Java设计模式 - 命令模式

命令模式的由来

在进行小规模开发时,直接通过声明某个类的实例,通过调用这个类的方法,以完成功能.

而在小规模开发转向大规模开发的时候,各种功能互相调用, 凌乱不堪.

此时, 通过将程序体分层, 规定某层的方法不能直接调用另外某些层的方法.这样能够显著减少程序体之间方法调用的复杂度, 更加有利于管理.

而命令模式,就是将调用类(Invoker)下发的命令(Command), 抽象成一个接口, 通过这个接口的实现类去调用被调用类(Receiver)的具体方法.

下图示: LogoutService.java需要调用UserDataManagement.java中的deleteUser()以及UserSessionManagement.java中的deleteSession()两个方法已达到注销当前用户的目的。

使用命令模式之后，将Command抽象成为一个接口userDao，LogoutService.java通过调用userDao的实现类，即可调用UserDataManagement.java中的deleteUser()以及UserSessionManagement.java中的deleteSession()两个方法。

Java代码实现

包的结构

- LogoutService.java

调用类，在命令模式中被称为Invoker。

package services;

import Dao.userDao;
import Dao.userDataDao;
import Dao.userSessionDao;

/**
 * This is service which impl user to logout
 * 1.delete from session
 * 2.delete from database
 * 3.return to login page
 * Created by yl28537 on 6/09/2018.
 */
public class LogoutService {

    private userDao userSessionDao = new userSessionDao();
    private userDao userDataDao = new userDataDao();

     public void logout(){
         //1. delete from session
         userSessionDao.delete();
         //2. delete from database
         userDataDao.delete();
         //3. return to login page
         System.out.println("return to login page");

     }

}

- userDao.java

Command接口，拥有方法的声明。

package Dao;

/**
 * Created by yl28537 on 6/09/2018.
 */
public interface userDao {

    public void create();

    public void delete();
}

- userDataDao.java

package Dao;

import utils.UserDataManagment;

/**
 * Created by yl28537 on 6/09/2018.
 */
public class userDataDao implements userDao {

    private UserDataManagment userDataManagment = new UserDataManagment();

    @Override
    public void create() {
        userDataManagment.createUser();
    }

    @Override
    public void delete() {
        userDataManagment.deleteUser();
    }
}

- userSessionDao.java

package Dao;

import utils.UserSessionManagment;

/**
 * Created by yl28537 on 6/09/2018.
 */
public class userSessionDao implements userDao {

    private UserSessionManagment userSessionManagment =  new UserSessionManagment();

    @Override
    public void create() {
        userSessionManagment.createSession();
    }

    @Override
    public void delete() {
        userSessionManagment.deleteSession();
    }
}

- UserDataManagment.java

package utils;

/**
 * This is a util class that will have all kinds of function to manage database
 */
public class UserDataManagment {

    public void createUser(){
        System.out.println("User created!");
    }

    public void deleteUser(){
        System.out.println("User deleted!");
    }

    public void selectUser(){
        System.out.println("User selected!");
    }
}

- UserSessionManagement.java

package utils;

/**
 * This is a util class that will have all kinds of method to manage session
 */
public class UserSessionManagment {

    public void createSession(){
        System.out.println("Session created!");
    }

    public void deleteSession(){
        System.out.println("Session deleted!");
    }

    public void printSession(){
        System.out.println("Print Session!");
    }
}

命令模式其他内容

宏命令

对于Command接口有多种实现。在Invoker中声明不同的command对象以达到宏命令的目的。