將synchronized(String)與String 混合使用時需要注意String的常量池特性帶來的影響

package extthread;

import service.Service;

public class ThreadA extends Thread {
	private Service service;
	public ThreadA(Service service) {
		super();
		this.service = service;
	}

	@Override
	public void run() {
		service.print("AA");
	}
}
 

package extthread;

import service.Service;

public class ThreadB extends Thread {
	private Service service;
	public ThreadB(Service service) {
		super();
		this.service = service;
	}

	@Override
	public void run() {
		service.print("AA");
	}
}

package service;

public class Service {
	public static void print(String stringParam) {
		try {
			synchronized (stringParam) {
				while (true) {
					System.out.println(Thread.currentThread().getName());
					Thread.sleep(1000);
				}
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}
 

package test;

import service.Service;
import extthread.ThreadA;
import extthread.ThreadB;

public class Run {

	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("B");
		b.start();

	}

}
 

執行緒B 根本沒有執行。因為持有同一個String鎖