单例模式

public class Singleton {

    private volatile static Singleton instance = null;

    

    private Singleton () {

    }

    

    public static Singleton getSingleton() {

        if(instance == null) {

            synchronized(Singleton.class) {

                if(instance == null) {

                    instance = new Singleton();

                }

            }

        }

        return instance;

    }

}


//jdk1.5前

public class Singleton {

    private static class SingletonInstance {

        private static final Singleton instance = new Singleton();

    }

    

    private Singleton {

    }

    

    public static Singleton getSingleton() {

        return SingletonInstance.instance;

    }

}