本节摘要：介绍线程优先级，代码示例

一、线程优先级

线程优先级的范围是1~10，默认优先级是5，线程优先级的主要作用是让调度器优先调度高优先级线程，但这不意味着低优先级线程得不到执行，通常情况不需要修改线程优先级，Thread类中有3个优先级的常量：

二、源码分析

public final void setPriority(int newPriority) {
      ThreadGroup g;//线程所属的线程组
      checkAccess();
      //设置的级别不能大于10且不能小于1
      if (newPriority > MAX_PRIORITY || newPriority < MIN_PRIORITY) {
            throw new IllegalArgumentException();
      }
      if((g = getThreadGroup()) != null) {
            //设置的级别不能大于所属线程组的最大级别
            if (newPriority > g.getMaxPriority()) {
                newPriority = g.getMaxPriority();
            }
          setPriority0(priority = newPriority);//本地方法
      }
}

三、线程优先级示例

public class ThreadPrioritiesDemo extends Thread {
      private int priority;
      public ThreadPrioritiesDemo(String name, int priority) {
            super(name);
            this.priority = priority;
      }
    public static void main(String[] args) {
          Thread t1 = new ThreadPrioritiesDemo("thread1", 1);
          Thread t2 = new ThreadPrioritiesDemo("thread2", 10);
          t1.start();
          t2.start();
    }
    @Override
    public void run() {
            Thread.currentThread().setPriority(priority);
                    for (int i = 0; i < 1000; i++) {
                        System.out.println(this + ":" + i);
                         if (i % 2 == 0)
                              Thread.yield();
                      }
          }
}

输出结果（截取部分）：
Thread[thread1,1,main]:670
Thread[thread2,10,main]:293
Thread[thread2,10,main]:294
Thread[thread1,1,main]:671
Thread[thread1,1,main]:672
Thread[thread2,10,main]:295
Thread[thread2,10,main]:296
Thread[thread1,1,main]:673
Thread[thread1,1,main]:674
Thread[thread2,10,main]:297
Thread[thread2,10,main]:298
Thread[thread1,1,main]:675
Thread[thread1,1,main]:676
Thread[thread2,10,main]:299

3.1、结果说明

新开启2个线程，线程1优先级为1，线程2优先级为10，两个线程交替执行，高优先级线程（t2）并不是一直优先于t1执行

四、总结

高优先级线程并不是一直优于低优先级线程执行

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