多线程最容易犯的错集合

发表于 分类于 阅读次数： 本文字数： 455 阅读时长 ≈ 2 分钟

ScheduleService/ScheduledThreadPoolExecutor没有try-catch导致定时任务不执行

现象

定时执行的任务如果没有对执行逻辑try-catch,后续所有定时任务都会取消，并且线程卡在AQS等待唤醒

1
2
3
4
5
6
7
8
9
10
11
12
"starrocks-full-timeout-flush-trigger-6-thd-0" #56 prio=5 os_prio=0 tid=0x00007f2eed8b8800 nid=0x3c1bff waiting on condition [0x00007f2eba8f8000]
   java.lang.Thread.State: WAITING (parking)
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x00000003c08d63c0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
	at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1081)
	at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)
	at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:750)

原理

ScheduledThreadPool中的定时任务都是封装成ScheduledFutureTask，本质是FutureTask。源码执行逻辑如下：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
public void run() {
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
        Callable<V> c = callable;
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                setException(ex);
            }
            if (ran)
                set(result);
        }
    } finally {
        // runner must be non-null until state is settled to
        // prevent concurrent calls to run()
        runner = null;
        // state must be re-read after nulling runner to prevent
        // leaked interrupts
        int s = state;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}


protected void setException(Throwable t) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = t;
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        finishCompletion();
    }
}

如果执行异常，这个异常只是赋值给outcome，不会有任何异常打印。

最佳实践

多线程异步处理的时候，都显式使用try-catch以及日志打印

submit和execute异常处理不正确导致吞异常

现象

使用submit，即使配置了exception handler，仍然没有进行正确的异常处理

原理

execute service使用execute的时候jvm才会去调用uncaughtExceptionHandler，使用submit的时候是封装成FutureTask的，exception赋值给outcome，只有使用future.get的时候才可以获取到exception

1
2
3
4
5
6
7
/**
 * Dispatch an uncaught exception to the handler. This method is
 * intended to be called only by the JVM.
 */
private void dispatchUncaughtException(Throwable e) {
    getUncaughtExceptionHandler().uncaughtException(this, e);
}

最佳实践

任何时候都记得在异步线程中使用try-catch处理异常