java設計模式-單例設計模式的4種實現

（1）餓漢式(線程安全)

該方式沒有線程安全問題，當類被卸載時，靜態變量被摧毀，並釋放所佔有的內存，因此在某些特定條件下會耗費內存。

package cn.edu.bjfu.main;

public class MainTest {

	public static void main(String[] args) {
		Teacher t1 = Teacher.getInstance();
		Teacher t2 = Teacher.getInstance();
		System.out.println(t1==t2);
	}
}
class Teacher {
	//本類創建一個對象
	private static Teacher t = new Teacher();
	//不讓別人創建對象
	private Teacher() {}
	//使外界可以通過類名.	來調用
	public static Teacher getInstance( ) {
		return t;
	}
}

（2）懶漢式(線程不安全的)

package cn.edu.bjfu.main;

public class MainTest {

	public static void main(String[] args) {
		Teacher t1 = Teacher.getInstance();
		Teacher t2 = Teacher.getInstance();
		System.out.println(t1==t2);
	}
}
class Teacher {
	//本類創建一個對象
	private static Teacher t = null;
	//不讓別人創建對象
	private Teacher() {}
	//使外界可以通過類名.	來調用
	public static Teacher getInstance() {
		if(t == null) {
			t = new Teacher();
		}
		return t;
	}
}

因此在某些特定條件下會節約了內存。在多線程環境中，這種實現方法是完全錯誤的，根本不能保證單例的狀態。

（3）懶漢式(優化爲線程安全)

package cn.edu.bjfu.main;

public class MainTest {

	public static void main(String[] args) {
		Teacher t1 = Teacher.getInstance();
		Teacher t2 = Teacher.getInstance();
		System.out.println(t1==t2);
	}
}
class Teacher {
	//本類創建一個對象
	private static Teacher t = null;
	//不讓別人創建對象
	private Teacher() {}
	//使外界可以通過類名.	來調用
	public static synchronized Teacher getInstance() {
		if(t == null) {
			t = new Teacher();
		}
		return t;
	}
}

在多線程情形下，synchronized方法通常效率低。

（4）DCL雙檢鎖機制改進優化懶漢式

package cn.edu.bjfu.main;

public class MainTest {

	public static void main(String[] args) {
		Teacher t1 = Teacher.getInstance();
		Teacher t2 = Teacher.getInstance();
		System.out.println(t1==t2);
	}
}
class Teacher {
	//本類創建一個對象
	private static Teacher t = null;
	//不讓別人創建對象
	private Teacher() {}
	//使外界可以通過類名.	來調用
	public static Teacher getInstance() {
		if(t == null) {
			synchronized (Teacher.class) {
				//某個線程取到了類的鎖，實例化對象前第二次檢查t是否已經被實例化出來，如果沒有，new一個。
				if(t == null) {
					t = new Teacher();
				}
			}
		}
		return t;
	}
}