java 线程设置优先级 java多线程优先级

1.8　线程的优先级

　在操作系统中，线程可以划分优先级，优先级较高的线程得到的cpu的资源较多，也就是cpu优先执行优先级较高的线程对象中的任务．

　在Java中，线程优先级分为1~10个等级，如果小于1大于10，则JDK抛出异常,源码如下:

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);
        }
    }

测试用例:

public class PriorityThread1 extends Thread{
    @Override
    public void run() {
        System.out.println("PriorityThread1 run priority=" + this.getPriority());
        PriorityThread2 thread2 = new PriorityThread2();
        thread2.start();
    }
    public static void main(String[] args) {
        System.out.println("main thread begin priority = " + Thread.currentThread().getPriority());
//        Thread.currentThread().setPriority(6);
        System.out.println("main thread end priority =" + Thread.currentThread().getPriority());
        new PriorityThread1().start();
    }
}

public class PriorityThread2 extends Thread{
    @Override
    public void run() {
        System.out.println("PriorityThread2 run priority=" + this.getPriority());
    }
}

打印结果如下(优先级被继承)：

main thread begin priority = 5
main thread end priority =5
PriorityThread1 run priority=5
PriorityThread2 run priority=5

去掉代码中的注释打印结果如下(优先级被修改继续被继承):

main thread begin priority = 5
main thread end priority =6
PriorityThread1 run priority=6
PriorityThread2 run priority=6

1.10.2 优先级具有规则性

　虽然可以设置优先级，但是没有看到优先级带来的效果

public class PriorityThread1 extends Thread{
    @Override
    public void run() {
        long begin = 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 = addResult + i;
            }
        }
        long end = System.currentTimeMillis();
        System.out.println("❤　❤　❤　❤　❤ thread１ use time " +(end - begin)+ " millisecond");
    }
    public static void main(String[] args) {
       for (int i=0; i<5; i++) {
           PriorityThread1 thread1 = new PriorityThread1();
           thread1.setPriority(10);
           thread1.start();
           PriorityThread2 thread2 = new PriorityThread2();
           thread2.setPriority(1);
           thread2.start();
       }
    }
}

public class PriorityThread2 extends Thread{
    @Override
    public void run() {
        long begin = 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 = addResult + i;
            }
        }
        long end = System.currentTimeMillis();
        System.out.println("☆　☆　☆　☆　☆ thread2 use time " +(end - begin)+ " millisecond");
    }
}

打印结果如下:

☆　☆　☆　☆　☆ thread2 use time 217 millisecond
☆　☆　☆　☆　☆ thread2 use time 291 millisecond
❤　❤　❤　❤　❤ thread１ use time 302 millisecond
❤　❤　❤　❤　❤ thread１ use time 295 millisecond
❤　❤　❤　❤　❤ thread１ use time 320 millisecond
❤　❤　❤　❤　❤ thread１ use time 272 millisecond
❤　❤　❤　❤　❤ thread１ use time 338 millisecond
☆　☆　☆　☆　☆ thread2 use time 316 millisecond
☆　☆　☆　☆　☆ thread2 use time 286 millisecond
☆　☆　☆　☆　☆ thread2 use time 341 millisecond

由结果可以得知，高优先级的线程总是大部分先执行完，但不代表高优先级的线程全部执行完(也有可能是高优先级的最后才执行完，但是大部分都是先执行的)．另外不要以为高优先级的线程被main线程调用就会先执行完，出现这样的结果全是因为其优先级是最高值10造成的．但线程优先级的等级差距很大时，谁先执行完和代码的调用顺序无关,cpu尽量将执行资源让给优先级高的线程．

1.10.3　优先级具有随机性

　前面案例介绍了线程的优先级较高则优先执行完run()方法中的任务，但这个结果不能说的太肯定，因为线程的优先级还具有随机性，也就是优先级较高的线程不一定每一次都先执行完

1.11　守护线程

在Java线程中有两种线程，一种是用户线程，另一种是守护线程．守护线程是一种特殊的线程，当进程中不存在非守护线程了，则守护线程自动销毁．典型的守护线程就是垃圾回收线程．用这个比较通俗的比喻来解释一下"守护线程":任何一个守护线程都是整个JVM中所有非守护线程的保姆，只要当前的JVM实例中存在任何一个非守护线程没有结束，守护线程就在工作，只有当最后一个非守护线程结束时，守护线程才随着JVM一同结束工作．Daemon的作用就是为其它线程的运行提供便利服务，守护线程最典型的应用就是GC，它是一个很称职的守护者．

public class DaemonThread extends Thread{
    private static int count = 0;
    @Override
    public void run() {
        while(true) {
            try {
                count ++;
                System.out.println(count);
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) {
        try {
            DaemonThread thread = new DaemonThread();
            thread.setDaemon(true);
            thread.start();
            Thread.sleep(5000);
            System.out.println("main thread is exist, so daemon is stop working");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}