關鍵字synchronized擁有鎖重入的功能，也就是在使用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再次得到該物件的鎖的，這也證明在一個synchronized方法 / 塊內部呼叫本類的其他synchronized方法 / 塊時，是永遠可以得到鎖的。

“可重入鎖”的概念是：自己可以再次獲取自己的內部鎖。

Demo程式碼如下：

public class DemoServiceReentrenceLock {
	synchronized public void service1() {
		try {
			System.out.println(Thread.currentThread().getName()+ " - " +"service1 獲取鎖" + " - " + System.nanoTime());
			System.out.println(Thread.currentThread().getName()+ " - " +"service1" + " - " + System.currentTimeMillis());
			Thread.sleep(5000);
			service2();
			System.out.println(Thread.currentThread().getName()+ " - " +"service1 釋放鎖" + " - " + System.nanoTime());
			
		} catch (Exception e) {
			// TODO: handle exception
		}
	}
	synchronized public void service2() {
		try {
			System.out.println(Thread.currentThread().getName()+ " - " +"service2 獲取鎖" + " - " + System.nanoTime());
			System.out.println(Thread.currentThread().getName()+ " - " +"service2" + " - " + System.currentTimeMillis());
//			Thread.sleep(3000);
			service3();
			System.out.println(Thread.currentThread().getName()+ " - " +"service2 釋放鎖" + " - " + System.nanoTime());
		} catch (Exception e) {
			// TODO: handle exception
		}
	}
	synchronized public void service3() {
		System.out.println(Thread.currentThread().getName()+ " - " +"service3 獲取鎖" + " - " + System.nanoTime());
		System.out.println(Thread.currentThread().getName()+ " - " +"service3" + " - " + System.currentTimeMillis());
		System.out.println(Thread.currentThread().getName()+ " - " +"service3釋放鎖" + " - " + System.nanoTime());
	}
}
 

public class TestDemoServiceReentrenceLock extends Thread{
	private DemoServiceReentrenceLock service;
	public TestDemoServiceReentrenceLock(DemoServiceReentrenceLock dLock) {
		this.service = dLock;
	}
	@Override
	public void run() {
		// TODO Auto-generated method stub
		super.run();
		/*
		 * 若在此處例項化物件，建立新執行緒時會建立新物件，每個執行緒操作的是不同的物件
		 */
//		DemoServiceReentrenceLock service = new DemoServiceReentrenceLock(); 
		service.service1();
	}
	
	public static void main(String[] args) {
		DemoServiceReentrenceLock demoServiceReentrenceLock = new DemoServiceReentrenceLock();
		TestDemoServiceReentrenceLock demo = new TestDemoServiceReentrenceLock(demoServiceReentrenceLock);
		
//		demo.start();
		Thread t1 = new Thread(demo);
		Thread t2 = new Thread(demo);
		Thread t3 = new Thread(demo);
		
		t1.start();
		t2.start();
		t3.start();
	}
}
 

//執行結果：
Thread-1 - service1 獲取鎖 - 463506790016015
Thread-1 - service1 - 1540134076620
Thread-1 - service2 獲取鎖 - 463511789963967
Thread-1 - service2 - 1540134081620
Thread-1 - service3 獲取鎖 - 463511790385900
Thread-1 - service3 - 1540134081620
Thread-1 - service3釋放鎖 - 463511790751518
Thread-1 - service2 釋放鎖 - 463511790895718
Thread-1 - service1 釋放鎖 - 463511791731904
Thread-3 - service1 獲取鎖 - 463511792423465
Thread-3 - service1 - 1540134081622
Thread-3 - service2 獲取鎖 - 463516794723832
Thread-3 - service2 - 1540134086625
Thread-3 - service3 獲取鎖 - 463516794978527
Thread-3 - service3 - 1540134086625
Thread-3 - service3釋放鎖 - 463516795099263
Thread-3 - service2 釋放鎖 - 463516795154724
Thread-3 - service1 釋放鎖 - 463516795207199
Thread-2 - service1 獲取鎖 - 463516796485370
Thread-2 - service1 - 1540134086627
Thread-2 - service2 獲取鎖 - 463521795939716
Thread-2 - service2 - 1540134091627
Thread-2 - service3 獲取鎖 - 463521796082209
Thread-2 - service3 - 1540134091627
Thread-2 - service3釋放鎖 - 463521796205930
Thread-2 - service2 釋放鎖 - 463521796272484
Thread-2 - service1 釋放鎖 - 463521796344157