Preface : Learning objectives: streamline program logic and improve code readability. Content positioning: people who hope to write more elegant code through learning the delegation model.
1. Definition of delegation mode
- The basic function of the delegate pattern is to be responsible for task scheduling and task assignment. It is very similar to the agent pattern, which can be regarded as a full-power agent of a static agent under special circumstances, but the delegate pattern focuses on the process, while the delegate pattern focuses on result.
- Not one of the design patterns in GOF 23
- Behavioural model
2. Common application scenarios
- DispatcherServlet
- The class name ends with Delegate
- Including Dispatcher is generally a delegation mode
- BeanDefinitionParserDelegate in Spring Framework IOC: Spring provides a variety of ways (java, xml, annotations) to configure beans, this class is responsible for distribution processing, and the injected bean information will eventually be loaded as a BeanDefition object (description object information)
Application scenario : Solve multi-tasks, and do not want to let users know about these tasks or specific implementations (do not let users directly interact), create an intermediate class, realize full authority, and achieve the purpose of having only one entry for users.
Three, simple code case
Example : The boss needs to assign tasks to employees, but he can't directly let the boss understand each employee's good function. You can create an intermediate manager position to understand the staff's expertise and distribute tasks.
boss
public class Boss {
/**
* 发任务
* @param command
* @param leader
*/
public void command(String command,Leader leader){
leader.doing(command);
}
}
Manager
public class Leader {
// 预先知道每个员工的特长、特征，分发任务
private Map<String,IEmployee> register = new HashMap<String,IEmployee>();
public Leader(){
register.put("加密",new EmployeeA());
register.put("架构",new EmployeeB());
}
public void doing(String command){
// 交给指定的员工去做
register.get(command).doing(command);
}
}
Employee interface
public interface IEmployee {
/**
* 做事
* @param command
*/
void doing(String command);
}
Employee A
public class EmployeeA implements IEmployee {
@Override
public void doing(String command) {
System.out.println("我是员工A,我开始干活了，我擅长加密，执行");
}
}
Employee B
public class EmployeeB implements IEmployee {
@Override
public void doing(String command) {
System.out.println("我是员工B,我擅长架构，我开始干活了");
}
}
Test class
public class DelegateTest {
public static void main(String[] args) {
new Boss().command("架构",new Leader());
}
}
Four, SpringMvc's DispatcherServlet simple implementation
DispatcherServlet is an implementation of the front controller delegation model, providing a centralized access point for Spring Web MVC and responsible for the assignment of responsibilities
web.xml
<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd" >
<web-app>
<display-name>Archetype Created Web Application</display-name>
<servlet>
<servlet-name>delegateServlet</servlet-name>
<servlet-class>com.zc.pattern.delegate.mvc.DispatcherServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>delegateServlet</servlet-name>
<url-pattern>/*</url-pattern>
</servlet-mapping>
<!-- http://localhost:8080/getMemberById.do -->
</web-app>
DispatcherServlet
package com.zc.pattern.delegate.mvc;
import com.zc.pattern.delegate.mvc.controllers.MemberController;
import com.zc.pattern.delegate.mvc.controllers.OrderController;
import com.zc.pattern.delegate.mvc.controllers.SystemController;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
public class DispatcherServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// 完成调度
doDispach(req,resp);
}
private void doDispach(HttpServletRequest req, HttpServletResponse resp) throws IOException {
String uri = req.getRequestURI();
if ("getMemberById".equals(uri)){
String mid = req.getParameter("mid");
new MemberController().getMemberById(mid);
}else if ("getOrderById".equals(uri)){
String oid = req.getParameter("oid");
new OrderController().getOrderById(oid);
}else if ("getSystemById".equals(uri)){
new SystemController().logout();
}else {
resp.getWriter().write("404 not found");
}
}
}
MemberController
public class MemberController {
public void getMemberById(String mid){
System.out.println("获取员工id");
}
}
OrderController
public class OrderController {
public void getOrderById(String oid){
System.out.println("获取订单id");
}
}
SystemController
public class SystemController {
public void logout(){
System.out.println("退出");
}
}
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