java 多线程4: java线程的优先级

Java线程的优先级取值范围是1 （Thread.MIN_PRIORITY ） 到 10 （Thread.MAX_PRIORITY ）。如果没有设置， 线程默认的优先级是NORM_PRIORITY。这个值是5。

getPriority()和setPriority(int newPriority) 是实例方法

这两个方法用于获取和设置线程的优先级，优先级高的CPU得到的CPU资源比较多，设置优先级有助于帮"线程规划器"确定下一次选择哪一个线程优先执行。换句话说，两个在等待CPU的线程，优先级高的线程越容易被CU选择执行。下面来看一下例子，并得出几个结论：

1.线程的继承性

public class MyThread09_0 extends Thread
{
    public void run()
    {
        System.out.println("MyThread9_0 run priority = " +
                this.getPriority());
    }
}

public class MyThread09_1 extends Thread
{
    public void run()
    {
        System.out.println("MyThread9_1 run priority = " +
                this.getPriority());
        MyThread09_0 thread = new MyThread09_0();
        thread.start();
    }
}

public static void main(String[] args)
{
    System.out.println("main thread begin, priority = " +
            Thread.currentThread().getPriority());
    System.out.println("main thread end, priority = " +
            Thread.currentThread().getPriority());
    MyThread09_1 thread = new MyThread09_1();
    thread.start();
}

看一下运行结果：

main thread begin, priority = 5
main thread end, priority = 5
MyThread9_1 run priority = 5
MyThread9_0 run priority = 5

从这个例子我们得出结论：线程默认优先级为5，如果不手动指定，那么线程优先级具有继承性，比如线程A启动线程B，那么线程B的优先级和线程A的优先级相同。

如果A设置了线程优先级为6 ， 在A中启动B ，则B的优先级也是一样的

2.线程的规则性

下面的例子演示了设置线程优先级带来的效果：

public class MyThread10_0 extends Thread
{
    public void run()
    {
        long beginTime = System.currentTimeMillis();
        for (int j = 0; j < 100000; j++){}
        long endTime = System.currentTimeMillis();
        System.out.println("◆◆◆◆◆ thread0 use time = " +
                (endTime - beginTime));
    }
}

public class MyThread10_1 extends Thread
{
    public void run()
    {
        long beginTime = System.currentTimeMillis();
        for (int j = 0; j < 100000; j++){}
        long endTime = System.currentTimeMillis();
        System.out.println("◇◇◇◇◇ thread1 use time = " +
                (endTime - beginTime));
    }
}

public static void main(String[] args)
{
    for (int i = 0; i < 5; i++)
    {
        MyThread10_0 mt0 = new MyThread10_0();
        mt0.setPriority(5);
        mt0.start();
        MyThread10_1 mt1 = new MyThread10_1();
        mt1.setPriority(4);
        mt1.start();
    }
}

看下运行结果：

◆◆◆◆◆ thread0 use time = 0
◆◆◆◆◆ thread0 use time = 0
◆◆◆◆◆ thread0 use time = 1
◆◆◆◆◆ thread0 use time = 2
◆◆◆◆◆ thread0 use time = 2
◇◇◇◇◇ thread1 use time = 0
◇◇◇◇◇ thread1 use time = 1
◇◇◇◇◇ thread1 use time = 5
◇◇◇◇◇ thread1 use time = 2
◇◇◇◇◇ thread1 use time = 0

看到黑色菱形（线程优先级高的）先打印完，再多试几次也是一样的。为了产生一个对比效果，把yMyThread10_0的优先级设置为4，看下运行结果：

◆◆◆◆◆ thread0 use time = 0
◇◇◇◇◇ thread1 use time = 1
◇◇◇◇◇ thread1 use time = 1
◆◆◆◆◆ thread0 use time = 0
◇◇◇◇◇ thread1 use time = 0
◆◆◆◆◆ thread0 use time = 1
◆◆◆◆◆ thread0 use time = 1
◇◇◇◇◇ thread1 use time = 2
◇◇◇◇◇ thread1 use time = 1
◆◆◆◆◆ thread0 use time = 0

看到，马上差别就出来了。从这个例子我们得出结论：线程的优先级是有一定的规则性的，CPU会尽量将执行资源让给优先级比较高的线程。

3.线程的随机性

上面说到的第二点就是验证线程的规则性，在优先级越相近的情况，线程执行的顺序是随机的，CPU会尽量将执行资源让给优先级比较高的线程这一说法是对的，但是实际运行的情况是多种多样的，所以线程充满了随机性