“synchronized(非this对象x)”格式的写法是将x对象本身作为“对象监视器”，这样就可以得出以下3个结论：
- 当多个线程同时执行synchronized(x){}同步代码块时呈同步效果
- 当其他线程执行x对象中synchronized同步方法时呈同步效果
- 当其他线程执行x对象方法里面的synchronized(this)代码块时也呈现同步效果
- 但需要注意：如果其他线程调用不加synchronized关键字的方法时，还是异步调用。
当多个线程同时执行synchronized(x){}同步代码块时呈同步效果
/**
* @author wuyoushan
* @date 2017/3/29.
*/
public class MyObject {
}
/**
* @author wuyoushan
* @date 2017/4/10.
*/
public class Service {
public void testMethod1(MyObject object){
synchronized (object){
try{
System.out.println("testMethod1 getLock time="
+System.currentTimeMillis()+"run ThreadName="
+Thread.currentThread().getName());
Thread.sleep(2000);
System.out.println("testMethod1 releaseLock time="
+System.currentTimeMillis()+"run ThreadName="
+Thread.currentThread().getName());
}catch (InterruptedException e){
e.printStackTrace();
}
}
}
}
/**
* @author wuyoushan
* @date 2017/4/4.
*/
public class ThreadA extends Thread{
private Service service;
private MyObject object;
public ThreadA(Service service, MyObject object) {
super();
this.service = service;
this.object = object;
}
@Override
public void run() {
super.run();
service.testMethod1(object);
}
}
/**
* @author wuyoushan
* @date 2017/4/4.
*/
public class ThreadB extends Thread{
private Service service;
private MyObject object;
public ThreadB(Service service, MyObject object) {
super();
this.service = service;
this.object = object;
}
@Override
public void run() {
super.run();
service.testMethod1(object);
}
}
/**
* @author wuyoushan
* @date 2017/3/20.
*/
public class Run {
public static void main(String[] args) throws InterruptedException {
Service service=new Service();
MyObject object=new MyObject();
ThreadA a=new ThreadA(service,object);
a.setName("a");
a.start();
ThreadB b=new ThreadB(service,object);
b.setName("b");
b.start();
}
}
程序的运行结果是：
testMethod1 getLock time=1492734868657run ThreadName=a
testMethod1 releaseLock time=1492734870658run ThreadName=a
testMethod1 getLock time=1492734870658run ThreadName=b
testMethod1 releaseLock time=1492734872658run ThreadName=b
同步的原因是使用了同一个“对象监视器”。如果使用不同的“对象监视器”会出现什么样的效果呢？
修改类文件Run.java，代码如下：
/**
* @author wuyoushan
* @date 2017/3/20.
*/
public class Run {
public static void main(String[] args) throws InterruptedException {
Service service=new Service();
MyObject object1=new MyObject();
MyObject object2=new MyObject();
ThreadA a=new ThreadA(service,object1);
a.setName("a");
a.start();
ThreadB b=new ThreadB(service,object2);
b.setName("b");
b.start();
}
}
运行结果为:
testMethod1 getLock time=1492735877980run ThreadName=a
testMethod1 getLock time=1492735877981run ThreadName=b
testMethod1 releaseLock time=1492735879982run ThreadName=b
testMethod1 releaseLock time=1492735879982run ThreadName=a
当其他线程执行x对象中synchronized同步方法时呈同步效果
/**
* @author wuyoushan
* @date 2017/3/29.
*/
public class MyObject {
synchronized public void speedPrintString(){
System.out.println("speedPrintString getLock time="
+System.currentTimeMillis()+"run ThreadName="
+Thread.currentThread().getName());
System.out.println("--------------------");
System.out.println("speedPrintString releaseLock time="
+System.currentTimeMillis()+"run ThreadName="
+Thread.currentThread().getName());
}
}
/**
* @author wuyoushan
* @date 2017/4/10.
*/
public class Service {
public void testMethod1(MyObject object){
synchronized (object){
try{
System.out.println("testMethod1 getLock time="
+System.currentTimeMillis()+"run ThreadName="
+Thread.currentThread().getName());
Thread.sleep(5000);
System.out.println("testMethod1 releaseLock time="
+System.currentTimeMillis()+"run ThreadName="
+Thread.currentThread().getName());
}catch (InterruptedException e){
e.printStackTrace();
}
}
}
}
/**
* @author wuyoushan
* @date 2017/4/4.
*/
public class ThreadA extends Thread{
private Service service;
private MyObject object;
public ThreadA(Service service, MyObject object) {
super();
this.service = service;
this.object = object;
}
@Override
public void run() {
super.run();
service.testMethod1(object);
}
}
/**
* @author wuyoushan
* @date 2017/4/4.
*/
public class ThreadB extends Thread{
private MyObject object;
public ThreadB(MyObject object) {
super();
this.object = object;
}
@Override
public void run() {
super.run();
object.speedPrintString();
}
}
/**
* @author wuyoushan
* @date 2017/3/20.
*/
public class Run {
public static void main(String[] args) throws InterruptedException {
Service service=new Service();
MyObject object=new MyObject();
ThreadA a=new ThreadA(service,object);
a.setName("a");
a.start();
Thread.sleep(100);
ThreadB b=new ThreadB(object);
b.setName("b");
b.start();
}
}
程序的运行结果为:
testMethod1 getLock time=1492736535766run ThreadName=a
testMethod1 releaseLock time=1492736540766run ThreadName=a
speedPrintString getLock time=1492736540766run ThreadName=b
--------------------
speedPrintString releaseLock time=1492736540766run ThreadName=b
验证第3个结论
当其他线程执行x对象方法里面的synchronized(this)代码块时也呈现同步效果。
修改MyObject.java的类，代码如下。
/**
* @author wuyoushan
* @date 2017/3/29.
*/
public class MyObject {
synchronized public void speedPrintString(){
synchronized (this) {
System.out.println("speedPrintString getLock time="
+ System.currentTimeMillis() + "run ThreadName="
+ Thread.currentThread().getName());
System.out.println("--------------------");
System.out.println("speedPrintString releaseLock time="
+ System.currentTimeMillis() + "run ThreadName="
+ Thread.currentThread().getName());
}
}
}
代码的运行结果如下:
testMethod1 getLock time=1492916942775run ThreadName=a
testMethod1 releaseLock time=1492916947775run ThreadName=a
speedPrintString getLock time=1492916947775run ThreadName=b
--------------------
speedPrintString releaseLock time=1492916947775run ThreadName=b
摘选自 java多线程核心编程技术-2.2.8
