It turns out that the HTTP protocol is so simple! Understand HttpServletRequest in one article

1. Introduction to HttpServletRequest

The HttpServletRequest object represents the client's request. When the client accesses the server through the HTTP protocol, all the information in the HTTP request header is encapsulated in this object. Through the methods provided by this object, all the information requested by the client can be obtained.

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2. How to introduce js and css files into jsp pages

Create a new web project in eclipse, the directory structure is as follows:

At the beginning of the jsp page, get the root path of the project:

 <%
 String path = request.getContextPath();
 String basePath = request.getScheme() + "://"  
 + request.getServerName() + ":" + request.getServerPort()
 + path + "/" ;
 %>

In , insert the following code:

 <base href= "<%=basePath%>" />

The purpose of this code is to set the root path of the entire page to the project path.

3. Common Request Methods

1. Get client information

getRequestURL() returns the complete URL when the client makes the request. getRequestURI() returns the resource name part of the request line. getQueryString() returns the parameter part of the request line. getRemoteAddr() returns the IP address of the client making the request. getPathInfo() returns the additional path information in the request URL. The additional path information is the content in the request URL after the Servlet path and before the query parameters, which starts with "/". getRemoteHost() returns the full host name of the client making the request. getRemotePort() returns the network port number used by the client. getLocalAddr() returns the IP address of the WEB server. getLocalName() returns the host name of the WEB server.

 private void RequestMessages(HttpServletRequest req, HttpServletResponse resp) throws IOException{
 String reqUrl = req.getRequestURL().toString(); //Get the requested URL address
 String reqUri = req.getRequestURI(); //Get the requested resource
 String queryString = req.getQueryString(); //Get the parameters included in the requested URL address
 String remoteAddr = req.getRemoteAddr(); //Get the visitor's IP address
 String remoteHost = req.getRemoteHost();
 int remotePort = req.getRemotePort();
 String remoteUser = req.getRemoteUser();
 String method = req.getMethod(); //The method used to get the request URL address
 String pathInfo = req.getPathInfo();
 String localAddr = req.getLocalAddr(); //Get the IP address of the WEB server
 String localName = req.getLocalName(); //Get the host name of the WEB server
 resp.setCharacterEncoding( "UTF-8" ); //Set the characters to be output to the client browser in "UTF-8" encoding
 //Control the browser to display data in UTF-8 encoding by setting the response header. If this sentence is not added, the browser will display garbled characters
 resp.setHeader( "content-type" , "text/html;charset=UTF-8" );
 PrintWriter out = resp.getWriter();
 out .write( "The client information obtained is as follows: " );
 out .write( "<br/>" );
 out .write( "Requested URL: " + reqUrl);
 out .write( "<br/>" );
 out .write( "Requested resource: " + reqUri);
 out .write( "<br/>" );
 out .write( "Parameters included in the requested URL: " + queryString);
 out .write( "<br/>" );
 out .write( "Visitor's IP address: " + remoteAddr);
 out .write( "<br/>" );
 out .write( "Visitor's host name: " + remoteHost);
 out .write( "<br/>" );
 out .write( "Port number used: " + remotePort);
 out .write( "<br/>" );
 out .write( "remoteUser：" +remoteUser);
 out .write( "<br/>" );
 out .write( "Request method: " + method);
 out .write( "<br/>" );
 out .write( "pathInfo：" +pathInfo);
 out .write( "<br/>" );
 out .write( "localAddr：" +localAddr);
 out .write( "<br/>" );
 out .write( "localName：" +localName);
 }

