package extthread;

import test.MyObject;

public class MyThread extends Thread {

	@Override
	public void run() {
		System.out.println(MyObject.getInstance().hashCode());
	}

}

package test;

public class MyObject {

	private volatile static MyObject myObject;

	private MyObject() {
	}

	// 使用雙檢測機制來解決問題
	// 即保證了不需要同步程式碼的非同步
	// 又保證了單例的效果
	public static MyObject getInstance() {
		try {
			if (myObject != null) {
			} else {
				// 模擬在建立物件之前做一些準備性的工作
				Thread.sleep(3000);
				synchronized (MyObject.class) {
					if (myObject == null) {
						myObject = new MyObject();
					}
				}
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		return myObject;
	}
	// 此版本的程式碼稱為：
	// 雙重檢查Double-Check Locking

}
 

package test.run;

import extthread.MyThread;

public class Run {

	public static void main(String[] args) {
		
		
		MyThread t1 = new MyThread();
		MyThread t2 = new MyThread();
		MyThread t3 = new MyThread();

		t1.start();
		t2.start();
		t3.start();

	}

}

單例設計模式DCL雙檢查鎖機制

執行結果：
322722178
322722178
322722178