Java多线程系列--“基础篇”10之 线程优先级和守护线程

Every thread has a priority. Threads with higher priority are executed in preference to threads with lower priority. Each thread may or may not also be marked as a daemon. When code running in some thread creates a new Thread object, the new thread has its priority initially set equal to the priority of the creating thread, and is a daemon thread if and only if the creating thread is a daemon.

When a Java Virtual Machine starts up, there is usually a single non-daemon thread (which typically calls the method named main of some designated class). The Java Virtual Machine continues to execute threads until either of the following occurs:

The exit method of class Runtime has been called and the security manager has permitted the exit operation to take place.
All threads that are not daemon threads have died, either by returning from the call to the run method or by throwing an exception that propagates beyond the run method. 
Marks this thread as either a daemon thread or a user thread. The Java Virtual Machine exits when the only threads running are all daemon threads.

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：
(01) 调用了exit()方法，并且exit()有权限被正常执行。
(02) 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。

 1 class MyThread extends Thread{  
 2     public MyThread(String name) {
 3         super(name);
 4     }
 5 
 6     public void run(){
 7         for (int i=0; i<5; i++) {
 8             System.out.println(Thread.currentThread().getName()
 9                     +"("+Thread.currentThread().getPriority()+ ")"
10                     +", loop "+i);
11         }
12     } 
13 }; 
14 
15 public class Demo {  
16     public static void main(String[] args) {  
17 
18         System.out.println(Thread.currentThread().getName()
19                 +"("+Thread.currentThread().getPriority()+ ")");
20 
21         Thread t1=new MyThread("t1");    // 新建t1
22         Thread t2=new MyThread("t2");    // 新建t2
23         t1.setPriority(1);                // 设置t1的优先级为1
24         t2.setPriority(10);                // 设置t2的优先级为10
25         t1.start();                        // 启动t1
26         t2.start();                        // 启动t2
27     }  
28 }

main(5)
t1(1), loop 0
t2(10), loop 0
t1(1), loop 1
t2(10), loop 1
t1(1), loop 2
t2(10), loop 2
t1(1), loop 3
t2(10), loop 3
t1(1), loop 4
t2(10), loop 4

 1 // Demo.java
 2 class MyThread extends Thread{  
 3     public MyThread(String name) {
 4         super(name);
 5     }
 6 
 7     public void run(){
 8         try {
 9             for (int i=0; i<5; i++) {
10                 Thread.sleep(3);
11                 System.out.println(this.getName() +"(isDaemon="+this.isDaemon()+ ")" +", loop "+i);
12             }
13         } catch (InterruptedException e) {
14         }
15     } 
16 }; 
17 
18 class MyDaemon extends Thread{  
19     public MyDaemon(String name) {
20         super(name);
21     }
22 
23     public void run(){
24         try {
25             for (int i=0; i<10000; i++) {
26                 Thread.sleep(1);
27                 System.out.println(this.getName() +"(isDaemon="+this.isDaemon()+ ")" +", loop "+i);
28             }
29         } catch (InterruptedException e) {
30         }
31     } 
32 }
33 public class Demo {  
34     public static void main(String[] args) {  
35 
36         System.out.println(Thread.currentThread().getName()
37                 +"(isDaemon="+Thread.currentThread().isDaemon()+ ")");
38 
39         Thread t1=new MyThread("t1");    // 新建t1
40         Thread t2=new MyDaemon("t2");    // 新建t2
41         t2.setDaemon(true);                // 设置t2为守护线程
42         t1.start();                        // 启动t1
43         t2.start();                        // 启动t2
44     }  
45 }

main(isDaemon=false)
t2(isDaemon=true), loop 0
t2(isDaemon=true), loop 1
t1(isDaemon=false), loop 0
t2(isDaemon=true), loop 2
t2(isDaemon=true), loop 3
t1(isDaemon=false), loop 1
t2(isDaemon=true), loop 4
t2(isDaemon=true), loop 5
t2(isDaemon=true), loop 6
t1(isDaemon=false), loop 2
t2(isDaemon=true), loop 7
t2(isDaemon=true), loop 8
t2(isDaemon=true), loop 9
t1(isDaemon=false), loop 3
t2(isDaemon=true), loop 10
t2(isDaemon=true), loop 11
t1(isDaemon=false), loop 4
t2(isDaemon=true), loop 12