2. Get the client request header

• getHeader(string name) method:String
• getHeaders(String name) method:Enumeration
• getHeaderNames() Method

 private void RequestHead(HttpServletRequest req, HttpServletResponse resp) throws IOException{
 resp.setCharacterEncoding( "UTF-8" ); //Set the characters to be output to the client browser in "UTF-8" encoding
 //Control the browser to display data in UTF-8 encoding by setting the response header
 resp.setHeader( "content-type" , "text/html;charset=UTF-8" );
 PrintWriter out = resp.getWriter();
 Enumeration<String> reqHeadInfos = req.getHeaderNames(); //Get all request headers
 out .write( "All the request header information obtained from the client is as follows: " );
 out .write( "<br/>" );
 while (reqHeadInfos.hasMoreElements()) {
 String headName = (String) reqHeadInfos.nextElement();
 String headValue = req.getHeader(headName); //Get the value of the corresponding request header according to the name of the request header
 out .write(headName+ ":" +headValue);
 out .write( "<br/>" );
 }
 out .write( "<br/>" );
 out .write( "The value of the client's Accept-Encoding request header obtained: " );
 out .write( "<br/>" );
 String value = req.getHeader( "Accept-Encoding" ); //Get the value corresponding to the Accept-Encoding request header
 out .write(value);
 Enumeration<String> e = req.getHeaders( "Accept-Encoding" );
 while (e.hasMoreElements()) {
 String string = (String) e.nextElement();
 System. out .println(string);
 }
 }

3. Get client request parameters

getParameter(String name) Gets the request parameter according to name (commonly used) getParameterValues(String name) Gets the request parameter list according to name (commonly used) getParameterMap() returns a Map type value, which records the mapping relationship between the request parameters and request parameter values ​​in the request submitted by the front end (such as jsp page). (Commonly used when writing frameworks)

 <%@ page language= "java" contentType= "text/html; charset=UTF-8"  
 pageEncoding= "UTF-8" %>
 <%
 String path = request.getContextPath();
 String basePath = request.getScheme() + "://"  
 + request.getServerName() + ":" + request.getServerPort()
 + path + "/" ;
 %>
 <html>
 <head>
 <base href= "<%=basePath%>" />
 <meta http-equiv= "Content-Type" content= "text/html;charset=UTF-8" >
 <title>Form Submission</title>
 <link href= "css/bootstrap.css" rel= "stylesheet" >
 <script src= "js/jquery-3.2.1.js" ></script>
 <script src= "js/bootstrap.js" ></script>
 </head>
 <body>
 <form class= "form-horizontal"   action = "<%=request.getContextPath()%>/GetParameterRequest.html" role= "form" method= "post" >
 <div class= "form-group" >
 <label for = "firstname" class= "col-sm-1 control-label" >name</label>
 <div class= "col-sm-3" >
 <input type= "text" class= "form-control"   name = "name"  
 placeholder= "Please enter your name" >
 </div>
 </div>
 <div class= "form-group" >
 <label for = "lastname" class= "col-sm-1 control-label" >Age</label>
 <div class= "col-sm-3" >
 <input type= "text" class= "form-control"   name = "age"  
 placeholder= "Please enter your age" >
 </div>
 </div>
 <div class= "form-group" >
 <label for = "lastname" class= "col-sm-1 control-label" >Gender</label>
 <div class= "col-sm-3" >
 <input type= "radio"   name = "sex" value= "male" checked>male
 <input type= "radio"   name = "sex" value = "female" >female
 </div>
 </div>
 <div class= "form-group" >
 <label for = "lastname" class= "col-sm-1 control-label" >Hobby</label>
 <div class= "col-sm-3" >
 <input type= "checkbox"   name = "aihao" value = "sing" >sing
 <input type= "checkbox"   name = "aihao" value = "Internet access" >Internet access
 <input type= "checkbox"   name = "aihao" value = "game" >game
 <input type= "checkbox"   name = "aihao" value = "reading" > reading
 </div>
 </div>
 <div class= "form-group" >
 <div class= "col-sm-offset-1 col-sm-3" >
 <button type= "submit" class= "btn btn-default" >Submit</button>
 <button type= "reset" class= "btn btn-default" >Reset</button>
 </div>
 </div>
 </form>
 </body>
 </html>

