Java设计模式---单例模式

单例模式的几种实现方法,具体如下:

懒汉模式

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

优点
- 可以延迟加载

缺点
- 多线程不安全

饿汉模式

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

优点
- 多线程安全

缺点
- 加载类时就初始化完成,无法延时加载

双重检查

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

优点
- 多线程安全
- 延迟加载

缺点
- 同步耗时

静态内部类

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

优点
- 多线程安全
- 延迟加载
- 耗时短(与双重检查相比)

用缓存实现

public class Singleton {    private static final String KEY = "instance";    private static Map<String, Singleton> map = new HashMap<>();    private Singleton(){}    public static Singleton getInstance(){        Singleton singleton ;        if (map.get(KEY) == null){            singleton = new Singleton();            map.put(KEY, singleton);        } else {            singleton = map.get(KEY);        }        return singleton;    }}

优点
- 线程安全

缺点
- 占用内存较大

枚举模式

public enum Singleton {    instance;    public void operate(){}}

优点
- 简洁

缺点
- 占用内存大(Android官方不推荐使用枚举)

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