设计模式java----单例模式

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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;
    }
}