Synchronized是Java中解决并发问题的一种最常用的方法，也是最简单的一种方法。Synchronized的作用主要有三个：（1）确保线程互斥的访问同步代码（2）保证共享变量的修改能够及时可见（3）有效解决重排序问题。从语法上讲，Synchronized总共有三种用法：
（1）修饰普通方法
（2）修饰静态方法
（3）修饰代码块
| synchronized形式（均写在在类A中） | 锁 | 锁类型 |
|---|---|---|
| synchronized(obj){} | obj | 对象锁 |
| synchronized(A.class){} | A.class | 类锁 |
| synchronized(this){} | 当前类对象 | 对象锁 |
| synchronized void memberMethod(){} | 当前类对象 | 对象锁 |
| static synchronized void staticMethod(){} | 当前类 | 类锁 |
在多线程开发中，我们经常看到synchronized(this)、synchronized(*.class)、synchronized(任意对象)这几种类型同步方法。但是是否知道这几种写法有什么区别了？下面根据代码来分析：
synchronized代码块间的同步性
package com.zwz.thread.demo1;public class ObjectService { public void serviceMethodA(){ try { synchronized (this) { System.out.println("A begin time="+System.currentTimeMillis()); Thread.sleep(2000); System.out.println("A end time="+System.currentTimeMillis()); } } catch (InterruptedException e) { e.printStackTrace(); } } public void serviceMethodB(){ synchronized (this) { System.out.println("B begin time="+System.currentTimeMillis()); System.out.println("B end time="+System.currentTimeMillis()); } }}复制代码 package com.zwz.thread.demo1;public class ThreadA extends Thread { private ObjectService objectService; public ThreadA(ObjectService objectService){ super(); this.objectService=objectService; } @Override public void run() { super.run(); objectService.serviceMethodA(); }}复制代码 package com.zwz.thread.demo1;public class ThreadB extends Thread { private ObjectService objectService; public ThreadB(ObjectService objectService){ super(); this.objectService=objectService; } @Override public void run() { super.run(); objectService.serviceMethodB(); }}复制代码 package com.zwz.thread.demo1;public class MainTest { public static void main(String[] args) { ObjectService service=new ObjectService(); ThreadA a=new ThreadA(service); a.setName("a"); a.start(); ThreadB b=new ThreadB(service); b.setName("b"); b.start(); }}复制代码 运行结果：
验证synchronized (this)代码块是锁定当前对象
package com.zwz.thread.demo2;public class ObjectService { public void objectMethodA(){ System.out.println("run----objectMethodA"); } public void objectMethodB(){ synchronized (this) { try { for (int i = 1; i <= 10; i++) { System.out.println("synchronized thread name:"+Thread.currentThread().getName()+"-->i="+i); Thread.sleep(1000); } } catch (InterruptedException e) { e.printStackTrace(); } } }}复制代码 package com.zwz.thread.demo2;public class ThreadA extends Thread { private ObjectService objectService; public ThreadA(ObjectService objectService) { super(); this.objectService = objectService; } @Override public void run() { super.run(); objectService.objectMethodA(); }}复制代码 package com.zwz.thread.demo2;public class ThreadB extends Thread { private ObjectService objectService; public ThreadB(ObjectService objectService) { super(); this.objectService = objectService; } @Override public void run() { super.run(); objectService.objectMethodB(); }}复制代码 package com.zwz.thread.demo2;public class MainTest { public static void main(String[] args) throws InterruptedException { ObjectService service=new ObjectService(); ThreadB b=new ThreadB(service); b.start(); Thread.sleep(2000); ThreadA a=new ThreadA(service); a.start(); }}复制代码 运行结果：
可以看到objectMethodA方法异步执行了，下面我们将objectMethodA()加上同步。
package com.zwz.thread.demo2;public class ObjectService { public synchronized void objectMethodA(){ System.out.println("run----objectMethodA"); } public void objectMethodB(){ synchronized (this) { try { for (int i = 1; i <= 10; i++) { System.out.println("synchronized thread name:"+Thread.currentThread().getName()+"-->i="+i); Thread.sleep(1000); } } catch (InterruptedException e) { e.printStackTrace(); } } }}复制代码 运行结果：
结论：
上面三个小例子我们可以知道，多个线程调用同一个对象中的不同名称的synchronized同步方法或synchronized(this)同步代码块时，是同步的。
1、synchronized同步方法
①对其它的synchronized同步方法或synchronized(this)同步代码块调用是堵塞状态；
②同一时间只有一个线程执行synchronized同步方法中的代码。
2、synchronized(this)同步代码块
①对其它的synchronized同步方法或synchronized(this)同步代码块调用是堵塞状态；
②同一时间只有一个线程执行synchronized同步方法中的代码。
将任意对象作为对象监视器
package com.zwz.thread.demo3;public class ObjectService { private String uname; private String pwd; String lock=new String(); public void setUserNamePassWord(String userName,String passWord){ try { synchronized (lock) { System.out.println("thread name="+Thread.currentThread().getName() +" 进入代码快:"+System.currentTimeMillis()); uname=userName; Thread.sleep(3000); pwd=passWord; System.out.println("thread name="+Thread.currentThread().getName() +" 进入代码快:"+System.currentTimeMillis()+"入参uname:"+uname+"入参pwd:"+pwd); } } catch (InterruptedException e) { e.printStackTrace(); } }}复制代码 package com.zwz.thread.demo3;public class ThreadA extends Thread { private ObjectService objectService; public ThreadA(ObjectService objectService) { super(); this.objectService = objectService; } @Override public void run() { objectService.setUserNamePassWord("a", "aa"); }}复制代码 package com.zwz.thread.demo3;public class ThreadB extends Thread { private ObjectService objectService; public ThreadB(ObjectService objectService) { super(); this.objectService = objectService; } @Override public void run() { objectService.setUserNamePassWord("b", "bb"); }}复制代码 package com.zwz.thread.demo3;public class MainTest { public static void main(String[] args) { ObjectService service=new ObjectService(); ThreadA a=new ThreadA(service); a.setName("A"); a.start(); ThreadB b=new ThreadB(service); b.setName("B"); b.start(); }}复制代码 运行结果：
