一：餓漢式

public class MyObject {

	// 立即載入方式==餓漢模式
	private static MyObject myObject = new MyObject();

	private MyObject() {
	}

	public static MyObject getInstance() {
		// 此程式碼版本為立即載入
		// 此版本程式碼的缺點是不能有其它例項變數
		// 因為getInstance()方法沒有同步
		// 所以有可能出現非執行緒安全問題
		return myObject;
	}
}

public class MyThread extends Thread {

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

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

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

	}

多執行緒獲取單例，列印的hashcode是同一個值，說明物件是同一個 

二：懶漢式（1）

public class MyObject {

	private static MyObject myObject;

	private MyObject() {
	}

	// 設定同步方法效率太低了
	// 整個方法被上鎖
	synchronized 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);
				myObject = new MyObject();
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		return myObject;
	}

}
 

public class MyThread extends Thread {

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

}

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

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

	}
 

二：懶漢式（2）

public class MyThread extends Thread {

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

}

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

}

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

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

	}

三：使用靜態內建類實現單例模式

public class MyObject {

	// 內部類方式
	private static class MyObjectHandler {
		private static MyObject myObject = new MyObject();
	}

	private MyObject() {
	}

	public static MyObject getInstance() {
		return MyObjectHandler.myObject;
	}

}

public class MyThread extends Thread {

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

}

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

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

	}

四：使用靜態程式碼塊實現單例模式

public class MyObject {

	private static MyObject instance = null;

	private MyObject() {
	}

	static {
		instance = new MyObject();
	}

	public static MyObject getInstance() {
		return instance;
	}

}

public class MyThread extends Thread {

	@Override
	public void run() {
		for (int i = 0; i < 5; i++) {
			System.out.println(MyObject.getInstance().hashCode());
		}
	}
}

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

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

	}

五：使用列舉實現單例模式

public enum MyObject {
	connectionFactory;

	private Connection connection;

	private MyObject() {
		try {
			System.out.println("呼叫了MyObject的構造");
			String url = "jdbc:sqlserver://localhost:1079;databaseName=ghydb";
			String username = "sa";
			String password = "";
			String driverName = "com.microsoft.sqlserver.jdbc.SQLServerDriver";
			Class.forName(driverName);
			connection = DriverManager.getConnection(url, username, password);
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		} catch (SQLException e) {
			e.printStackTrace();
		}
	}

	public Connection getConnection() {
		return connection;
	}
}

public class MyThread extends Thread {

	@Override
	public void run() {
		for (int i = 0; i < 5; i++) {
			System.out.println(MyObject.connectionFactory.getConnection()
					.hashCode());
		}
	}
}

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

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

	}

六：序列化與反序列化的單例模式實現

public class MyObject implements Serializable {

	private static final long serialVersionUID = 888L;

	// 內部類方式
	private static class MyObjectHandler {
		private static final MyObject myObject = new MyObject();
	}

	private MyObject() {
	}

	public static MyObject getInstance() {
		return MyObjectHandler.myObject;
	}

	protected Object readResolve() throws ObjectStreamException {
		System.out.println("呼叫了readResolve方法！");
		return MyObjectHandler.myObject;
	}

}

public class SaveAndRead {

	public static void main(String[] args) {
		try {
			MyObject myObject = MyObject.getInstance();
			
			FileOutputStream fosRef = new FileOutputStream(new File(
					"myObjectFile.txt"));
			ObjectOutputStream oosRef = new ObjectOutputStream(fosRef);
			oosRef.writeObject(myObject);
			oosRef.close();
			fosRef.close();
			System.out.println(myObject.hashCode());
		} catch (FileNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

		try {
			FileInputStream fisRef = new FileInputStream(new File(
					"myObjectFile.txt"));
			ObjectInputStream iosRef = new ObjectInputStream(fisRef);
			MyObject myObject = (MyObject) iosRef.readObject();
			iosRef.close();
			fisRef.close();
			System.out.println(myObject.hashCode());
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		}

	}

}