Java如何停止线程，确定你知道的都是正确的么？

回想当年刚开始做Java开发的时候，如何停止线程运行可是难倒了一批人，停止线程的方法众说纷纭：

1. 调用Thread.stop() :

Thread thread = new Thread(){
    @Override
    public void run() {
        for (int i=0; i < 999 && !isCanceled; i++) {
            System.out.println("哈哈，我还没死...");
        }
    }
};
thread.start();

// kill you now
thread.stop();

点评: 不用多说，直接拉出去斩了

2. 线程外面定义boolean变量标识为是否已经取消，线程里每个循环地方加判断，变量变成了取消则停止循环:

private volatile boolean isCanceled = false;

new Thread(){
    @Override
    public void run() {
        for (int i=0; i < 999 && !isCanceled; i++) {
            System.out.println("哈哈，我还没死...");
        }
    }
}.start();

// kill thread
isCanceled = true;

点评：太片面，线程里Thread.sleep(999999)取消不了的

3. 通过调用interrupt() ：

Thread thread = new Thread(){
    @Override
    public void run() {
        sleep(99999);
        System.out.println("刚睡醒，我还没死！！！");
    }
};
thread.start();

// kill you now
thread.interrupt();

点评：也是太片面，for循环取消不了

4. 以上方案意思是有那么一点了，只是零零碎碎，如何将两个方案结合是今天的第一个话题。

在Thread内提供了一个方法叫interrupted()，其返回值即告知是否线程被interrupt()执行过，简单说：无需自己定义boolean变量，可以用它代替，如果线程能被interrupt()而停止正好，如果是内部循环操作可以用它作为循环停止的条件：

Thread thread = new Thread(){
    @Override
    public void run() {
        for (int i=0; i < 999 && !Thread.interrupted(); i++) {
            System.out.println("哈哈，我还没死...");
        }
    }
};

其实，还有一种情况是停止不了的，类似socket/http通信这种不能被interrputted的情况，当然也有巧妙的办法：重载Thread的interrput()方法：

// 重载interrupt 通过关闭socket起到关闭线程的作用
@Override
public void interrupt() {
    try {
        socket.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

今天的第二个话题是，使用单个线程的情况在项目中极少，很多情况使用线程池托管的，那么线程池里如何停止某个task呢？

JDK5提供了ExecuteService，并通过Executors提供了多个候选的线程池选择方案，ExecutorService提交task有两种方式：execute()和submit(), 我们这里对submit()比较感兴趣，因为其中submit()会返回Feature，不管submit的是 Runnable还是Callable都会有Feature返回：

public interface ExecutorService extends Executor {
    /**
     * Submits a value-returning task for execution and returns a
     * Future representing the pending results of the task. The
     * Future's {@code get} method will return the task's result upon
     * successful completion.
     *
     * <p>
     * If you would like to immediately block waiting
     * for a task, you can use constructions of the form
     * {@code result = exec.submit(aCallable).get();}
     *
     * <p>Note: The {@link Executors} class includes a set of methods
     * that can convert some other common closure-like objects,
     * for example, {@link java.security.PrivilegedAction} to
     * {@link Callable} form so they can be submitted.
     *
     * @param task the task to submit
     * @param <T> the type of the task's result
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    <T> Future<T> submit(Callable<T> task);


    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future's {@code get} method will
     * return {@code null} upon <em>successful</em> completion.
     *
     * @param task the task to submit
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    Future<?> submit(Runnable task);

    /**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future's {@code get} method will
     * return the given result upon successful completion.
     *
     * @param task the task to submit
     * @param result the result to return
     * @param <T> the type of the result
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    <T> Future<T> submit(Runnable task, T result);
}

Feature对象提供了一系列API，如：

cancel(boolean mayInterruptIfRunning) ；
isCanceled()；
get()；
get(long timeout, TimeUnit unit)；
isDone()

看到第一个API，很是让人兴奋，难倒这真的可以一步到位？但很不好意思，依然不行，其实跟第一个话题的原理一摸一样，如果内部有阻塞线程的操作会因此而退出，如： wait(), join(), sleep() 等，但是遇到IO阻塞操作，比如进行socket通信，仅仅通过调用cancel(true)是不行的，不过可以重载interrupt() 关闭socket即可, 遇到大量循环也要通过Thread.interrupted()作为循环结束的条件：

class Task extends Thread{
    private Socket socket;

    public Task(){
        // init socket
    }

    // 重载interrupt 通过关闭socket起到关闭线程的作用
    @Override
    public void interrupt() {
        try {
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public void func () {
    ExecutorService timer = Executors.newCachedThreadPool();

    Future<?> future = timer.submit(new Task());
    future.cancel(true);
}

好了，夜深了...回忆到此结束