8000 单例模式 · Folgerjun/DesignPatterns-Java@c97cdca · GitHub
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单例模式
单例模式
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‎java-des/src/com/java/design/singleton/SingletonA.java

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package com.java.design.singleton;
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/**
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* 单例模式的实现：饿汉式,线程安全 但效率比较低
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*/
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public class SingletonA {
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private SingletonA() {
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}
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private static final SingletonA instance = new SingletonA();
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public static SingletonA getInstance() {
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return instance;
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}
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}

‎java-des/src/com/java/design/singleton/SingletonB.java

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package com.java.design.singleton;
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/**
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* 单例模式的实现：饱汉式,非线程安全
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*/
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public class SingletonB {
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// 防止实例化
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private SingletonB() {
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}
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// 定义一个SingletonTest类型的变量（不初始化，注意这里没有使用final关键字）
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private static SingletonB instance;
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// 定义一个静态的方法（调用时再初始化SingletonTest，但是多线程访问时，可能造成重复初始化问题）
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// 加上 synchronized 保证线程安全
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public static synchronized SingletonB getInstance() {
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if (instance == null) {
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instance = new SingletonB();
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}
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return instance;
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}
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}

‎java-des/src/com/java/design/singleton/SingletonC.java

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package com.java.design.singleton;
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/**
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* 单例模式最优方案 线程安全 并且效率高
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*/
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public class SingletonC {
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private SingletonC() {
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}
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// 定义一个静态私有变量(不初始化，不使用final关键字，使用volatile保证了多线程访问时instance变量的可见性，避免了instance初始化时其他变量属性还没赋值完时，被另外线程调用)
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private static volatile SingletonC instance;
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public static SingletonC getInstance() {
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if (instance == null) {
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// 同步代码块（对象未初始化时，使用同步代码块，保证多线程访问时对象在第一次创建后，不再重复被创建）
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synchronized (SingletonC.class) {
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if (instance == null) {
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instance = new SingletonC();
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}
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}
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}
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return instance;
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}
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}

‎java-des/src/com/java/design/singleton/SingletonD.java

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package com.java.design.singleton;
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/**
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* 静态内部类
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*
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* @author Administrator
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*
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*/
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public class SingletonD {
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private static class SingletonHolder {
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private static final SingletonD INSTANCE = new SingletonD();
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}
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private SingletonD() {
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}
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public static SingletonD getInstance() {
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return SingletonHolder.INSTANCE;
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}
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}

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