1，單例模式

解法一：只適用于單線程：

package com.singleton;import sun.security.jca.GetInstance.Instance;public class Singleton1 {private static Singleton1 instance = null;public Singleton1() {}public static Singleton1 getInstance() {if (instance == null) {instance = new Singleton1();}return instance;}public static void main(String[] args) {// TODO Auto-generated method stub Singleton1 s1 = getInstance();Singleton1 s2 = getInstance();System.out.println(s1==s2);}}

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解法二：適用于多線程但效率不高----在getInstance()上加synchronized加鎖

package com.singleton;public class Singleton1 {private static Singleton1 instance = null;public Singleton1() {}public static synchronized Singleton1 getInstance() {if (instance == null) {instance = new Singleton1();}return instance;}public static void main(String[] args) {// TODO Auto-generated method stub Singleton1 s1 = getInstance();Singleton1 s2 = getInstance();System.out.println(s1==s2);}}

解法三：改進的多線程單例----不需要整個方法加鎖，只需要創建時加鎖

package com.singleton;public class Singleton1 {private static Object lock = new Object();private static Singleton1 instance = null;public Singleton1() {}public static Singleton1 getInstance() {if (instance == null) {synchronized (lock) {if (instance == null) {instance = new Singleton1();}}}return instance;}public static void main(String[] args) {// TODO Auto-generated method stub Singleton1 s1 = getInstance();Singleton1 s2 = getInstance();System.out.println(s1==s2);}}

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解法四（強烈推薦）：利用靜態構造函數

package com.singleton;public class Singleton2 {private static Singleton2 instance = new Singleton2();public static Singleton2 getInstance() {return instance;}public static void main(String[] args) {// TODO Auto-generated method stub Singleton2 s1 = getInstance();Singleton2 s2 = getInstance();System.out.println(s1 == s2);}}

原理：類成員變量只在類加載時初始化一次。

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解法五（強烈推薦）：

package com.singleton;public class Singleton3 {private static class SingletonHolder {private final static Singleton3 instance = new Singleton3();}public Singleton3() {}public static Singleton3 getInstance() {return SingletonHolder.instance;}public static void main(String[] args) {Singleton3 s1 = getInstance();Singleton3 s2 = getInstance();System.out.println(s1 == s2);}}

因為SingletonHolder內部類只有在顯式被調用時才會加載，所以只有在調用getInstance方法時才會實例化instance，這樣就解決了解法四加載過早的問題。

轉載于:https://www.cnblogs.com/IvySue/p/7529001.html

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