Singleton 模式详解

 Singleton 模式详解

下面是我们常见的Singleton的写法

 package example.designmodel.singleton;public class Singleton1 {        private static Singleton1 instance;    public Singleton1(){            }    public static Singleton1 getinstance(){        if(instance == null){            instance = new Singleton1();        }            return instance;            }}

//problem ... 多个地方去调用getinstance的时候会出问题。还是会多创建instance。于是有个Singleton2

public class Singleton2 {        private static Singleton2 instance;    public Singleton2(){            }    public static synchronized Singleton2 getinstance(){        if(instance == null){            instance = new Singleton2();        }            return instance;            }}

//problem ... 一次只有一个可以调用getinstance，性能不是特别好，其实只有第一次需要同步，后面的调用都同步是一种累赘。于是有个Singleton3

public class Singleton3 {        private static Singleton3 instance = new Singleton3();    public Singleton3(){            }    public static  Singleton3 getinstance(){            return instance;    }}

//problem ... eagerly create急切创建实例 .如果您仍然关心性能，可使用Singleton4

package example.designmodel.singleton;public class Singleton4 {        private volatile static Singleton4 instance;    public Singleton4(){            }    public static  Singleton4 getinstance(){        if(instance == null){            synchronized(Singleton4.class){                if(instance == null){                    instance = new Singleton4();                }            }        }            return instance;    }}

//problem ... 双重加锁 JDK5以后适用。这里使用volatile标识，双重加锁