java手写单例模式

1 懒汉模式

public class Singleton {

private Singleton singleton = null;

private Singleton() {

}

public Singleton getInstance() {

if (singleton == null) {

singleton = new Singleton();
       } else {

return singleton;

}

}
}

问题在于不是线程安全的。要线程安全，在getInstance前面加上synchronized关键字即可，但是开销太大。

2 饿汉模式

public class Singleton {

private Singleton singleton = new Singleton();

private Singleton() {

}

public Singletion getSingleton() {

return singleton;
    }

}

3 静态内部类

public class Singleton {

private static class SingletonHolder {

private static Singleton instance = new Singleton();

}

public static Singleton getInstance() {

return SingletonHolder.instance;
    }

}