Use the getParameter method and getParameterValues ​​method to receive form parameters:

 public class GetParameterRequest extends HttpServlet{
 private static final long serialVersionUID = 3903946972744326948L;
 @Override
 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 this.doPost(req, resp);
 }
 @Override
 protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 //The client submits the form data in UTF-8 encoding, so the server needs to be set to receive it in UTF-8 encoding, otherwise garbled characters will be generated for Chinese data
 req.setCharacterEncoding( "UTF-8" );
 //Get the name
 String name = req.getParameter( "name" );
 //Get age
 String age = req.getParameter( "age" );
 //Get gender
 String sex = req.getParameter( "sex" );
 //Get the hobby. Because you can select multiple values, the value obtained is a string array, so you need to use the getParameterValues ​​method to obtain it.
 String[] aihaos = req.getParameterValues( "aihao" );
 String aihao = "" ;
 if(aihaos != null ){
 for ( int i = 0; i < aihaos.length; i++) {
 if(i == aihaos.length - 1){
 aihao += aihaos[i];
 } else {
 aihao += aihaos[i] + "," ;
 }
 }
 }
 System. out .println( "Name: " + name );
 System.out.println ( "Age:" + age);
 System.out.println ( "Gender:" + sex);
 System.out.println ( "Hobbies: " + aihao) ;
 req.setAttribute( "aihao" , aihao);
 //Set the server to output data to the client in UTF-8 encoding
 resp.setCharacterEncoding( "UTF-8" );
 this.getServletContext().getRequestDispatcher( "/request.jsp" ). forward (req, resp);
 }
 }

Response page:

 <%@ page language= "java" contentType= "text/html; charset=UTF-8"  
 pageEncoding= "UTF-8" %>
 <%
 String path = request.getContextPath();
 String basePath = request.getScheme() + "://"  
 + request.getServerName() + ":" + request.getServerPort()
 + path + "/" ;
 %>
 <html>
 <head>
 <base href= "<%=basePath%>" />
 <meta http-equiv= "Content-Type" content= "text/html;charset=UTF-8" >
 <title>Form Submission</title>
 <link href= "css/bootstrap.css" rel= "stylesheet" >
 <script src= "js/jquery-3.2.1.js" ></script>
 <script src= "js/bootstrap.js" ></script>
 </head>
 <body>
 < table class= "table" >
 <thead>
 <tr>
 <th>Name</th>
 <th>Results</th>
 </tr>
 </thead>
 <tbody>
 <tr>
 <td>Name</td>
 <td><%=request.getParameter( "name" ) %></td>
 </tr>
 <tr>
 <td>Age</td>
 <td><%=request.getParameter( "age" ) %></td>
 </tr>
 <tr>
 <td>Gender</td>
 <td><%=request.getParameter( "sex" ) %></td>
 </tr>
 <tr>
 <td>Hobbies</td>
 <td><%=request.getAttribute( "aihao" ) %></td>
 </tr>
 </tbody>
 </ table >
 </body>
 </html>

Submit the form below:

Background printing:

The results are as follows:

4. Request receiving form submission Chinese parameter garbled code problem

1. The garbled Chinese parameters in the form submitted via POST

There are the following forms:

 <%@ page language= "java" contentType= "text/html; charset=UTF-8"  
 pageEncoding= "UTF-8" %>
 <%
 String path = request.getContextPath();
 String basePath = request.getScheme() + "://"  
 + request.getServerName() + ":" + request.getServerPort()
 + path + "/" ;
 %>
 <html>
 <head>
 <base href= "<%=basePath%>" />
 <meta http-equiv= "Content-Type" content= "text/html;charset=UTF-8" >
 <title>Form Submission</title>
 <link href= "css/bootstrap.css" rel= "stylesheet" >
 <script src= "js/jquery-3.2.1.js" ></script>
 <script src= "js/bootstrap.js" ></script>
 </head>
 <body>
 <form class= "form-horizontal"   action = "<%=request.getContextPath()%>/PostRequest.html" role= "form" method= "post" >
 <div class= "form-group" >
 <label for = "firstname" class= "col-sm-1 control-label" >name</label>
 <div class= "col-sm-3" >
 <input type= "text" class= "form-control"   name = "name"  
 placeholder= "Please enter your name" >
 </div>
 </div>
 <div class= "form-group" >
 <div class= "col-sm-offset-1 col-sm-3" >
 <button type= "submit" class= "btn btn-default" >Submit</button>
 <button type= "reset" class= "btn btn-default" >Reset</button>
 </div>
 </div>
 </form>
 </body>
 </html>

