Java ExecutorService 多线程实践（一）
需要实现一个多线程并发的业务场景，启动若干子线程，最后要所有子线程运行结束才结束。（类似 .NET 里的 Task WaitAll )
Java 中的 ExecutorService 多线程编程模型提供这样一个机制，通过代码来介绍一下。
方法一：ExecutorService#awaitTermination
/**
* Blocks until all tasks have completed execution after a shutdown
* request, or the timeout occurs, or the current thread is
* interrupted, whichever happens first.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return true if this executor terminated and
* false if the timeout elapsed before termination
* @throws InterruptedException if interrupted while waiting
*/
boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException;
通过 ExecutorService#submit(或者execute) 加入子线程并启动，awaitTermination 阻塞等待所有子线程执行完毕。
sample：
ExecutorService es = Executors.newCachedThreadPool();
List> futureList = Lists.newArrayList();
futureList.add(es.submit(new Callable() {
@Override
public String call() throws Exception {
System.out.println("Threading1 is running");
Thread.sleep(1000);
return "1";
}
}));
futureList.add(es.submit(new Callable() {
@Override
public String call() throws Exception {
System.out.println("Threading2 is running");
Thread.sleep(3000);
return "2";
}
}));
futureList.add(es.submit(new Callable() {
@Override
public String call() throws Exception {
System.out.println("Threading3 is running");
Thread.sleep(2000);
return "3";
}
}));
es.shutdown();
try {
boolean completed = es.awaitTermination(2000, TimeUnit.MILLISECONDS);
System.out.println("all tasks completed: " + completed);
//es.wait(3000);
for(Future future : futureList) {
try {
if (future.isDone()) {
System.out.println("result: " + future.get());
}
} catch (CancellationException ex) {
ex.printStackTrace();
} finally {
future.cancel(true);
}
}
} catch (Exception e) {
e.printStackTrace();
} 输出结果：
Threading 1 is running
Threading 2 is running
Threading 3 is running
all tasks completed: false
result: 1
result: 3
只有1和3输出了结果，因为 awaitTermination 超时设置了 2000 ms，Threading2 模拟了 3000 ms 因此被超时取消了。通过 Future#isDone() 可以判断对应线程是否处理完毕，
这个场景里如果 isDone() == false 那么就可以认为是被超时干掉了。
需要注意的是，在添加完所有的子线程并启动后调用了 ExecutorService#shutdown 方法：一方面是不再接受新的子线程"提交"，另一方面ExecutorService 其实自己也是一个work线程，如果不shutdown 其实当前线程并不会结束。调用shutdown 和不调用shutdown 从main运行后控制台状态就可以看出差异。
最后在 finally 里调用了 Future#cancel() 主要是当await之后，被超时处理的线程可能还在运行，直接取消掉。
方法二：ExecutorService#invokeAll
invokeAll 很像上述实现的整体包装，但细节略有不同。首先将 Callable 统一创建好放在List里交给 invokeAll 方法执行并设置超时时间。
ExecutorService es = Executors.newCachedThreadPool();
List> tasks = Lists.newArrayList();
tasks.add(new Callable() {
@Override
public String call() throws Exception {
System.out.println("Threading 1 is running");
Thread.sleep(1000);
return "1";
}
});
tasks.add(new Callable() {
@Override
public String call() throws Exception {
System.out.println("Threading 2 is running");
Thread.sleep(3000);
return "2";
}
});
tasks.add(new Callable() {
@Override
public String call() throws Exception {
System.out.println("Threading 3 is running");
Thread.sleep(2000);
return "3";
}
});
try {
List> futureList = es.invokeAll(tasks, 2000, TimeUnit.MILLISECONDS);
es.shutdown();
for(Future future : futureList) {
try {
if (future.isDone()) {
System.out.println("result: " + future.get());
}
} catch (CancellationException ex) {
ex.printStackTrace();
}
}
} catch (Exception e) {
e.printStackTrace();
} Threading 1 is running
Threading 2 is running
Threading 3 is running
result: 1
result: 3
java.util.concurrent.CancellationExceptionExecutorService#invokeAll() 执行后所有 future.isDone() 都是 true，在 future.get() 拿结果的时候，被超时的Future会抛出 CancellationException 。因为在 invokeAll 内部调用了Future#cancel() 方法。
源码如下：
public List> invokeAll(Collection> tasks,
long timeout, TimeUnit unit)
throws InterruptedException {
if (tasks == null || unit == null)
throw new NullPointerException();
long nanos = unit.toNanos(timeout);
List> futures = new ArrayList>(tasks.size());
boolean done = false;
try {
for (Callable t : tasks)
futures.add(newTaskFor(t));
long lastTime = System.nanoTime();
// Interleave time checks and calls to execute in case
// executor doesn't have any/much parallelism.
Iterator> it = futures.iterator();
while (it.hasNext()) {
execute((Runnable)(it.next()));
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0)
return futures;
}
for (Future f : futures) {
if (!f.isDone()) {
if (nanos <= 0)
return futures;
try {
f.get(nanos, TimeUnit.NANOSECONDS);
} catch (CancellationException ignore) {
} catch (ExecutionException ignore) {
} catch (TimeoutException toe) {
return futures;
}
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
}
}
done = true;
return futures;
} finally {
if (!done)
for (Future f : futures)
f.cancel(true);
}
} 当然在 ExecutorService 编程模型外，自己定义Threading，通过CountDownLatch 控制也是可以实现的。