公用线程池要不要shutdown？

时间：2024-03-12 17:46:48

　　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 class ThreadPoolSingleTest {
    private  static ThreadPoolExecutor threadPoolExecutor=null;

    public static ThreadPoolExecutor getThreadPoolExecutor(){
        if(threadPoolExecutor==null){
            synchronized (ThreadPoolSingleTest.class){
                if(threadPoolExecutor==null){
                    threadPoolExecutor= new ThreadPoolExecutor(10,50,5, TimeUnit.MINUTES,new LinkedBlockingQueue<>());
                }
            }
        }
        return threadPoolExecutor;
    }
}

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

public class Test {
    public static void main(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 class ThreadPoolSingleTest {
    private  static ThreadPoolExecutor threadPoolExecutor=null;

    public static ThreadPoolExecutor 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<>());
                }
            }
        }
        return threadPoolExecutor;
    }
}

　　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。

 

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