下面我把String lock=new String();放在方法中会有啥结果了：
package com.zwz.thread.demo3;public class ObjectService { private String uname; private String pwd; public void setUserNamePassWord(String userName,String passWord){ try { String lock=new String(); synchronized (lock) { System.out.println("thread name="+Thread.currentThread().getName() +" 进入代码快:"+System.currentTimeMillis()); uname=userName; Thread.sleep(3000); pwd=passWord; System.out.println("thread name="+Thread.currentThread().getName() +" 进入代码快:"+System.currentTimeMillis()+"入参uname:"+uname+"入参pwd:"+pwd); } } catch (InterruptedException e) { e.printStackTrace(); } }}复制代码 运行结果：
结论：
多个线程持有对象监视器作为同一个对象的前提下，同一时间只有一个线程可以执行synchronized(任意自定义对象)同步代码快。 synchronized(任意自定义对象)与synchronized同步方法共用
package com.zwz.thread.demo4;public class ObjectService { private String lock=new String(); public void methodA(){ try { synchronized (lock) { System.out.println("a begin"); Thread.sleep(3000); System.out.println("a end"); } } catch (InterruptedException e) { e.printStackTrace(); } } public synchronized void methodB(){ System.out.println("b begin"); System.out.println("b end"); }}复制代码 package com.zwz.thread.demo4;public class ThreadA extends Thread { private ObjectService objectService; public ThreadA(ObjectService objectService) { super(); this.objectService = objectService; } @Override public void run() { objectService.methodA(); }}复制代码 package com.zwz.thread.demo4;public class ThreadB extends Thread { private ObjectService objectService; public ThreadB(ObjectService objectService) { super(); this.objectService = objectService; } @Override public void run() { objectService.methodB(); }}复制代码 package com.zwz.thread.demo4;public class MainTest { public static void main(String[] args) { ObjectService service=new ObjectService(); ThreadA a=new ThreadA(service); a.setName("A"); a.start(); ThreadB b=new ThreadB(service); b.setName("B"); b.start(); } }复制代码 运行结果:
使用synchronized(任意自定义对象)进行同步操作，对象监视器必须是同一个对象。不过不是同一个，运行就是异步执行了。
静态同步synchronized方法与synchronized(*.class)代码块
静态同步synchronized方法
package com.zwz.thread.demo6;public class ObjectService { public synchronized static void methodA(){ try { System.out.println("static methodA begin 线程名称:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis()); Thread.sleep(3000); System.out.println("static methodA end 线程名称:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis()); } catch (InterruptedException e) { e.printStackTrace(); } } public synchronized static void methodB(){ System.out.println("static methodB begin 线程名称:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis()); System.out.println("static methodB end 线程名称:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis()); }}复制代码 package com.zwz.thread.demo6;public class ThreadA extends Thread { @Override public void run() { ObjectService.methodA(); }}复制代码 package com.zwz.thread.demo6;public class ThreadB extends Thread { @Override public void run() { ObjectService.methodB(); }}复制代码 package com.zwz.thread.demo6;public class MainTest { public static void main(String[] args) { ThreadA a=new ThreadA(); a.setName("A"); a.start(); ThreadB b=new ThreadB(); b.setName("B"); b.start(); }}复制代码 运行结果：
结论： synchronized应用在static方法上，那是对当前对应的*.Class进行持锁。 synchronized(*.class)代码块
package com.zwz.thread.demo7;public class ObjectService { public void methodA(){ try { synchronized (ObjectService.class) { System.out.println("methodA begin 线程名称:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis()); Thread.sleep(3000); System.out.println("methodA end 线程名称:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis()); } } catch (InterruptedException e) { e.printStackTrace(); } } public void methodB(){ synchronized (ObjectService.class) { System.out.println("methodB begin 线程名称:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis()); System.out.println("methodB end 线程名称:"+Thread.currentThread().getName()+" times:"+System.currentTimeMillis()); } }}复制代码 package com.zwz.thread.demo7;public class ThreadA extends Thread { private ObjectService objectService; public ThreadA(ObjectService objectService) { super(); this.objectService = objectService; } @Override public void run() { objectService.methodA(); }}复制代码 package com.zwz.thread.demo7;public class ThreadB extends Thread { private ObjectService objectService; public ThreadB(ObjectService objectService) { super(); this.objectService = objectService; } @Override public void run() { objectService.methodB(); }}复制代码 package com.zwz.thread.demo7;public class MainTest { public static void main(String[] args) { ObjectService service=new ObjectService(); ThreadA a=new ThreadA(service); a.setName("A"); a.start(); ThreadB b=new ThreadB(service); b.setName("B"); b.start(); }}复制代码 运行结果:
上面测试方法是共同对象，下面我们分别实例化一个对象：
package com.zwz.thread.demo7;public class MainTest { public static void main(String[] args) { ObjectService service1=new ObjectService(); ObjectService service2=new ObjectService(); ThreadA a=new ThreadA(service1); a.setName("A"); a.start(); ThreadB b=new ThreadB(service2); b.setName("B"); b.start(); }}复制代码 运行结果：
结论： 同步synchronized(*.class)代码块的作用其实和synchronized static方法作用一样。Class锁对类的所有对象实例起作用。
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