上篇介绍了用synchronized修饰static方式来实现“Class 锁”，今天要介绍另一种实现方式，synchronized(class)代码块，写法不一样但是作用是一样的。下面我附上一段代码来看一下synchronized(class)代码块的基本用法，如下：
public static void main(String[] args) {
Service4 s1 = new Service4();
Service4 s2 = new Service4();
ThreadA a = new ThreadA(s1);
ThreadB b = new ThreadB(s2);
a.setName("A");
b.setName("B");
a.start();
b.start();
}
public static class ThreadA extends Thread {
private Service4 service;
public ThreadA(Service4 service) {
super();
this.service = service;
}
@Override
public void run() {
super.run();
service.printA();
}
}
public static class ThreadB extends Thread {
private Service4 service;
public ThreadB(Service4 service) {
super();
this.service = service;
}
@Override
public void run() {
super.run();
service.printB();
}
}
}
class Service4 {
static public void printA() {
synchronized (Service4.class) {
try {
System.out.println("线程：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printA");
Thread.sleep(3000);
System.out.println("线程：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static public void printB() {
synchronized (Service4.class) {
System.out.println("线程：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "进入printB");
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("线程：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "离开printB");
}
}
}
运行结果如下：synchronized(class)代码块的作用和synchronized static的作用是一样的
以前我说过，synchronized还可以传入其他的实例对象或者方法的形参，那么我现在要说一种把synchronized(class)和String一起使用的特殊情况，还是用代码讲解，下面我附上一段代码，如下：
public static void main(String[] args) {
Service5 service = new Service5();
ThreadA a = new ThreadA(service);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service);
b.setName("B");
b.start();
}
public static class ThreadB extends Thread {
private Service5 service;
public ThreadB(Service5 service) {
super();
this.service = service;
}
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
service.print("AA");
}
}
public static class ThreadA extends Thread {
private Service5 service;
public ThreadA(Service5 service) {
super();
this.service = service;
}
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
service.print("AA");
}
}
}
class Service5 {
public static void print(String string) {
try {
synchronized (string) {
while (true) {
System.out.println(Thread.currentThread().getName());
Thread.sleep(1000);
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
运行结果如下：可以看到输出结果会打印出无数个连续的A，这是由于在JVM中有String常量池缓存的功能，所以说printA()和printB()两个方法里传进来的“AA”是同一个值，因此两个线程持有相同的锁，所以总有一个线程执行不到。
上面已经看到了常量池带来的问题，因此大多情况下，都不用String最为对像锁，而改用其他的，比如new Object()实例化一个Object对象。可以看看下面的例子，运行后的区别在哪，如下：
public static void main(String[] args) {
Service6 service = new Service6();
ThreadA a = new ThreadA(service);
a.setName("A");
a.start();
ThreadB b = new ThreadB(service);
b.setName("B");
b.start();
}
public static class ThreadB extends Thread {
private Service6 service;
public ThreadB(Service6 service) {
super();
this.service = service;
}
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
service.print(new Object());
}
}
public static class ThreadA extends Thread {
private Service6 service;
public ThreadA(Service6 service) {
super();
this.service = service;
}
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
service.print(new Object());
}
}
}
class Service6 {
public static void print(Object object) {
try {
synchronized (object) {
while (true) {
System.out.println(Thread.currentThread().getName());
Thread.sleep(1000);
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
运行结果如下：可以看到运行结果是交叉的异步的，说明两个线程持有的锁不是同一把锁。
