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package com.ha.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import java.io.IOException;
import java.io.PrintWriter;


public class RegisterServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 1. Get the data requested by the user 
        String username = req.getParameter("username");
        String password = req.getParameter("password");

        // 【 Set the response header 】
        // setStatus:  Set the status code in the status line 
        // resp.setStatus(200);
        // setContentType:  Set... In the response header  Content-Type, Set the data format of the response client 
        resp.setContentType("text/html");  //  Equivalent to ：resp.setHeader("Content-Type", "text/html");
        // setContentLength:  Set the data length of the response client , You don't usually need to set it , The client will read the data （ This is generally used for verification ）
        // resp.setContentLength(1024);  //  Equivalent to ：resp.setHeader("Content-Length", "1024");
        // setHeader:  Set some other information 
        resp.setHeader("Connection", "keep-alive");
        
        // 【 Set response body 】
        // setCharacterEncoding: Set the data encoding format of the response client 
        resp.setCharacterEncoding("utf-8");

        //  adopt resp Object to get the output stream 
        //  Byte stream （ If you want to respond to the file data to the client , You need to use byte stream ）
        // ServletOutputStream outputStream = resp.getOutputStream();
        //  Character stream （ If you want to respond to file data -HTML file , Character stream is used ）
        PrintWriter printWriter = resp.getWriter();
        //  Data written through the stream , It will be transmitted to the client browser in the form of response text , If the browser can recognize the data , It will directly show 
        printWriter.println("<!DOCTYPE html>");
        printWriter.println("<html>");
        printWriter.println("<head>");
        printWriter.println("<meta chaeset='utf-8'>");
        printWriter.println("<title> Login user name and password </title>");
        printWriter.println("</head>");
        printWriter.println("<body>");
        printWriter.println("<label> user name ：" + username + "<label>");
        printWriter.println("<label> password ：" + password + "<label>");
        printWriter.println("</body>");
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

This is the response header ：

Two 、 The present servlet There are several problems

1、 In the above Java Business logic work such as judgment may be done in the program , It is also used to print the display effect .

2、 When there is a better page to display at the back , Will revise this Java Code .

This is to call the business logic and display the result page in the same Servlet in , There will be design problems ：（1） Not in line with the principle of single function 、 The idea of performing their respective duties ;（2） It is not conducive to subsequent maintenance

Business logic should be separated from displaying results

3、 ... and 、 Separate two Servlet Problems after

problem ： After the business logic and display results are separated , How to jump to the Servlet?

How the data results obtained by the business logic are transferred to the display of the results Servlet?

Four 、 forward

4.1 effect

The role of forwarding is on the server side , Send the request to other resources on the server , To jointly complete the processing of a request .

4.2 Page Jump

When invoking business logic Servlet in , Write the following code ：

request.getRequestDispatcher("/ The goal is URL-pattern").forward(request, response);

4.3 Operation and code

Create a new one Java file

package com.ha.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import java.io.IOException;


public class AServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.getRequestDispatcher("/rs").forward(req, resp);
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

Add the content of the configuration file

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
  version="3.1"
  metadata-complete="true">

  <servlet>
    <servlet-name>rs</servlet-name>
    <servlet-class>com.ha.servlet.RegisterServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>rs</servlet-name>
    <url-pattern>/rs</url-pattern>
  </servlet-mapping>


  <servlet>
    <servlet-name>a</servlet-name>
    <servlet-class>com.ha.servlet.AServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>a</servlet-name>
    <url-pattern>/a</url-pattern>
  </servlet-mapping>

</web-app>

visit /a

Completed the page from AServlet.java Jump to RegisterServlet.java

In the eyes of the client, always access a. Use forward When jumping , Is to jump inside the server , Address bar does not change , Belong to the same request .

4.4 Data transfer

1、forward To express a request , Is to jump inside the server , Can share the same - Time request Data in scope

（1）request Scope ： Have space to store data , The scope of action is one Valid requests ( One This request can be forwarded many times )

Data can be stored in request after , Get anywhere in a request

Can transfer any data ( Basic data type 、 object 、 Array 、 Collection etc. )

（2） Save the data ：request.setAttribute(key, value);

Stored in... As key value pairs request Scope .key by String type , value by 0bject type

（3） Take the data ：request.gettribute(key);

adopt String Type of key visit 0bject Type of value

2、 Complete process

package com.ha.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import java.io.IOException;


public class AServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setAttribute("username", " Juan ");
        req.setAttribute("password", "123456");
        req.getRequestDispatcher("/rs").forward(req, resp);
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

package com.ha.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import java.io.IOException;
import java.io.PrintWriter;


public class RegisterServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = (String)req.getAttribute("username");
        String password = (String)req.getAttribute("password");

        resp.setContentType("text/html");
        resp.setHeader("Connection", "keep-alive");
        resp.setCharacterEncoding("utf-8");
        
        PrintWriter printWriter = resp.getWriter();
        printWriter.println("<!DOCTYPE html>");
        printWriter.println("<html>");
        printWriter.println("<head>");
        printWriter.println("<meta chaeset='utf-8'>");
        printWriter.println("<title> Login user name and password </title>");
        printWriter.println("</head>");
        printWriter.println("<body>");
        printWriter.println("<label> user name ：" + username + "<label>");
        printWriter.println("<label> password ：" + password + "<label>");
        printWriter.println("</body>");
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
  version="3.1"
  metadata-complete="true">

  <servlet>
    <servlet-name>rs</servlet-name>
    <servlet-class>com.ha.servlet.RegisterServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>rs</servlet-name>
    <url-pattern>/rs</url-pattern>
  </servlet-mapping>


  <servlet>
    <servlet-name>a</servlet-name>
    <servlet-class>com.ha.servlet.AServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>a</servlet-name>
    <url-pattern>/a</url-pattern>
  </servlet-mapping>

</web-app>

Running results ：

4.5 Forwarding features

Forwarding is server behavior

Forwarding is that the browser only makes one access request

Forwarding browser address unchanged

The information transmitted between two hops is not lost , So you can go through request Transfer data 、

Forwarding can only forward requests to the same Web Components in the application

5、 ... and 、 Redirect

5.1 effect

Redirection works on the client , After the client sends the request to the server , The server responds to the client with a New request address , The client resends a new request .

5.2 Page Jump

When invoking business logic Servlet in , Write the following code

response.sendRedirect(" The goal is URL");

URL： Uniform resource identifiers (Uniform Resource Identifier), It is used to indicate a location in the server A resource , Resources in web The path in the project (/project/source)

5.3 Code implementation

visit /a Swap to /rs

5.4 Data transfer

sendRedirect When jumping , Address bar changes , Resend request on behalf of client . It belongs to two requests

response No scope , two request The data in the request cannot be shared

To transfer data ： adopt UR For data transmission ("/WebProject/b?usemame=tom");

get data ：request.getParameter("usemame");

5.5 Code

5.6 Redirection features

Redirection is client behavior .

Redirection is that the browser has made at least two access requests .

Redirect browser address change .

The information transmitted between redirections will be lost (request Range ).

Redirection can point to any resource , Include other resources in the current application 、 Resources in other applications on the same site 、 Resources from other sites .