Java 多线程基础（十一）线程优先级和守护线程

每个线程都有一个优先级。“高优先级线程”会优先于“低优先级线程”执行。每个线程都可以被标记为一个守护进程或非守护进程。在一些运行的主线程中创建新的子线程时，
子线程的优先级被设置为等于“创建它的主线程的优先级”，当且仅当“创建它的主线程是守护线程”时“子线程才会是守护线程”。

当Java虚拟机启动时，通常有一个单一的非守护线程（该线程通过是通过main()方法启动）。JVM会一直运行直到下面的任意一个条件发生，JVM就会终止运行：
①、调用了exit()方法，并且exit()有权限被正常执行。
②、所有的“非守护线程”都死了(即JVM中仅仅只有“守护线程”)。

每一个线程都被标记为“守护线程”或“用户线程”。当只有守护线程运行时，JVM会自动退出。 

public class Demo01 {
    private static Object obj = new Object();
    public static void main(String[] args) {
        
        Thread t1 = new ThreadA("t1");
        Thread t2 = new ThreadA("t2");
        
        t1.setPriority(1); // 设置优先级为1
        t2.setPriority(10);// 设置优先级为10
        
        t1.start();
        t2.start();
    }
}
class ThreadA extends Thread{
    public ThreadA(String name) {
        super(name);
    }
    public void run() {
        for(int i = 0;i < 3;i++)
            System.out.println(Thread.currentThread().getName() + " [ " 
                    + Thread.currentThread().getPriority() + " ] loop " + i);  
    }
}

// 运行结果
t1 [ 1 ] loop 0
t2 [ 10 ] loop 0
t2 [ 10 ] loop 1
t2 [ 10 ] loop 2
t1 [ 1 ] loop 1
t1 [ 1 ] loop 2

public class Demo {
    public static void main(String[] args) {

        System.out.println(Thread.currentThread().getName()
                +" [ isDaemon = "+Thread.currentThread().isDaemon()+ " ]");
        Thread t1=new ThreadA("t1");    
        Thread t2=new MyDaemon("t2");    
        t2.setDaemon(true);// 设置t2为守护线程
        t1.start();                        
        t2.start();                        
    }
}
class ThreadA extends Thread{
    public ThreadA(String name) {
        super(name);
    }

    public void run(){
        try {
            for (int i=0; i<5; i++) {
                Thread.sleep(3);
                System.out.println(this.getName() +"[ isDaemon = "+this.isDaemon()+ " ] " + "loop " + i);
            }
        } catch (InterruptedException e) {
        }
    }
};

class MyDaemon extends Thread{
    public MyDaemon(String name) {
        super(name);
    }

    public void run(){
        try {
            for (int i=0; i<10000; i++) {
                Thread.sleep(1);
                System.out.println(this.getName() +"[ isDaemon = " + this.isDaemon() +  " ] " +"loop "+i);
            }
        } catch (InterruptedException e) {
        }
    }
}

// 运行结果
main [ isDaemon = false ]
t2[ isDaemon = true ] loop 0
t2[ isDaemon = true ] loop 1
t2[ isDaemon = true ] loop 2
t1[ isDaemon = false ] loop 0
t2[ isDaemon = true ] loop 3
t2[ isDaemon = true ] loop 4
t1[ isDaemon = false ] loop 1
t2[ isDaemon = true ] loop 5
t2[ isDaemon = true ] loop 6
t2[ isDaemon = true ] loop 7
t1[ isDaemon = false ] loop 2
t2[ isDaemon = true ] loop 8
t2[ isDaemon = true ] loop 9
t2[ isDaemon = true ] loop 10
t1[ isDaemon = false ] loop 3
t2[ isDaemon = true ] loop 11
t2[ isDaemon = true ] loop 12
t2[ isDaemon = true ] loop 13
t2[ isDaemon = true ] loop 14
t1[ isDaemon = false ] loop 4
t2[ isDaemon = true ] loop 15