關鍵字synchronized 擁有鎖重入的功能，也就是使用synchronized時，當一個執行緒得到一個物件鎖後，再次請求此物件鎖時是可以再次得到該物件的鎖的。這也就證明在一個synchronized方法/快的內部呼叫本類的其他的synchronized方法/快時，是永遠可以得到鎖的。

package extthread;

import myservice.Service;

public class MyThread extends Thread {
	@Override
	public void run() {
		Service service = new Service();
		service.service1();
	}

}
 

package myservice;

public class Service {

	synchronized public void service1() {
		System.out.println("service1");
		service2();
	}

	synchronized public void service2() {
		System.out.println("service2");
		service3();
	}

	synchronized public void service3() {
		System.out.println("service3");
	}

}
 

package test;

import extthread.MyThread;

public class Run {
	public static void main(String[] args) {
		MyThread t = new MyThread();
		t.start();
	}
}

package extthread;

import myservice.Main;
import myservice.Sub;

public class MyThread extends Thread {
	@Override
	public void run() {
		Sub sub = new Sub();
		sub.operateISubMethod();
	}

}
 

package myservice;

public class Main {

	public int i = 10;

	synchronized public void operateIMainMethod() {
		try {
			i--;
			System.out.println("main print i=" + i);
			Thread.sleep(100);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

}

package myservice;

public class Sub extends Main {

	synchronized public void operateISubMethod() {
		try {
			while (i > 0) {
				i--;
				System.out.println("sub print i=" + i);
				Thread.sleep(100);
				this.operateIMainMethod();
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

}

package test;

import extthread.MyThread;

public class Run {
	public static void main(String[] args) {
		MyThread t = new MyThread();
		t.start();
	}
}