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 entrances = new ArrayList();
  
      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 yield
  Thread[pool-1-thread-1,5,main]: increment
  Thread[pool-1-thread-1,5,main]: Entrance 0: 1 Total: 1
  Thread[pool-1-thread-5,5,main]: increment
  Thread[pool-1-thread-5,5,main]: Entrance 4: 1 Total: 2
  Thread[pool-1-thread-5,5,main]: Sleep 100ms
  Thread[pool-1-thread-4,5,main]: thread yield
  Thread[pool-1-thread-1,5,main]: Sleep 100ms
  Thread[pool-1-thread-4,5,main]: increment
  Thread[pool-1-thread-3,5,main]: increment
  Thread[pool-1-thread-3,5,main]: Entrance 2: 1 Total: 4
  Thread[pool-1-thread-3,5,main]: Sleep 100ms
  Thread[pool-1-thread-4,5,main]: Entrance 3: 1 Total: 3
  Thread[pool-1-thread-2,5,main]: increment
  Thread[pool-1-thread-2,5,main]: Entrance 1: 1 Total: 5
  Thread[pool-1-thread-2,5,main]: Sleep 100ms
  Thread[pool-1-thread-4,5,main]: Sleep 100ms
  Thread[pool-1-thread-3,5,main]: thread yield
  Thread[pool-1-thread-3,5,main]: increment
  Thread[pool-1-thread-2,5,main]: increment
  Thread[pool-1-thread-2,5,main]: Entrance 1: 2 Total: 7
  Thread[pool-1-thread-2,5,main]: Sleep 100ms
  Thread[pool-1-thread-4,5,main]: increment
  Thread[pool-1-thread-4,5,main]: Entrance 3: 2 Total: 8
  Thread[pool-1-thread-4,5,main]: Sleep 100ms
  Thread[pool-1-thread-5,5,main]: thread yield
  Thread[pool-1-thread-3,5,main]: Entrance 2: 2 Total: 6
  Thread[pool-1-thread-3,5,main]: Sleep 100ms
  Thread[pool-1-thread-5,5,main]: increment
  Thread[pool-1-thread-1,5,main]: thread yield
  Thread[pool-1-thread-5,5,main]: Entrance 4: 2 Total: 9