线程池中Callable异常处理分析

前言

  分析前几天遇到的一个老代码留下的坑。线程池中运行Callable线程时抛出的异常捕获不到，简化的逻辑如图,环境是jdk8： 运行结果：

解决方案

线程池返回Future<>，调用其get()在Callable中 try-catch可能抛错的异常 运行结果：

源码分析

  不难发现线程池提交时创建的类为FutureTask。

public <T> Future<T> submit(Callable<T> task) { if (task == null) throw new NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task); execute(ftask); return ftask; } protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) { return new FutureTask<T>(callable); }

看FutureTask.run()之前，先简单结束一下其关的属性。

state：线程的状态。主要有如下几种： NEW： 新建COMPLETING: 运行在NORMAL: 正常完成EXCEPTIONAL: 异常CANCELLED: 取消INTERRUPTING: 被中断的中间状态INTERRUPTED: 被中断的最终状态 outcome： get()返回值 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); } }

注意这里

try { result = c.call(); ran = true; } catch (Throwable ex) { result = null; ran = false; setException(ex); } protected void setException(Throwable t) { if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { outcome = t; UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state finishCompletion(); } }

这里线程在运行时抛出异常时，FutureTask把异常信息赋值给outcome，并将state设为EXCEPTIONAL。

public V get() throws InterruptedException, ExecutionException { int s = state; if (s <= COMPLETING) s = awaitDone(false, 0L); return report(s); } @SuppressWarnings("unchecked") private V report(int s) throws ExecutionException { Object x = outcome; if (s == NORMAL) return (V)x; if (s >= CANCELLED) throw new CancellationException(); throw new ExecutionException((Throwable)x); }

在调用get()时，如果运行时抛出异常，此时会抛出异常。

总结

  这种坑还是代码规范的问题。Callable返回结果并没有被使用可以用Runnable代替;try-catch代码的习惯。

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