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一、懒汉式单例

 

在第一次调用的时候实例化自己，Singleton的唯一实例只能通过getInstance()方法访问、线程不安全

/** * Created by Admin on 2017/3/19. */public class Singleton {    private static Singleton instance;    private Singleton(){}    public static Singleton getInstance() {        if(instance==null){            instance=new Singleton();        }        return instance;    }}

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解决方案：

 

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1. 在getInstance方法上加同步

/** * Created by Admin on 2017/3/19. */public class Singleton {    private static Singleton instance;    private Singleton(){}    private static synchronized Singleton getInstance() {        if(instance==null){            instance=new Singleton();        }        return instance;    }}

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  2.双重检查锁定

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

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3.3、静态内部类

这实现了线程安全，又避免了同步带来的性能影响

/** * Created by Admin on 2017/3/19. */public class Singleton {    private  Singleton(){}    private static class singleLazy{        private static final Singleton instance=new Singleton();    }    private static final Singleton getInstance() {                return singleLazy.instance;    }}

 

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二、饿汉式单例

饿汉式单例类.在类初始化时，已经自行实例化

/** * Created by Admin on 2017/3/19. */public class Singleton {    private  Singleton(){}        private static final Singleton instance=new Singleton();    private static Singleton getInstance() {                return instance;    }}