为什么不允许使用Executors创建线程池

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阿里的Java开发手册中这样说:“线程池不允许使用Executors 去创建,而是通过 ThreadPoolExecutor 的方式,这样的处理方式让写的同学更加明确线程池的运行规则,规避资源耗尽的风险。”

1. FixedThreadPool 和 SingleThreadPool

允许的请求队列长度为 Integer.MAX_VALUE,可能会堆积大量的请求,从而导致 OOM

看看下面两个方法的实现:

 public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }
 
    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }

FixedThreadPoolSingleThreadPool都使用了LinkedBlockingQueue

Java中的阻塞队列BlockingQueue主要有两种实现,分别是ArrayBlockingQueueLinkedBlockingQueue

ArrayBlockingQueue是一个用数组实现的有界阻塞队列,必须设置容量。

LinkedBlockingQueue是一个用链表实现的有界阻塞队列,容量可以选择进行设置,不设置的话,将是一个无边界的阻塞队列,最大长度为Integer.MAX_VALUE

这里的问题就出在:“不设置的话,将是一个无边界的阻塞队列,最大长度为Integer.MAX_VALUE”。也就是说,如果我们不设置LinkedBlockingQueue的容量的话,其默认容量将会是Integer.MAX_VALUE

newFixedThreadPool中创建LinkedBlockingQueue时,并未指定容量。此时,LinkedBlockingQueue就是一个无边界队列,对于一个无边界队列来说,是可以不断的向队列中加入任务的,这种情况下就有可能因为任务过多而导致内存溢出问题。

上面提到的问题主要体现在newFixedThreadPoolnewSingleThreadExecutor两个工厂方法上,并不是说newCachedThreadPoolnewScheduledThreadPool这两个方法就安全了,这两种方式创建的最大线程数可能是Integer.MAX_VALUE,而创建这么多线程,必然就有可能导致OOM

示例

public class ExecutorsDemo {
    private static ExecutorService executor = Executors.newFixedThreadPool(15);
    public static void main(String[] args) {
        for (int i = 0; i < Integer.MAX_VALUE; i++) {
            executor.execute(new SubThread());
        }
    }
}
 
class SubThread implements Runnable {
    @Override
    public void run() {
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            //do nothing
        }
    }
}

通过指定JVM参数:-Xmx8m -Xms8m 运行以上代码,会抛出OOM:

Exception in thread "main" java.lang.OutOfMemoryError: GC overhead limit exceeded
    at java.util.concurrent.LinkedBlockingQueue.offer(LinkedBlockingQueue.java:416)
    at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1371)
    at com.hollis.ExecutorsDemo.main(ExecutorsDemo.java:16)

2. CachedThreadPo和ScheduledThreadPool

允许的创建线程数量为 Integer.MAX_VALUE, 可能会创建大量的线程,从而导致 OOM

    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }
 
   public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
        }

示例

public class ThreadPoolDemo {
    private static ExecutorService executor = Executors.newScheduledThreadPool(5);
    public static void main(String[] args) {
        for (int i = 0; i < Integer.MAX_VALUE; i++) {
            executor.execute(new SubThread());
        }
    }
}
class SubThread implements Runnable {
    @Override
    public void run() {
        try {
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName());
        } catch (InterruptedException e) {
            //do nothing
        }
    }
}

通过指定JVM参数:-Xmx8m -Xms8m 运行以上代码,会抛出OOM

pool-1-thread-5
pool-1-thread-4
Exception in thread "main" java.lang.OutOfMemoryError: GC overhead limit exceeded
    at java.util.concurrent.Executors.callable(Executors.java:407)
    at java.util.concurrent.FutureTask.<init>(FutureTask.java:152)
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.<init>(ScheduledThreadPoolExecutor.java:209)
    at java.util.concurrent.ScheduledThreadPoolExecutor.schedule(ScheduledThreadPoolExecutor.java:532)
    at java.util.concurrent.ScheduledThreadPoolExecutor.execute(ScheduledThreadPoolExecutor.java:622)
    at ThreadPoolDemo.main(ThreadPoolDemo.java:11)
pool-1-thread-2

3.建议使用ThreadPoolExecutor创建