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捕获Java线程池执行任务抛出的异常

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Java中线程执行的任务接口java.lang.Runnable 要求不抛出Checked异常,

public interface Runnable {

    public abstract void run();
}

那么如果 run() 方法中抛出了RuntimeException,将会怎么处理了?

通常java.lang.Thread对象运行设置一个默认的异常处理方法:

java.lang.Thread.setDefaultUncaughtExceptionHandler(UncaughtExceptionHandler)

而这个默认的静态全局的异常捕获方法是直接输出异常堆栈。

当然,我们可以覆盖此默认实现,只需要一个自定义的java.lang.Thread.UncaughtExceptionHandler接口实现即可。

public interface UncaughtExceptionHandler {

    void uncaughtException(Thread t, Throwable e);
}

而在线程池中却比较特殊。默认情况下,线程池 java.util.concurrent.ThreadPoolExecutor 会Catch住所有异常, 当任务执行完成(java.util.concurrent.ExecutorService.submit(Callable))获取其结果 时(java.util.concurrent.Future.get())会抛出此RuntimeException。

/**
 * Waits if necessary for the computation to complete, and then
 * retrieves its result.
 *
 * @return the computed result
 * @throws CancellationException if the computation was cancelled
 * @throws ExecutionException if the computation threw an exception
 * @throws InterruptedException if the current thread was interrupted while waiting
 */
V get() throws InterruptedException, ExecutionException;

其中 ExecutionException 异常即是java.lang.Runnable 或者 java.util.concurrent.Callable 抛出的异常。

也就是说,线程池在执行任务时捕获了所有异常,并将此异常加入结果中。这样一来线程池中的所有线程都将无法捕获到抛出的异常。 从而无法通过设置线程的默认捕获方法拦截的错误异常。

也不同通过自定义线程来完成异常的拦截。

好在java.util.concurrent.ThreadPoolExecutor 预留了一个方法,运行在任务执行完毕进行扩展(当然也预留一个protected方法beforeExecute(Thread t, Runnable r)):

protected void afterExecute(Runnable r, Throwable t) { }

此方法的默认实现为空,这样我们就可以通过继承或者覆盖ThreadPoolExecutor 来达到自定义的错误处理。

解决办法如下:

ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(11, 100, 1, TimeUnit.MINUTES, //
        new ArrayBlockingQueue<Runnable>(10000),//
        new DefaultThreadFactory()) {

    protected void afterExecute(Runnable r, Throwable t) {
        super.afterExecute(r, t);
        printException(r, t);
    }
};

private static void printException(Runnable r, Throwable t) {
    if (t == null && r instanceof Future<?>) {
        try {
            Future<?> future = (Future<?>) r;
            if (future.isDone())
                future.get();
        } catch (CancellationException ce) {
            t = ce;
        } catch (ExecutionException ee) {
            t = ee.getCause();
        } catch (InterruptedException ie) {
            Thread.currentThread().interrupt(); // ignore/reset
        }
    }
    if (t != null)
        log.error(t.getMessage(), t);
}

此办法的关键在于,事实上 afterExecute 并不会总是抛出异常 Throwable t,通过查看源码得知,异常是封装在此时的Future对象中的, 而此Future对象其实是一个java.util.concurrent.FutureTask的实现,默认的run方法其实调用的 java.util.concurrent.FutureTask.Sync.innerRun()。

void innerRun() {
    if (!compareAndSetState(0, RUNNING))
        return;
    try {
        runner = Thread.currentThread();
        if (getState() == RUNNING) // recheck after setting thread
            innerSet(callable.call());
        else
            releaseShared(0); // cancel
    } catch (Throwable ex) {
        innerSetException(ex);
    }
}

void innerSetException(Throwable t) {
    for (;;) {
        int s = getState();
        if (s == RAN)
            return;
        if (s == CANCELLED) {
            // aggressively release to set runner to null,
            // in case we are racing with a cancel request
            // that will try to interrupt runner
            releaseShared(0);
            return;
        }
        if (compareAndSetState(s, RAN)) {
            exception = t;
            result = null;
            releaseShared(0);
            done();
            return;
        }
    }
}

这里我们可以看到它吃掉了异常,将异常存储在java.util.concurrent.FutureTask.Sync的exception字段中:

/** The exception to throw from get() */
private Throwable exception;

当我们获取异步执行的结果时, java.util.concurrent.FutureTask.get()

public V get() throws InterruptedException, ExecutionException {
    return sync.innerGet();
}

java.util.concurrent.FutureTask.Sync.innerGet()

V innerGet() throws InterruptedException, ExecutionException {
    acquireSharedInterruptibly(0);
    if (getState() == CANCELLED)
        throw new CancellationException();
    if (exception != null)
        throw new ExecutionException(exception);
    return result;
}

异常就会被包装成ExecutionException异常抛出。

也就是说当我们想线程池 ThreadPoolExecutor(java.util.concurrent.ExecutorService)提交任务时, 如果不理会任务结果(Feture.get()),那么此异常将被线程池吃掉。

<T> Future<T> submit(Callable<T> task);
Future<?> submit(Runnable task);

而java.util.concurrent.ScheduledThreadPoolExecutor是继承ThreadPoolExecutor的,因此情况类似。

结论,通过覆盖ThreadPoolExecutor.afterExecute 方法,我们才能捕获到任务的异常(RuntimeException)。

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