JAVA线程池如何优雅关闭

JAVA线程池如何优雅关闭

	Effective JAVA 第三版并发部分提起了线程池优雅关闭的问题，意识到之前的线程关闭知识还不完善。讨论如下：

1.shutdown()

• 基本意思是：启动有序关闭，其中先前提交的任务将被执行关闭，但不会接受任何新任务。
  如果已经关闭，调用没有额外的作用。
  此方法不等待以前提交的任务完成执行。 使用awaitTermination做到这一点。
• shutdown只负责任务关闭，不负责任务是否执行完毕。比如有线程往数据库写数据，当线程池任务shutdown时，数据未完全写入数据库。此时就会造成预期与实际的数据不一致。
• 源码

  	
      /**
       * Initiates an orderly shutdown in which previously submitted
       * tasks are executed, but no new tasks will be accepted.
       * Invocation has no additional effect if already shut down.
       *
       * <p>This method does not wait for previously submitted tasks to
       * complete execution.  Use {@link #awaitTermination awaitTermination}
       * to do that.
       *
       * @throws SecurityException {@inheritDoc}
       */
      private final ReentrantLock mainLock = new ReentrantLock();
      public void shutdown() {
          final ReentrantLock mainLock = this.mainLock;
          mainLock.lock();
          try {
              checkShutdownAccess();
              advanceRunState(SHUTDOWN);
              interruptIdleWorkers();
              onShutdown(); // hook for ScheduledThreadPoolExecutor
          } finally {
              mainLock.unlock();
          }
          tryTerminate();
      }
  
  

2.awaitTermination(long timeout, TimeUnit unit)

• 基本意思是：在执行shutdown 或shutdownNow后，线程任务执行并未完全终止，如果任务执行完毕返回true，否则返回false 。
• 说白了就是一个线程活动监测器，等所有任务执行完毕，优雅关闭。
• 源码

     /**
       * Returns true if this executor is in the process of terminating
       * after {@link #shutdown} or {@link #shutdownNow} but has not
       * completely terminated.  This method may be useful for
       * debugging. A return of {@code true} reported a sufficient
       * period after shutdown may indicate that submitted tasks have
       * ignored or suppressed interruption, causing this executor not
       * to properly terminate.
       *
       * @return {@code true} if terminating but not yet terminated
       */
  public boolean awaitTermination(long timeout, TimeUnit unit)
          throws InterruptedException {
          long nanos = unit.toNanos(timeout);
          final ReentrantLock mainLock = this.mainLock;
          mainLock.lock();
          try {
              for (;;) {
                  if (runStateAtLeast(ctl.get(), TERMINATED))
                      return true;
                  if (nanos <= 0)
                      return false;
                  nanos = termination.awaitNanos(nanos);
              }
          } finally {
              mainLock.unlock();
          }
      }
  

3.优雅关闭

• 使用

  pool.shutdown();
  while(!pool.awaitTermination(100, TimeUnit.MILLISECONDS)) {
  	 System.out.println("线程池尚未关闭！");
  }