线程的优先级和守护线程

概述

线程的优先级
线程优先级的特性
守护线程
总结

第1节线程的优先级

在Java 中，线程优先级的范围是1～10，默认的优先级是5。
“高优先级线程”会优先于“低优先级线程”执行。

第2节线程优先级的特性

1.线程A启动线程B，线程A和B具有相同的优先级

2.CPU尽量将执行的资源让给优先级高的线程用，但是不一定是优先级较大的线程先执行完。

3.即使线程设有优先级，并不能保证执行先后，线程运行具有随机性。

线程优先级的继承特性

代码语言：javascript复制

public class MyThread1 extends Thread {
    @Override
    public void run() {
        super.run();
        //输出线程级别
        System.out.println("MyThread1 Priority = "   this.getPriority());
        //启动线程MyThread2
        MyThread2 myThread2 = new MyThread2();
        myThread2.start();
    }
}

代码语言：javascript复制

public class MyThread2 extends Thread {
    @Override
    public void run() {
        super.run();
        System.out.println("MyThread2 Priority = "   this.getPriority());
    }
}

代码语言：javascript复制

public class ThreadPriority {
    public static void main(String[] args) {
        System.out.println("once Main Thread Priority = "   Thread.currentThread().getPriority());
        //待会打开下面注释再看结果
        Thread.currentThread().setPriority(10);
        System.out.println("twice Main Thread Priority = "   Thread.currentThread().getPriority());
        MyThread1 myThread1 = new MyThread1();
        myThread1.start();
    }
}

执行结果如下：

代码语言：javascript复制

once Main Thread Priority = 5
twice Main Thread Priority = 10
MyThread1 Priority = 10
MyThread2 Priority = 10

验证线程规则性和随机性

代码语言：javascript复制

public class MyThread1 extends Thread {
    @Override
    public void run() {
        long start = System.currentTimeMillis();
        System.out.println("------1------ thread 1 start running");
        long count = 0;
        for (int i = 0; i < 10; i  ) {
            for (int j = 0; j < 50000; j  ) {
                Random random = new Random();
                random.nextInt();
                count = count   i;
            }
        }
        long end = System.currentTimeMillis();
        System.out.println("------1------ thread 1 use time = "   (end - start));
    }
}

代码语言：javascript复制

public class MyThread2 extends Thread {
    @Override
    public void run() {
        long start = System.currentTimeMillis();
        System.out.println("------2------ thread 2 start running");
        long count = 0;
        for (int i = 0; i < 10; i  ) {
            for (int j = 0; j < 50000; j  ) {
                Random random = new Random();
                random.nextInt();
                count = count   i;
            }
        }
        long end = System.currentTimeMillis();
        System.out.println("------2------ thread 2 use time = "   (end - start));
    }
}

代码语言：javascript复制

public class ThreadPriority {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i  ) {
            MyThread1 myThread1 = new MyThread1();
            myThread1.setPriority(1);
            MyThread2 myThread2 = new MyThread2();
            myThread2.setPriority(10);
            myThread1.start();
            myThread2.start();
        }
    }
}

执行结果如下：

代码语言：javascript复制

------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 start running
------2------ thread 2 start running
------1------ thread 1 use time = 889
------2------ thread 2 use time = 938
------2------ thread 2 use time = 904
------1------ thread 1 use time = 901
------2------ thread 2 use time = 947
------1------ thread 1 use time = 969
------2------ thread 2 use time = 974
------2------ thread 2 use time = 975
------1------ thread 1 use time = 981
------1------ thread 1 use time = 986
------1------ thread 1 use time = 1011
------2------ thread 2 use time = 1012
------1------ thread 1 use time = 1017
------2------ thread 2 use time = 1035
------2------ thread 2 use time = 1036
------2------ thread 2 use time = 1041
------2------ thread 2 use time = 1042
------1------ thread 1 use time = 1041
------1------ thread 1 use time = 1043
------1------ thread 1 use time = 1043