在JVM中有string常量池缓存的功能。 

package com.leran.thread.demo1;

public class Test {

public static void main(String[] args) {

String a = "a";

String b = "a";

System.out.println(a == b);

}

}

结果:true;

for  example:

 

public class Service {

public static void print(String stringParam) {

try {

synchronized (stringParam) {

while (true) {

System.out.println(Thread.currentThread().getName());

Thread.sleep(1000);

}

}

 

} catch (Exception e) {

e.printStackTrace();

}

 

}

 

}

package com.leran.thread.demo1;

public class ThreadA extends Thread {

private Service service;

public ThreadA(Service service){

super();

this.service=service;

}

@Override

public void run() {

service.print("AA");

}

}

 

 

package com.leran.thread.demo1;

public class ThreadB extends Thread {

private Service service;

public ThreadB(Service service){

super();

this.service=service;

}

@Override

public void run() {

service.print("AA");

}

}

 

package com.leran.thread.demo1;

public class Test {

public static void main(String[] args) {

Service service = new Service();

ThreadA a = new ThreadA(service);

a.setName("A");

a.start();

ThreadB b = new ThreadB(service);

b.setName("A");

b.start();

}

}

 

输出结果是:　　A A A A A.....

出现这样的结果就是因为String的两个值都是AA,两个线程持有相同的锁，所以造成线程B不能执行。这就是String常量池所带来的问题。因此

在大多数情况下，同步synchronized代码块都不使用String作为锁对象，而改用其他，比如new Object()实例化Object对象，但它不放入缓存池中。