Java synchronized

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());
		}
	}
}

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();
	}
}

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();
	}
}

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();
	}
}

运行结果：

结论： 当一个线程访问ObjectService的一个synchronized (this)同步代码块时，其它线程对同一个ObjectService中其它的synchronized (this)同步代码块的访问将是堵塞，这说明synchronized (this)使用的对象监视器是一个。

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();
			}
		}
	}
}

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();
	}
}

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();
	}
}

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();
	}
}

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();
			}
		}
	}
}

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();
		}
	}
}

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");
	}
}

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");
	}
}

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();
	}
}

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();
		}
	}
}

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");
	}
}

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();
	}
}

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();
	}
}

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();
	}
	
}

运行结果:

结论：

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());
	}
}

package com.zwz.thread.demo6;

public class ThreadA extends Thread {

	@Override
	public void run() {
		ObjectService.methodA();
	}
}

package com.zwz.thread.demo6;

public class ThreadB extends Thread {
	@Override
	public void run() {
		ObjectService.methodB();
	}
}

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();
	}
}

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());
		}
	}
}

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();
	}
}

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();
	}
}

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();
	}
}

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();
	}
}

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。

参考: blog.csdn.net/luckey_zh/a…