最近学习了线程过后，又想学学线程池，在写测试代码的时候想到一个问题，线程太多可能会导致内存占满的问题，那线程池要不要关闭呢？怎么关闭呢？

已知关闭有两种方法，shutdown()和shutdownNow()。shutdown()方法会关闭线程池，不再接受新的任务，已接受的任务会继续执行，直到完成。shutdownNow()方法也类似，不过它会去尝试终止正在执行的任务。如果任务都已提交或者都执行完，当然shutdown就没问题啦。那还有线程没execute线程池就被shutdown呢？

我们先写一个线程池的公用单例。

/*** @ClassName ThreadPoolSingleTest

* @Description 线程池单例模式

* @Auther zhui

* @Date 2020/7/2 8:47

* @Version 1.0

**/

public classThreadPoolSingleTest {private static ThreadPoolExecutor threadPoolExecutor=null;public staticThreadPoolExecutor getThreadPoolExecutor(){if(threadPoolExecutor==null){synchronized (ThreadPoolSingleTest.class){if(threadPoolExecutor==null){

threadPoolExecutor= new ThreadPoolExecutor(10,50,5, TimeUnit.MINUTES,new LinkedBlockingQueue<>());

}

}

}returnthreadPoolExecutor;

}

}

然后测试如果还未execute，线程池就被别的线程shutdown了怎么办？

public classTest {public static voidmain(String[] args) {

ThreadPoolExecutor threadPoolExecutor=ThreadPoolSingleTest.getThreadPoolExecutor();//这个线程执行完成后就shutdown线程

threadPoolExecutor.execute(()->{try{

Thread.sleep(2000);

}catch(Exception e){}

});//模拟其他线程拿到了线程池实例，但是还未提交任务

new Thread(()->{

ThreadPoolExecutor threadPoolExecutor1=ThreadPoolSingleTest.getThreadPoolExecutor();

out.println("new Thread已经拿到了ThreadPoolExecutor");try{//等它睡醒，懵逼的发现线程池被shutdown了

Thread.sleep(5000);

}catch(Exception e){}

threadPoolExecutor1.execute(()->{

out.println("执行");

});

}).start();

threadPoolExecutor.shutdown();

}

}

运行结果还是报错了，线程池被关掉了，无法提交任务。

Exception in thread "Thread-0" java.util.concurrent.RejectedExecutionException: Task 测试.Test$$Lambda$3/1588496549@50697d00 rejected from java.util.concurrent.ThreadPoolExecutor@5456e3d5[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1]

at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063)

at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830)

at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379)

at 测试.Test.lambda$main$2(Test.java:310)

at java.lang.Thread.run(Thread.java:748)

做到这里，就知道了，公用线程池是不能被shutdown的，毕竟在业务上，一个地方手贱的shutdown了，其他持有线程池对象却没来得及提交任务的代码就GameOver了。但是但是就真的有人手贱要shutdown怎么办？哎，那没办法，单例模式再多加一个判断吧。

public classThreadPoolSingleTest {private static ThreadPoolExecutor threadPoolExecutor=null;public staticThreadPoolExecutor getThreadPoolExecutor(){//为null和被shutdown都实例化对象

if(threadPoolExecutor==null||threadPoolExecutor.isShutdown()){synchronized (ThreadPoolSingleTest.class){if(threadPoolExecutor==null||threadPoolExecutor.isShutdown()){

threadPoolExecutor= new ThreadPoolExecutor(10,50,5, TimeUnit.MINUTES,new LinkedBlockingQueue<>());

}

}

}returnthreadPoolExecutor;

}

}

以上方法只是防止被shutdown后执行任务失败，但是还是会有错误的风险，所以最好还是不要随便的shutdown线程池了。