[Js-设计模式]单例模式（饿汉，懒汉，登记式）

·单例模式的特点：

　　1. 单例类只能有一个实例。

　　2. 单例类必须自己创建自己的唯一实例。

　　3. 单例类必须给所有其他对象提供这一实例。

·各种单例模式的特点：

　　·懒汉式是延时加载,在需要的时候才创建对象,避免内存浪费，但存在线程安全问题。

　　·饿汉式线程安全，类一加载就实例化对象，所以要提前占用系统资源。

　　·登记式单例模式克服了饿汉以及懒汉单例的不可继承问题，其子类在登记时才被创建，并且子类的实例化方式只能是饿汉式。

·各种单例模式的类图：

　　·饿汉式单例模式

　　

　　·懒汉式单例模式：

　　

　　·登记式单例模式：

　　

· 各种单例模式的实现（Java）：

package com.neu.core;

/**
 * Only once instance of the class may be created during the execution of any
 * given program. Instances of this class should be acquired through the
 * getInstance() method. Notice that there are no public constructors for this
 * class.
 */
public class EagerSingleton {
    private static final EagerSingleton m_instance = new EagerSingleton();

    protected EagerSingleton() { }

    public static EagerSingleton getInstance() {
        return m_instance;
    }
}

=======================神奇的分割线==========================

package com.neu.core;

/**
 * Only once instance of the class may be created during the execution of any
 * given program. Instances of this class should be acquired through the
 * getInstance() method. Notice that there are no public constructors for this
 * class.
 */
public class LazySingleton {
    private static LazySingleton m_instance = null;

    private LazySingleton() { }

    synchronized public static LazySingleton getInstance() {
        if (m_instance == null) {
            m_instance = new LazySingleton();
        }
        return m_instance;
    }
}

=======================神奇的分割线==========================

package com.neu.core;

import java.util.HashMap;

public class RegSingleton {
    static private HashMap m_registry = new HashMap();

    static {
        RegSingleton x = new RegSingleton();
        m_registry.put(x.getClass().getName(), x);
    }

    protected RegSingleton() {
    }

    static public RegSingleton getInstance(String name) {
        if (name == null) {
            name = "com.neu.core.RegSingleton";
        }

        // System.out.println("From RegSingleton: requesting for " + name );

        if (m_registry.get(name) == null) {
            try {
                m_registry.put(name, Class.forName(name).newInstance());
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return (RegSingleton) (m_registry.get(name));
    }

    public String about() {
        return "Hello, I am RegSingleton.";
    }

}

package com.neu.core;

/**
 * This class is a subclass of RegSingleton
 */
public class RegSingletonChild extends RegSingleton {
    public RegSingletonChild() {
    }

    static public RegSingletonChild getInstance() {
        return (RegSingletonChild) RegSingleton.getInstance("com.javapatterns.singleton.demos.RegSingletonChild");
    }

    public String about() {
        return "Hello, I am RegSingletonChild.";
    }

}