synchronized(5)修饰语句块之：synchronized(XXX.class)

synchronized(XXX.class)有两种写法

　　synchronized(XXX.class)或者synchronized(obj.getClass())

• Class也是一个类xxx.class和obj.getClass()得到的是Class的一个实例对象.
• 比如String.class是Class类的一个实例，Object.class也是Class的一个实例……。
• 至于XX.class显然是一个Class的实例，而不是一个类。
• synchronized (XXX.class)和synchronized（this）这样的语句在语法上是同一类型的，它们本质上是synchronized(对象),只不过锁的是Class的一个实例xxx.class。

• synchronized(XXX.class)会阻塞后面返回相同class的synchronized(XXX.class)代码.

举例:

 public class Thread9 {
     private String a = "";
     private String b = "";
     private String c = "";

     private void sync_fun1() throws InterruptedException {
         synchronized (a.getClass()) {
             int i = ;
             while (i-- > ) {
                 System.out.println(Thread.currentThread().getName() + " : " + i);
                 try {
                     Thread.sleep();
                 } catch (InterruptedException ie) {
                 }
             }
         }
     }

     private void sync_fun2() throws InterruptedException {
         synchronized (b.getClass()) {
             int i = ;
             while (i-- > ) {
                 System.out.println(Thread.currentThread().getName() + " : " + i);
                 try {
                     Thread.sleep();
                 } catch (InterruptedException ie) {
                 }
             }
         }
     }

     private synchronized void sync_fun3() {
         int i = ;
         while (i-- > ) {
             System.out.println(Thread.currentThread().getName() + " : " + i);
             try {
                 Thread.sleep();
             } catch (InterruptedException ie) {
             }
         }
     }

     public static void main(String args[]) {
         Thread t1 = new Thread(new Runnable() {
             public void run() {
                 try {
                     new Thread9().sync_fun1();
                 } catch (InterruptedException e) {
                     e.printStackTrace();
                 }
             }
         }, "t1");
         Thread t2 = new Thread(new Runnable() {
             public void run() {
                 try {
                     new Thread9().sync_fun2();
                 } catch (InterruptedException e) {
                     e.printStackTrace();
                 }
             }
         }, "t2");
         Thread t3 = new Thread(new Runnable() {
             public void run() {
                 new Thread9().sync_fun3();
             }
         }, "not sync class");

         t1.start();
         t2.start();
         t3.start();
     }
 }

结果

t1 : 4
not sync class : 4
t1 : 3
not sync class : 3
t1 : 2
not sync class : 2
t1 : 1
not sync class : 1
t1 : 0
not sync class : 0
t2 : 4
t2 : 3
t2 : 2
t2 : 1
t2 : 0