0; i < 20; i++) {
   		executor.execute(new Runnable() {
   			@Override
   			public void run() {
   				try {
   					System.out.println(this.hashCode() / 1000);
   					for (int j = 0; j < 10; j++) {
   						System.out.println(this.hashCode() + ":" + j);
   						Thread.sleep(this.hashCode() % 2);
   					}
   				} catch (InterruptedException e) {
   					e.printStackTrace();
   				}
   				System.out.println(String.format("thread %d finished", this.hashCode()));
   			}
   		});
   	}
   }

任务已经执行完毕了，但是线程池中的线程并没有被回收,程序任然处于运行状态。但是在ThreadPoolExecutor的参数里面设置了超时时间的，好像没起作用，原因如下：
工作线程回收需要满足三个条件：

1. 参数allowCoreThreadTimeOut为true
2. 该线程在keepAliveTime时间内获取不到任务，即空闲这么长时间
3. 当前线程池大小 > 核心线程池大小corePoolSize。

解决一：allowCoreThreadTimeOut设置为true

public static void method1() {
          BlockingQueue queue = new LinkedBlockingQueue();
          ThreadPoolExecutor executor = new ThreadPoolExecutor(3, 6, 1, TimeUnit.SECONDS, queue);  
          executor.allowCoreThreadTimeOut(true);
          for ( int i = 0; i < 20; i++) {
              executor.execute( new Runnable() {
                  public void run() {
                      try {
                           System. out.println( this.hashCode()/1000);
                              for ( int j = 0; j < 10; j++) {
                                System. out.println( this.hashCode() + ":" + j);
                                Thread. sleep(this.hashCode()%2);
                           } 
                      } catch (InterruptedException e) {
                          e.printStackTrace();
                      }
                      System. out.println(String. format("thread %d finished", this.hashCode()));
                  }
              });
          }
      }

需要注意的是，allowCoreThreadTimeOut 的设置需要在任务执行之前，一般在new一个线程池后设置；在allowCoreThreadTimeOut设置为true时，ThreadPoolExecutor的keepAliveTime参数必须大于0。

解决二：调用shutdown方法

public static void method1() {
          BlockingQueue queue = new LinkedBlockingQueue();
          ThreadPoolExecutor executor = new ThreadPoolExecutor(3, 6, 1, TimeUnit.SECONDS, queue);
          for ( int i = 0; i < 20; i++) {
              executor.execute( new Runnable() {
                  public void run() {
                      try {
                          System. out.println( this.hashCode()/1000);
                            for ( int j = 0; j < 10; j++) {
                               System. out.println( this.hashCode() + ":" + j);
                               Thread. sleep(this.hashCode()%2);
                          } 
                      } catch (InterruptedException e) {
                          e.printStackTrace();
                      }
                      System. out.println(String. format("thread %d finished", this.hashCode()));
                  }
              });
          }         
          executor.shutdown();
      }

在任务执行完后，调用shutdown方法，将线程池中的空闲线程回收。该方法会使得keepAliveTime参数失效。