ThreadPoolExecutor afterExecute无

先说结论, 如果afterExecute方法里拿不到你提交的Runnable, 试试用execute提交任务.

简介

为了方便码农监控线程池里庄稼的长势, ThreadPoolExecutor给我们提供了两个钩子方法.

void beforeExecute(Thread t, Runnable r)
在Runnable被线程池运行之前调用, 可见这时已分配线程. 如果这个方法里抛出异常, 不会触发afterExecute.

void afterExecute(Runnable r, Throwable t)
在Runnable被线程池运行之后调用, 如果没有异常t=null.

具体可以看源代码的注释, 另外重写时记得先调用原super方法, 例如super.afterExecute(r, t);

我遇到的坑

我们在提交的Runnable上等待了对象, 希望通过线程执行时手工释放锁来提交下一个Runnable, 使线程池持续运行. 实际情况是有时Runnable莫名其妙的死了, 然后就没有然后了.

所以我希望在afterExecute里notify一下wait在上面的提交代码. 但刚开始用submit提交的Runnable, afterExecute里拿到的Runnable基本是空的. 后来发现submit时由于要处理Future, 线程池会在我们的Runnable外面再包一层对象, 而且最后把我们的对象搞丢了.

    public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }

所以, 解决办法就是如果想用afterExecute搞事情, 就用execute方法提交Runnable.

实例

简单介绍一下情况就上代码了, 测试代码输出是这样的, 看懂了就不用往下看了.

=====submit task 1=====
pool-1-thread-1 beforeExeccute got runnable: FutureTask
Running task, id: 1
afterExecute got runnable: FutureTask
=====execute task 2=====
pool-1-thread-1 beforeExeccute got runnable: TestTask
Running task, id: 2
afterExecute got runnable: TestTask
Got task id: 2

测试代码

public class ThreadPoolAfterExecuteTest {

    public static void main(String[] args) throws InterruptedException {
        ThreadPoolExecutor workerPool = new ThreadPoolExecutor(1, 1, 60, TimeUnit.SECONDS,
                new ArrayBlockingQueue<Runnable>(10)) {
            @Override
            protected void beforeExecute(Thread t, Runnable r) {
                super.beforeExecute(t, r);
                out(t.getName()+" beforeExeccute got runnable: " + r.getClass().getSimpleName());
            }

            @Override
            protected void afterExecute(Runnable r, Throwable t) {
                super.afterExecute(r, t);
                out("afterExecute got runnable: " + r.getClass().getSimpleName());
                if (r instanceof TestTask) {
                    System.out.println("Got task id: " + ((TestTask) r).id);
                }
                synchronized (r) {
                    r.notifyAll();
                }
            }
        };
        out("=====submit task 1=====");
        workerPool.submit(new TestTask(1));
        TimeUnit.SECONDS.sleep(1);
        out("=====execute task 2=====");
        workerPool.execute(new TestTask(2));

    }

    static class TestTask implements Runnable {
        private long id;

        public TestTask(long id) {
            this.id = id;
        }

        @Override
        public void run() {
            out("Running task, id: " + id);
        }

    }

    static void out(String text) {
        System.out.println(text);
    }
}

这样写有没有问题?

反正ActiveMQ也是这样用的
https://github.com/apache/activemq/blob/290a93d91f09ff86d0f977324dc91f4d3f26e8a8/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/KahaDBStore.java#L1741

https://github.com/apache/activemq/blob/290a93d91f09ff86d0f977324dc91f4d3f26e8a8/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/KahaDBStore.java#L339