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。
