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

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：



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。