单例模式

#懒汉式，线程安全
public class Singleton{
    private static Singleton instance;
    private Singleton(){}
    public static synchronized Singleton getInstance(){
        if (instance == null){
            instance = new Singleton();
    }
    return instance;  
    }
}

#优点：第一次调用才初始化，避免内存浪费。
#缺点：必须加锁 synchronized 才能保证单例，但加锁会影响效率。

#饿汉式
public class Singleton{
    private static Singleton instance = new Singleton();
    private Singleton(){}
    public static Singleton getInstance(){
        return instance;
    }
}

#双检锁/双重校验锁
public class Singleton{
    private volatile static Singleton singleton;
    private Singleton(){}
    public static Singleton getSingleton(){
    if (singleton==null){
        synchronized (Singleton.class){
        if (single == null){
        singleton = new Singleton();
        }
        }
    }
    return singleton;
    }  
}