单例

发现博客园可以很好的设置自己的博客文章的展示，很开心，然后特此发一篇

其实这几种写法大家应该都会的，就权当拿来记录一下吧，以后复习巩固也比较方便。 
这篇文章中的代码，来自一篇视频（我想找视频贴上，但我没找到，以后有机会看到再贴吧，实不好意思），上面有讲到，我觉得很好，择录了一些下来。

1.

//懒人写法|原始

//缺点：类加载的时候就初始化实例，会拖慢应用启动速度

public class PlainOldSingleton {

    private static PlainOldSingleton INSTANCE = new PlainOldSingleton();

    private PlainOldSingleton(){}

    public static PlainOldSingleton getInstance(){

        return INSTANCE;

    }

}

2.

//懒加载

//缺点：不是线程安全的，有可能线程A和线程B来同时访问INSTANCE的话，会new出两个实例，如果有多个线程的话，就会new出多个实例

public class LazyNotThreadSafe{

    private static LazyNotThreadSafe INSTANCE;

    private LazyNotThreadSafe(){}

    public static LazyNotThreadSafe getInstance(){

        if(INSTANCE == null){

            INSTANCE = new LazyNotThreadSafe();

        }

        return INSTANCE;

    }

}

3.

//同步锁

//缺点：每次获取都会加锁，会有一定的性能开销

//优点：线程安全

public class LazyThreadSafeSynchronized{

    private static LazyThreadSafeSynchronized INSTANCE;

    private LazyThreadSafeSynchronized(){}

    public static synchronized LazyThreadSafeSynchronized getInstance(){

        if(INSTANCE == null){

            INSTANCE = new LazyThreadSafeSynchronized();

        }

        return INSTANCE;

    }

}

4.

//双重验证

//面试多考，volatile是java1.5之后的一个内容，要理解这个需要懂Java虚拟机

public class LazyThreadSafeDoubleCheck{

    private static voltatile LazyThreadSafeDoubleCheck INSTANCE;

    private LazyThreadSafeDoubleCheck(){}

    public static LazyThreadSafeDoubleCheck getInstance(){

        if(INSTANCE == null){

            synchronized (LazyThreadSafeDoubleCheck.class){

                if(INSTANCE == null){

                    INSTANCE = new LazyThreadSafeDoubleCheck();

                }

            }

        }

        return INSTANCE;

    }

}

5.

//静态内部类

//优雅，实现了懒加载和线程安全，线程安全利用了虚拟机的机制

public class LazyThreadSafeStaticInnerClass{

    private static class Holder{

        private static LazyThreadSafeStaticInnerClass INSTANCE == new LazyThreadSafeStaticInnerClass();

    }

    private LazyThreadSafeStaticInnerClass(){}

    public static LazyThreadSafeStaticInnerClass getInstance(){

        return Holder.INSTANCE;

    }

}