【design patterns】设计模式

/**
 * 懒汉式：初始化的时候就会构造一个实例，消耗内存,但是不用考虑多线程的问题
 * @author 
 */
public class Singleton {
    private Singleton(){

    }
    private static Singleton singleton = new Singleton();
    public static Singleton getInstance(){
        return singleton;
    }
}

/**
 * 饿汉式：延迟初始化这个单例实例   ，要考虑多线程同时请求初始化的问题
 * 延迟初始化原因：①静态初始化时，没有足够的信息对单例进行初始化
 *              ②与获取资源有关，例如数据库连接，尤其在一个特定会话中，它包含的应用程序并不需要该对象实例时
 *
 */
public class SingletonOfHungry {
    private static Object classLock = SingletonOfHungry.class;

    private SingletonOfHungry() {
    }
    private static SingletonOfHungry singletonOfHungry = null;
    public static SingletonOfHungry getInstance(){
        synchronized (classLock){//同步
            if (singletonOfHungry == null){
                singletonOfHungry = new SingletonOfHungry();
            }
            return singletonOfHungry;
        }
    }
}

/**
 * 未初始化饿汉式单例工厂
 */
public class Factory {
    private long num;
    private static Object classLock = Factory.class;

    private Factory(){
        num = 0;
    }
    private static Factory factory;
    public static Factory getFactory(){
        synchronized (classLock){
            if (factory==null){
                factory = new Factory();
            }
            return factory;
        }
    }

    public void recordNum(){
        synchronized (classLock){
            num++;
        }
    }
}

public static Factory getFactory(){
        if(factory==null){     //当实例化之后就不需要再加锁
            synchronized (classLock){
                if (factory==null){    //防止在加锁后，被其他线程实例化，此时也不需要实例化
                    factory = new Factory();
                }
            }
        }
        return factory;
    }