Java并发编程基础（二）启动和终止线程

通过调用线程的start()方法进行启动，随着run()方法的执行完毕，线程也随之终止。

一、启动线程

线程对象在初始化完成后，调用start()方法就可以启动这个线程。线程start()方法的含义是：当前线程（即parent线程）同步告知Java虚拟机，只要线程规划器空闲，应立即启动调用start()方法的线程。

启动一个线程前，最好为这个线程命名，方便用jstack等工具分析程序进行问题排查。

二、理解中断

中断好比其他线程对该线程打了个招呼，前天线程通过调用该线程的interrupt()方法对其进行中断操作。
线程通过isInterrupted()来进行判断是否被中断，也可以调用静态方法Thread.interrupted()对当前线程的中断标识进行复位。
从Java的API中可以看到，许多声明抛出InterruptedException的方法（例如Thread.sleep(long millis)方法）在抛出InterruptedException之前，Java虚拟机会先将该线程的中断标识位清除，然后抛出InterruptedException，此时调用isInterrupted()方法将会返回false。
通过下面的示例可以证明：

public class Interrupted {
    
    public static void main(String[] args) throws Exception {
        // sleepThread不停的尝试睡眠
        Thread sleepThread = new Thread(new SleepRunner(), "SleepThread");
        sleepThread.setDaemon(true);
        // busyThread不停的运行
        Thread busyThread = new Thread(new BusyRunner(), "BusyThread");
        busyThread.setDaemon(true);
        sleepThread.start();
        busyThread.start();
        // 休眠5秒, 让sleepThread和busyThread充分运行
        TimeUnit.SECONDS.sleep(5);
        sleepThread.interrupt();
        busyThread.interrupt();
        System.out.println("SleepThread interrupted is " + sleepThread.isInterrupted());
        System.out.println("BusiThread interrupted is " + busyThread.isInterrupted());
        // 防止sleepThread和busiThread立即退出
        SleepUtils.second(2);
    }
    
    static class SleepRunner implements Runnable {
        @Override
        public void run() {
            while (true) {
                SleepUtils.second(10);
            }
        }
    }
    
    static class BusyRunner implements Runnable {
        @Override
        public void run() {
            while (true) {
                
            }
        }
    }

}

输出如下：

SleepThread interrupted is false
BusyThread interrupted is true

三、安全地终止线程

除中断操作外，还可以利用boolean变量来控制是否需要停止任务并终止该线程。
示例代码，创建了一个线程CountThread，它不断地进行变量累加，而主线程尝试对其进行中断操作和停止操作：

public class Shutdown {
    
    public static void main(String[] args) throws Exception {
        Runner one = new Runner();
        Thread countThread = new Thread(one, "CountThread");
        countThread.start();
        // 睡眠1秒, main线程对CountThread进行中断, 使CountThread能够感知中断而结束
        TimeUnit.SECONDS.sleep(1);
        countThread.interrupt();
        
        Runner two = new Runner();
        countThread = new Thread(two, "CountThread");
        countThread.start();
        // 睡眠1秒, main线程对Runner two进行取消, 使CountThread能够感知on为false而结束
        TimeUnit.SECONDS.sleep(1);
        two.cancel();
    }
    
    private static class Runner implements Runnable {
        private long i;
        private volatile boolean on = true;
        @Override
        public void run() {
            while (on && !Thread.currentThread().isInterrupted()) {
                i++;
            }
            System.out.println("Count i = " + i);
        }
        
        public void cancel() {
            on = false;
        }
    }

}