Java的單例模式實現

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    只能產生一個執行個體的類是實現了Singleton(單例)模式的類。以下為C#實現單例模式的方式

方式一隻使用於單線程環境

// 把建構函式設為私人函數以禁止他人建立執行個體// 定義一個靜態執行個體，在需要的時候建立該執行個體// 在Singleton的靜態屬性Instance中，只有在instance為null的時候才建立一個執行個體以避免重複建立// 把建構函式定義為私人函數public final class Singleton1 {private Singleton1() {}private static Singleton1 _instance = null;public static Singleton1 getInstance() {if (_instance == null)_instance = new Singleton1();return _instance;}public static void main(String[] args) {Singleton1 s1 = Singleton1.getInstance();Singleton1 s2 = Singleton1.getInstance();System.out.println(s1.hashCode());System.out.println(s2.hashCode());}}

方式二 加同步鎖

import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public final class Singleton2 {private Singleton2(){}static Lock lock = new ReentrantLock();private static Singleton2 _instance = null;public static Singleton2 getInstance(){lock.lock();try {if(_instance == null)_instance = new Singleton2();} finally {lock.unlock();}return _instance;}public static void main(String[] args) {Singleton2 s1 = Singleton2.getInstance();Singleton2 s2 = Singleton2.getInstance();System.out.println(s1.hashCode());System.out.println(s2.hashCode());}}

可行的解法 加同步鎖前後兩次判斷執行個體是否已存在

import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;public final class Singleton3 {private Singleton3(){}static Lock lock = new ReentrantLock();private static Singleton3 _instance = null;public static Singleton3 getInstance(){if(_instance == null){lock.lock();try{if(_instance == null)_instance = new Singleton3();}finally {lock.unlock();}}return _instance;}public static void main(String[] args) {Singleton3 s1 = Singleton3.getInstance();Singleton3 s2 = Singleton3.getInstance();System.out.println(s1.hashCode());System.out.println(s2.hashCode());}}

推薦的解法一利用靜態建構函式

public final class Singleton4 {private Singleton4(){}private static Singleton4 _instance = new Singleton4();public static Singleton4 getInstance(){return _instance;}public static void main(String[] args) {Singleton4 s1 = Singleton4.getInstance();Singleton4 s2 = Singleton4.getInstance();System.out.println(s1.hashCode());System.out.println(s2.hashCode());}}

推薦的解法二 實現按需建立執行個體

public class Singleton5 {private Singleton5(){}public static Singleton5 getInstance(){return Nested.instance;}private static class Nested{private Nested(){}public static final Singleton5 instance = new Singleton5();}public static void main(String[] args) {Singleton5 s1 = Singleton5.getInstance();Singleton5 s2 = Singleton5.getInstance();System.out.println(s1.hashCode());System.out.println(s2.hashCode());}}

1. 擴充 

    定義一個表示總統的類型President可以從該類型繼承出FrenchPresident和AmericanPresident等類型。這些衍生類別型都只能產生一個執行個體        

public class President {private String _name;public President(){}public String getName(){return _name;}public void setName(String name){_name = name;}public static void main(String[] args) {}}public class FrenchPresident extends President{private FrenchPresident(){}public static FrenchPresident getInstance(){return Nested.instance;}private static class Nested{private Nested(){}public static final FrenchPresident instance = new FrenchPresident();}public static void main(String[] args) {FrenchPresident s1 = FrenchPresident.getInstance();FrenchPresident s2 = FrenchPresident.getInstance();System.out.println(s1.hashCode());System.out.println(s2.hashCode());}}public class AmericanPresident {private AmericanPresident() {}public static AmericanPresident getInstance() {return Nested.instance;}private static class Nested {private Nested() {}public static final AmericanPresident instance = new AmericanPresident();}public static void main(String[] args) {AmericanPresident s1 = AmericanPresident.getInstance();AmericanPresident s2 = AmericanPresident.getInstance();System.out.println(s1.hashCode());System.out.println(s2.hashCode());}}

本文出自 “許大樹” 部落格，請務必保留此出處http://abelxu.blog.51cto.com/9909959/1965630

Java的單例模式實現