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一、概述

       单例模式（Singleton），保证一个类仅有一个实例，并提供一个访问它的全局访问点 。

二、UML图示

三、代码实现

1、singleton类

public class Singleton {    private static Singleton instance;    private Singleton() {    }    public static Singleton getInstance(){        if(null == instance){            instance = new Singleton();        }        return instance;    }}

2、测试

public class Test {    public static void main(String[] args) {        Singleton singleton = Singleton.getInstance();        Singleton singleton1 = Singleton.getInstance();        System.out.println(singleton == singleton1);    }}输出：true

3、多线程下的singleton类

public class Singleton {    private static Singleton instance;    //进程辅助对象    public static Object syncRoot = new Object();    private Singleton() {    }    public static Singleton getInstance(){        if(null == instance){            synchronized (syncRoot){                if(null == instance){                    instance = new Singleton();                }            }        }        return instance;    }}

4.单例模式的多种写法

/** * 1.饿汉式：线程安全，耗费资源 */class HugerSingleton1 {    //该对象的引用不可修改    private static final HugerSingleton1 ourInstance = new HugerSingleton1();    public static HugerSingleton1 getInstance() {        return ourInstance;    }    private HugerSingleton1() {}}/** * 2.饿汉式：在静态代码块实例对象 */class HugerSingleton2 {    private static HugerSingleton2 ourInstance;    static {        ourInstance = new HugerSingleton2();    }    public static HugerSingleton2 getInstance() {        return ourInstance;    }    private HugerSingleton2() {}}/** * 3.懒汉式：非线程安全 */class Singleton1 {    private static Singleton1 ourInstance;    public static Singleton1 getInstance() {        if (null == ourInstance) {            ourInstance = new Singleton1();        }        return ourInstance;    }    private Singleton1() {}}/** * 4.线程安全的懒汉式：给方法加锁，每次都枷锁，耗时 */class Singleton2 {    private static Singleton2 ourInstance;    public synchronized static Singleton2 getInstance() {        if (null == ourInstance) {            ourInstance = new Singleton2();        }        return ourInstance;    }    private Singleton2() {}}/** * 5.线程安全的懒汉式：双重检查锁（同步代码块），代码较复杂 */class Singleton3 {    private static Singleton3 ourInstance;    public static Singleton3 getInstance() {        if (null == ourInstance) {            synchronized (Singleton3.class) {                if (null == ourInstance) {                    ourInstance = new Singleton3();                }            }        }        return ourInstance;    }    private Singleton3() {}}/** * 6.线程安全的懒汉式：静态内部类（第一推荐） */class Singleton4 {    static class SingletonHolder {        private static Singleton4 ourInstance = new Singleton4();    }    public static Singleton4 getInstance() {        return SingletonHolder.ourInstance;    }    private Singleton4() {    }}

注：参考文献《大话设计模式》程杰著。