java多线程系列8-线程的优先级

在java中设置线程优先级使用setPriority，在jdk中的源代码如下：

public final void setPriority(int newPriority) {
        ThreadGroup g;
        checkAccess();
        if (newPriority > MAX_PRIORITY || newPriority < MIN_PRIORITY) {
            throw new IllegalArgumentException();
        }
        if((g = getThreadGroup()) != null) {
            if (newPriority > g.getMaxPriority()) {
                newPriority = g.getMaxPriority();
            }
            setPriority0(priority = newPriority);
        }
    }

在java中，线程的优先级分为1～10这10个等级，小于1或大于10，则会抛出IllegalArgumentException异常

在JDK中使用3个常量来预定义优先级：

  /**
     * The minimum priority that a thread can have.
     */
    public final static int MIN_PRIORITY = 1;

   /**
     * The default priority that is assigned to a thread.
     */
    public final static int NORM_PRIORITY = 5;

    /**
     * The maximum priority that a thread can have.
     */
    public final static int MAX_PRIORITY = 10;

线程优先级的继承性

在java中，线程的优先级具有继承性，例如A线程启动B线程，则A和B的优先级是一样的。举个例子：

public class MyThread1 extends Thread {
    @Override
    public void run() {
        System.out.println("MyThread1 run priority=" + this.getPriority());
        MyThread2 thread2 = new MyThread2();
        thread2.start();
    }
}
public class MyThread2 extends Thread {
    @Override
    public void run() {
        System.out.println("MyThread2 run priority=" + this.getPriority());
    }
}
public class Run {
    public static void main(String[] args) throws InterruptedException {
        System.out.println("main thread begin priority="
                + Thread.currentThread().getPriority());
        System.out.println("main thread end priority="
                + Thread.currentThread().getPriority());
        MyThread1 thread1 = new MyThread1();
        thread1.start();
    }
}

运行结果如下：

main thread begin priority=5
main thread end priority=5
MyThread1 run priority=5
MyThread2 run priority=5

优先级具有规则性

虽然setPriority()方法可以设置线程的优先级，但是没有看到设置优先级所带来的效果

举例如下：

public class MyThread1 extends Thread {
    @Override
    public void run() {
        long beginTime = System.currentTimeMillis();
        long addResult = 0;
        for (int i = 0; i < 10; i++) {
            for (int j = 0; j < 50000; j++) {
                Random random = new Random();
                random.nextInt();
                addResult += j;
            }
        }
        long endTime = System.currentTimeMillis();
        System.out.println("* * * * * thread 1 use time=" + (endTime - beginTime));
    }
}
public class MyThread2 extends Thread {
    @Override
    public void run() {
        long beginTime = System.currentTimeMillis();
        long addResult = 0;
        for (int i = 0; i < 10; i++) {
            for (int j = 0; j < 50000; j++) {
                Random random = new Random();
                random.nextInt();
                addResult += j;
            }
        }
        long endTime = System.currentTimeMillis();
        System.out.println("* * * * * thread 2 use time=" + (endTime - beginTime));
    }
}
public class Run {
    public static void main(String[] args) throws InterruptedException {

        for(int i=0;i<5;i++){
            MyThread1 thread1=new MyThread1();
            thread1.setPriority(10);
            thread1.start();
            MyThread2 thread2 =new MyThread2();
            thread2.setPriority(1);
            thread2.start();
        }
    }
}

运行结果如下：

* * * * * thread 1 use time=174
* * * * * thread 1 use time=221
* * * * * thread 1 use time=224
* * * * * thread 2 use time=360
* * * * * thread 1 use time=202
* * * * * thread 2 use time=185
* * * * * thread 1 use time=169
* * * * * thread 2 use time=466
* * * * * thread 2 use time=425
* * * * * thread 2 use time=98

从上面的结果可以看出，高优先级的线程总是大部分先执行完，但是不是所有的先执行完。先执行完也不是因为先调用，如果更改优先级，先执行完和和代码的调用顺序无关。

优先级具有一定的规则性，CPU总是尽量将执行资源让给优先级比较高的线程

优先级具有随机性

优先级较高的线程不一定每一次都先执行完，举个例子：

public class MyThread1 extends Thread {
    @Override
    public void run() {
        long beginTime = System.currentTimeMillis();
        for (int i = 0; i < 1000; i++) {
            Random random = new Random();
            random.nextInt();
        }
        long endTime = System.currentTimeMillis();
        System.out.println("* * * * * thread 1 use time="
                + (endTime - beginTime));
    }
}
public class MyThread2 extends Thread {
    @Override
    public void run() {
        long beginTime = System.currentTimeMillis();
        for (int i = 0; i < 1000; i++) {
            Random random = new Random();
            random.nextInt();
        }
        long endTime = System.currentTimeMillis();
        System.out.println("* * * * * thread 2 use time="
                + (endTime - beginTime));
    }
}
public class Run {
    public static void main(String[] args) throws InterruptedException {
        for (int i = 0; i < 5; i++) {
            MyThread1 thread1 = new MyThread1();
            thread1.setPriority(5);
            thread1.start();
            MyThread2 thread2 = new MyThread2();
            thread2.setPriority(6);
            thread2.start();
        }
    }
}

运行结果如下：

* * * * * thread 1 use time=5
* * * * * thread 2 use time=4
* * * * * thread 1 use time=5
* * * * * thread 1 use time=4
* * * * * thread 1 use time=4
* * * * * thread 2 use time=6
* * * * * thread 2 use time=3
* * * * * thread 2 use time=5
* * * * * thread 1 use time=2
* * * * * thread 2 use time=6

线程的优先级与打印顺序无关，它们的关系具有不确定性和随机性

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