Java线程的优先级

ゝ一纸荒年。 2022-05-24 03:19 240阅读 0赞

**“黄梅时节家家雨,青草池塘处处蛙”**  
线程是有优先级的,也就是线程的执行顺序;先来看下对于设置线程优先级的源码:  
![这里写图片描述][70]  
其中:MAX\_PRIORITY和MIN\_PRIORITY分别是最高级10和最低级1,当然还有默认级别是5;  
![这里写图片描述][70 1]

接着看下线程优先级的一些特性:

1.  **线程优先级的继承特性:**也就是如果线程A启动线程B,那么线程A和B的优先级是一样的;
2.  **线程优先级的规则性:**即线程会优先级的大小顺序执行,但是不一定是优先级较大的先执行完,因为线程的优先级还有下面第三个特性:
3.  **线程优先级的随机特性;**
    
    验证线程优先级的继承特性:

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(8);
            System.out.println("twice Main Thread Priority = " + Thread.currentThread().getPriority());
            MyThread1 myThread1 = new MyThread1();
            myThread1.start();
        }
    }
    
    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();
        }
    }
    class MyThread2 extends Thread { 
        @Override
        public void run() {
            super.run();
            System.out.println("MyThread2 Priority = " + this.getPriority());
        }
    }

打开设置线程优先级之前的结果:  
![这里写图片描述][70 2]  
打开设置线程优先级之后的结果:  
![这里写图片描述][70 3]  
**因此证明线程的优先级是具有继承性的;**  
验证线程具有规则性:

import java.util.Random;
    
    
    public class ThreadPriority { 
        public static void main(String[] args) {
            for (int i = 0; i < 5; i++) {
                MyThread1 myThread1 = new MyThread1();
                myThread1.setPriority(10);
                myThread1.start();
                MyThread2 myThread2 = new MyThread2();
                myThread2.setPriority(1);
                myThread2.start();
            }
        }
    }
    
    class MyThread1 extends Thread { 
        @Override
        public void run() {
            long start = System.currentTimeMillis();
            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("############# thread 1 use time = " + (end - start));
        }
    }
    class MyThread2 extends Thread { 
        @Override
        public void run() {
            long start = System.currentTimeMillis();
            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("$$$$$$$$$$$$$thread 2 use time = " + (end - start));
        }
    }

![这里写图片描述][70 4]  
从运行结果中可以发现,MyThread1总是大部分先执行完(可以多次运行看结果,会发现线程1都是先全部执行完).并不是说要等线程1全部执行完才执行线程2;  
而且,你可以把线程1和2的优先级换一下,线程1设置为1,线程2设置为10,再看结果:

MyThread1 myThread1 = new MyThread1();
        myThread1.setPriority(1);
        myThread1.start();
        MyThread2 myThread2 = new MyThread2();
        myThread2.setPriority(10);
        myThread2.start();

![这里写图片描述][70 5]  
由此可以证明并不是按照在代码的中顺序执行的.而是根据线程的优先级;  
再看线程的随机性:  
直接把刚才证明线程规则性的代码中,分别设置两个线程优先级为5和6,比较接近的两个值:

MyThread1 myThread1 = new MyThread1();
     myThread1.setPriority(5);
     myThread1.start();
     MyThread2 myThread2 = new MyThread2();
     myThread2.setPriority(6);
     myThread2.start();

![这里写图片描述][70 6]  
**由此可以得出,线程的优先级和打印顺序无关,不能将这两者相关联,它们的关系具有不确定性和随机性;**

[70]: /images/20220524/af3d3ea38a134acbba362ff89664cb55.png
[70 1]: /images/20220524/d3fadb4eee3248e185afa2a59d022ae9.png
[70 2]: /images/20220524/ed624ced38b24a00bca12ac84751a01a.png
[70 3]: /images/20220524/ed62da7b34344a26b3806f3a47081610.png
[70 4]: /images/20220524/bf8a5b2ed92b41c59dd16363494f33a8.png
[70 5]: /images/20220524/74e119feb35842548225c65e37b0b627.png
[70 6]: /images/20220524/c1407eb502e4492d84b553d6f721df5e.png