Java中synchronized(this)、synchronized(class)与synchronized(Object)的区别

深藏阁楼爱情的钟 2022-05-20 09:50 169阅读 0赞

本文转自：[https://blog.csdn.net/luckey\_zh/article/details/53815694][https_blog.csdn.net_luckey_zh_article_details_53815694]

在多线程开发中，我们经常看到synchronized(this)、synchronized(\*.class)与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：

![Center][]

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：

![Center 1][]

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：

![Center 2][]

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：

![Center 3][]

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：

![Center 4][]

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:

![Center 5][]

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：

![Center 6][]

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:

![Center 7][]

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：

![Center 8][]

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。

[https_blog.csdn.net_luckey_zh_article_details_53815694]: https://blog.csdn.net/luckey_zh/article/details/53815694
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