单例模式

1、定义
保证一个类仅有一个实例，并提供一个访问它的全局访问点。

懒汉式

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

双重检查加锁

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

饿汉式单例

public class Apple{    private static Apple instance ＝ new Apple();    public static Apple getInstance(){            return instance;        }    }}