Thread.yield后线程释放资源的时间节点

• 当synchronized方法中进行调用Thread.yield，线程之间的cpu调度是怎么样的？

  class Count {    private int count = 0;    private Random rand = new Random(47);    /**     * 这里为了实现多线程之间的同步，increment标注为synchronized，     * 同时里面调用yield，yield的作用就是暂时释放cpu时间，然后和所有     * 同一优先级的线程竞争。     * @return     */    public synchronized int increment() {        int temp = count;        if (rand.nextBoolean()) {            System.out.println(Thread.currentThread() + ": thread yield"); //print1            Thread.yield();        }        System.out.println(Thread.currentThread() + ": increment"); //print2        return (count = ++temp);    }    public int value() {        return count;    }}

  class Entrance implements Runnable {    /**     * 这里定义了两个私有静态成员变量，类所有，也就是说所有对象是共享的。     */    private static Count count = new Count();    private static List<Entrance> entrances = new ArrayList<Entrance>();    private int number = 0;    /**     * 定义了final的成员变量，这里不要初始化，因为final代表常量，赋值后就不可改     */    private final int id;    /**     * 使用volatitle定义canceled，因此canceled是直接从内存中进行读写，不会通过缓冲区，属于原子操作     */    private static volatile boolean canceled = false;    public static void cancel() {        canceled = true;    }    public Entrance(int id) {        this.id = id;        entrances.add(this);    }    @Override    public void run() {        while (!canceled) {            synchronized (this) {                ++number;            }            System.out.println(this + " Total: " +  count.increment()); //print3            try {                System.out.println(Thread.currentThread() + ": Sleep 100ms"); //print4                TimeUnit.MILLISECONDS.sleep(100);            } catch (InterruptedException e) {                System.out.println(e);            }        }        System.out.println("Stopping " + this);    }    public synchronized int getValue() {        return number;    }    public String toString() {        return Thread.currentThread() + ": Entrance " + id + ": " + getValue();    }    public static int getTotalCount() {        return count.value();    }    public static int sumEntrances() {        int sum = 0;        for (Entrance entrance: entrances) {            sum += entrance.getValue();        }        return sum;    }}

  public class OrnamentalGarden {    public static void main(String[] args) throws Exception {        ExecutorService exec = Executors.newCachedThreadPool();        for (int i = 0; i < 5; i++) {            exec.execute(new Entrance(i));        }        TimeUnit.SECONDS.sleep(3);        Entrance.cancel();        exec.shutdown();        if (exec.awaitTermination(250, TimeUnit.MILLISECONDS)) {            System.out.println("Some tasks were not terminateed!");        }        System.out.println("Total: " + Entrance.getTotalCount());        System.out.println("Sum of Entrances: " + Entrance.sumEntrances());    }}

  终端输出：    1、线程1，在increment中调用thread yield，然后当调用完成    System.out.println(this + " Total: " +  count.increment())，线程1释放，线程5占用；    2、线程5，没有调用thread yield，然后运行到sleep 100ms的时候释放cpu    3、线程4，调用thread yield后，立马释放了cpu    4、线程1，接着运行，进入sleep 100ms    5、线程4，继续运行increment方法，退出前又释放了    6、线程3，没有调用thread yield，然后运行到sleep 100ms的时候释放cpu    7、线程4，运行了一条语句，又释放了cpu    8、线程2，没有调用thread yield，然后运行到sleep 100ms的时候释放cpu结论：    thread yield条用后，当前线程什么时候释放cpu是由系统管理的，有可能调用后立马释放也有可过阵子释放，而且和synchronized method无关，synchronized处理的是线程之间的同步，而不是代码块的原子操作。

  Thread[pool-1-thread-1,5,main]: thread yieldThread[pool-1-thread-1,5,main]: incrementThread[pool-1-thread-1,5,main]: Entrance 0: 1 Total: 1Thread[pool-1-thread-5,5,main]: incrementThread[pool-1-thread-5,5,main]: Entrance 4: 1 Total: 2Thread[pool-1-thread-5,5,main]: Sleep 100msThread[pool-1-thread-4,5,main]: thread yieldThread[pool-1-thread-1,5,main]: Sleep 100msThread[pool-1-thread-4,5,main]: incrementThread[pool-1-thread-3,5,main]: incrementThread[pool-1-thread-3,5,main]: Entrance 2: 1 Total: 4Thread[pool-1-thread-3,5,main]: Sleep 100msThread[pool-1-thread-4,5,main]: Entrance 3: 1 Total: 3Thread[pool-1-thread-2,5,main]: incrementThread[pool-1-thread-2,5,main]: Entrance 1: 1 Total: 5Thread[pool-1-thread-2,5,main]: Sleep 100msThread[pool-1-thread-4,5,main]: Sleep 100msThread[pool-1-thread-3,5,main]: thread yieldThread[pool-1-thread-3,5,main]: incrementThread[pool-1-thread-2,5,main]: incrementThread[pool-1-thread-2,5,main]: Entrance 1: 2 Total: 7Thread[pool-1-thread-2,5,main]: Sleep 100msThread[pool-1-thread-4,5,main]: incrementThread[pool-1-thread-4,5,main]: Entrance 3: 2 Total: 8Thread[pool-1-thread-4,5,main]: Sleep 100msThread[pool-1-thread-5,5,main]: thread yieldThread[pool-1-thread-3,5,main]: Entrance 2: 2 Total: 6Thread[pool-1-thread-3,5,main]: Sleep 100msThread[pool-1-thread-5,5,main]: incrementThread[pool-1-thread-1,5,main]: thread yieldThread[pool-1-thread-5,5,main]: Entrance 4: 2 Total: 9