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。