面试题：如何优雅的停止一个正在运行的java线程

线程状态转换图：

几个容易混淆概念的方法

Thread.sleep()、 Thread.join() 或 Object.wait()，
他们都是阻塞方法并声明抛出 InterruptedException.
当线程调用这些方法进入阻塞状态后，如果外部调用了interrupt(),
线程进入就绪状态，并且这些方法抛出InterruptedException异常.

public void Thread.interrupt() // 设置线程的中断状态flag.
public boolean Thread.isInterrupted() // 获取线程的中断状态flag值
public static boolean Thread.interrupted() // 取线程的中断状态flag值，并重置flag值为false.
public final void join() throws InterruptedException  //指的是调用者进入阻塞状态, 而不是线程对象进入阻塞状态, 线程对象继续在执行， 不受影响， 看下面的例子.
public static void yield(); //线程从运行状态， 回到就绪状态.

//demo: join()
public class BottomBarManager {
    private final Object handlePopupMenuAction(int actionCode, Object... args) {
    switch (actionCode) {
            case Actions.MenuContainer.DOWNLOAD: // 下载
                ThreadTest tt = new ThreadTest();
                tt.start();
                try {
                    tt.join();  //当前线程等待tt线程执行完再往下执行
                    //作用是, 当前的UI线程进入阻塞状态， 直到等tt线程执行完再往下继续执行, tt线程该干什么还干什么， 不受影响.
                    //千万不要误理解成tt线程进入了阻塞状态.
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                JLog.i();
            break;


    }


    private class ThreadTest extends Thread {


        @Override
        public void run() {
            try {
                sleep(100000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            JLog.i();
        }
    }


}

中断线程的最佳实践是: 中断 + 条件变量。

最佳模板：

public class BestPractice extends Thread {
    private volatile boolean finished = false;   // ① volatile条件变量
    public void stopMe() {
        finished = true;    // ② 发出停止信号
        interrupt();
    }
    @Override
    public void run() {
        while (!finished) {    // ③ 检测条件变量
        try {
                // do dirty work   // ④业务代码
        } catch (InterruptedException e) {
                if (finished) {
                    return;
                }
                continue;
        }
        }
    }
}

当④处的代码阻塞于wait()或sleep()时，线程不能立刻检测到条件变量。因此②处的代码最好同时调用interrupt()方法。

典型的体现是在volley项目中。

// CacheDispatcher.java
public class CacheDispatcher extends Thread {
    private volatile boolean mQuit = false;


    public void quit() {
        mQuit = true;
        interrupt();
    }


    @Override
    public void run() {
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);


        // Make a blocking call to initialize the cache.
        mCache.initialize();


        while (true) {
            try {
                // Get a request from the cache triage queue, blocking until
                // at least one is available.
                final Request<?> request = mCacheQueue.take(); //****** block方法, it throws InterruptedException;
                request.addMarker("cache-queue-take");


                // Attempt to retrieve this item from cache.
                Cache.Entry entry = mCache.get(request.getCacheKey());
                if (entry == null) {
                    request.addMarker("cache-miss");
                    // Cache miss; send off to the network dispatcher.
                    mNetworkQueue.put(request);
                    continue;
                }


                // We have a cache hit; parse its data for delivery back to the request.
                request.addMarker("cache-hit");
                Response<?> response = null;
                try {
                    response = request.parseNetworkResponse(
                            new NetworkResponse(entry.data, entry.responseHeaders));
                } catch (Exception e) {
                    mDelivery.postError(request, new ParseError(e));
                    continue;
                } catch (Error error) {
                    mDelivery.postError(request, new ParseError(error));
                    continue;
                }
                request.addMarker("cache-hit-parsed");


            } catch (InterruptedException e) {
                // We may have been interrupted because it was time to quit.
                if (mQuit) {
                    return;
                }
                continue;
            } catch (OutOfMemoryError error) {
                if (mQuit) {
                    return;
                }
                continue;
            }
        }
    }


}

在quit()代码里, 设置条件变量, 同时调用interrupt()方法.