Backend receiving parameters:

 public class PostRequest extends HttpServlet{
 private static final long serialVersionUID = 3903946972744326948L;
 @Override
 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 this.doPost(req, resp);
 }
 @Override
 protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 String name = req.getParameter( "name" );
 System. out .println( "Name: " + name );
 }
 }

Submit data:

Running results:

The reason why garbled characters are generated is that the encodings used in communication between the server and the client are inconsistent. Therefore, the solution is to set a unified encoding between the client and the server, and then transmit and receive data according to this encoding.

Since the client transmits form data to the server in UTF-8 character encoding, the server also needs to be set to receive it in UTF-8 character encoding. The encoding format is unified through the setCharacterEncoding method:

 public class PostRequest extends HttpServlet{
 private static final long serialVersionUID = 3903946972744326948L;
 @Override
 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 this.doPost(req, resp);
 }
 @Override
 protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 //Set the server to receive data in UTF-8 encoding
 req.setCharacterEncoding( "UTF-8" );
 String name = req.getParameter( "name" );
 System. out .println( "Name: " + name );
 }
 }

Resubmit the form to solve the Chinese garbled code problem:

2. The garbled Chinese parameters when submitting the form via GET

There are the following forms:

 <%@ page language= "java" contentType= "text/html; charset=UTF-8"  
 pageEncoding= "UTF-8" %>
 <%
 String path = request.getContextPath();
 String basePath = request.getScheme() + "://"  
 + request.getServerName() + ":" + request.getServerPort()
 + path + "/" ;
 %>
 <html>
 <head>
 <base href= "<%=basePath%>" />
 <meta http-equiv= "Content-Type" content= "text/html;charset=UTF-8" >
 <title>Form Submission</title>
 <link href= "css/bootstrap.css" rel= "stylesheet" >
 <script src= "js/jquery-3.2.1.js" ></script>
 <script src= "js/bootstrap.js" ></script>
 </head>
 <body>
 <form class= "form-horizontal"   action = "<%=request.getContextPath()%>/GetRequest.html" role= "form" method= "get" >
 <div class= "form-group" >
 <label for = "firstname" class= "col-sm-1 control-label" >name</label>
 <div class= "col-sm-3" >
 <input type= "text" class= "form-control"   name = "name"  
 placeholder= "Please enter your name" >
 </div>
 </div>
 <div class= "form-group" >
 <div class= "col-sm-offset-1 col-sm-3" >
 <button type= "submit" class= "btn btn-default" >Submit</button>
 <button type= "reset" class= "btn btn-default" >Reset</button>
 </div>
 </div>
 </form>
 </body>
 </html>

Backend receiving parameters:

 public class GetRequest extends HttpServlet{
 private static final long serialVersionUID = 3903946972744326948L;
 @Override
 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 this.doPost(req, resp);
 }
 @Override
 protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 String name = req.getParameter( "name" );
 System. out .println( "Name: " + name );
 }
 }

Submit data:

Running results:

The reason why garbled characters are generated is that for data transmitted in get mode, the default character encoding ISO8859-1 is used to receive data. The client transmits data to the server in UTF-8 encoding, while the request object on the server uses ISO8859-1 character encoding to receive data. The encodings used in communication between the server and the client are inconsistent, so garbled Chinese characters are generated.

Solution:

After receiving the data, first get the byte array of the original data received by the request object in ISO8859-1 character encoding, and then construct a string using the byte array in the specified encoding

 public class GetRequest extends HttpServlet{
 private static final long serialVersionUID = 3903946972744326948L;
 @Override
 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 this.doPost(req, resp);
 }
 @Override
 protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
 String name = req.getParameter( "name" );
 // Byte array of the raw data received in ISO8859-1 character encoding, and then construct a string with the specified encoding through the byte array
 name = new String( name .getBytes( "ISO8859-1" ) , "UTF-8" );
 System. out .println( "Name: " + name );
 }
 }