设计模式之：单例模式

实现单例应该注意以下4点：

1. 构造函数不对外开放，一般为private;
2. 通过一个静态方法或者枚举返回单例类对象；
3. 确保单例的对象有且仅有一个，尤其是在多线程环境下；
4. 䧁单例类对象在反序列化时不会被重新构建对象。

1. 饿汉单列模式

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

2. 懒汉单列模式

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

3. 懒汉单列模式（双重校验）

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

4. 静态内部类单列模式

public class Singleton {
    private Singleton() {};
    public static Singleton getInstance () {
        return SingletonHolder.instance;
    }
    private static class SingletonHolder {
        private static final Singleton instance = new Singleton();
    }
}

5. 枚举单列模式

public enum Singleton {
    INSTANCE;
}

6. 容器实现单列模式

public class Singleton {
    private Singleton() {};
    private static Map<String, Object> objects = new HashMap<String, Object>();
    public static void registe (String key, Object instance) {
        if (!objects.containsKey(key)) objects.put(key, instance);
    }
    public static Object get () {
        return objects.get(key);
    }
}

注：。。。