Web service connector: Servlet

summary

Servlet（Server Applet） yes Java Servlet For short , Called server applet or service connector . It runs on the server Java Applet , Platform - and protocol-independent , The main function is to interactively browse and generate data （ That is to realize the interaction between the front and back ends ）, Generate dynamic Web Content .

Working principle diagram

among ,Tomcat Can pass HTTP Provide HTML Request access to static content such as pages ; It has also been realized. Servlet standard , Can run Servlet Program ; At the same time through Servlet Container call Servlet Processing dynamic requests , therefore Tomcat It can be Web Server or Servlet Container or Web Lightweight application servers .

Workflow

1） client （ It's usually a browser ） visit Web The server , send out HTTP request .
2）Web After the server receives the request , Pass request to Servlet Containers .
3）Servlet Container resolution request , load Servlet, meanwhile Servlet Containers create one HttpRequest Object and a HttpResponse object .HttpRequest Object encapsulates the passed request information .
4）Servlet Container call HttpServlet Object's service() Method , hold HttpRequest Object and the HttpResponse Object as a parameter to HttpServlet object .HttpServlet call HttpRequest Object about methods , obtain Http Request information ; call HttpResponse Object about methods , Generating response data .
5）Servlet Container handle HttpServlet The response result of is returned to Web The server .

Servlet Life cycle of

Servlet The life cycle of Servlet The process from Startup to shutdown , as follows ：

• Load and instantiate ： If Servlet The container has not instantiated one Servlet object , At this point, the container loads and instantiates a Servlet. If there is already one Servlet object , No new instances will be created at this time .
• initialization ： Is producing Servlet After the instance , The container is responsible for calling the Servlet Example of init() Method , Yes Servlet To initialize .
• Response request ： When Servlet The container received a Servlet When asked , Then run the corresponding Servlet Example of service() Method ,service() Method calls the corresponding doGet or doPost Method to handle user requests . Then enter the corresponding method to call the logical layer method , Response to customers .
• The destruction ： When Servlet stay Web Execute before the application is uninstalled , Release Servlet Occupied computer resources .
  Be careful ：(1)(2)(4) stay Servlet It will only be executed once in the whole life cycle of .

The code is as follows ,ServletLifeCycle.java：

package com.edu.ServletStudy;

import javax.servlet.*;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class ServletLifeCycle implements Servlet {
    
    public ServletLifeCycle() {
    
        // The constructor is called , Create an instance object 
        System.out.println(" First step ： Instance object ");
    }

    @Override
    public void init(ServletConfig servletConfig) throws ServletException {
    
        // Yes servlet To initialize , Only once 
        System.out.println(" The second step ： initialization ");
    }

    @Override
    public ServletConfig getServletConfig() {
    
        // Return to one ServletConfig Object is used to return initialization parameters and ServletContext
        return null;
    }

    @Override
    public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException {
    
        System.out.println(" The third step ： Provide services ");
        // Chinese code scrambling 
        response.setContentType("text/html;charset=utf-8");
        request.setCharacterEncoding("utf-8");
        // Information in response to the request 
        HttpServletResponse res = (HttpServletResponse)response ;
        res.getWriter().write("Hello Servlet, I am Q teacher ");
    }

    @Override
    public String getServletInfo() {
    
        // Provide relevant servlet Information about 
        return null;
    }

    @Override
    public void destroy() {
    
        System.out.println(" The destruction ");
    }
}

web.xml：

<?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_4_0.xsd" version="4.0">
    <!-- To configure servlet-->
    <servlet>
        <!--servlet Internal name -->
        <servlet-name>ServletLifeCycle</servlet-name>
        <!--servlet The full name of the class -->
        <servlet-class>com.edu.ServletStudy.ServletLifeCycle</servlet-class>
    </servlet>

    <!--servlet Mapping configuration -->
    <servlet-mapping>
        <!--servlet The internal name of , Keep consistent with the above -->
        <servlet-name>ServletLifeCycle</servlet-name>
        <!--servlet Mapping path for （ The address is usually action What's in it , Get data request ）-->
        <url-pattern>/ServletLifeCycle</url-pattern>
    </servlet-mapping>
</web-app>

The operation is as shown in the figure ： Console

page

When you click stop ,Servlet Will perform destroy() Method

ServletConfig object

ServletConfig Object is mainly used to configure servlet Is the initialization parameter of . The specific implementation is realized by Servlet Decided by the container developer . We use Tomcat For example .

The code is as follows ,web.xml：

<?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_4_0.xsd" version="4.0">
    <servlet>
        <servlet-name>ServletLifeCycle</servlet-name>
        <servlet-class>com.edu.ServletStudy.ServletLifeCycle</servlet-class>
        <!-- You can use one or more <init-param> Label configuration book Servlet Is the initialization parameter of -->
        <init-param>
            <!-- key -->
            <param-name>username</param-name>
            <!-- value -->
            <param-value> ah Q teacher </param-value>
        </init-param>
        <init-param>
            <param-name>pwd</param-name>
            <param-value>123</param-value>
        </init-param>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>ServletLifeCycle</servlet-name>
        <url-pattern>/ServletLifeCycle</url-pattern>
    </servlet-mapping>
</web-app>

ServletLifeCycle.java：

package com.edu.ServletStudy;

import javax.servlet.*;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Enumeration;

public class ServletLifeCycle implements Servlet {
    
    @Override
     // When Servlet Container initialization Servlet when ,Servlet The container will give Servlet Of init( ) Way to introduce a  ServletConfig object 
    public void init(ServletConfig servletConfig) throws ServletException {
    
        // Get configuration servlet Keys for all initialization parameters 
        Enumeration<String> enumeration = servletConfig.getInitParameterNames();
        // Traverse the output keys and values 
        while (enumeration.hasMoreElements()){
    
            String names = (String)enumeration.nextElement();
            String values = servletConfig.getInitParameter(names); // obtain servlet Configuration initialization parameters 
            System.out.println("ParamName = " + names + "ParamValue = " + values);
        }
    }

    @Override
    public ServletConfig getServletConfig() {
    
        return null;
    }

    @Override
    public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException {
    }

    @Override
    public String getServletInfo() {
    
        return null;
    }

    @Override
    public void destroy() {
    }
}

The console is shown in the picture ：

ServletContext object

ServletContext Is in Web When container starts , Every Web The application will create a corresponding ServletContext object （ Also known as Context object ）. One Web All in the application Servlet Share the same ServletContext object , therefore Servlet Objects can pass through ServletContext Object to communicate .

ServletContext In the object , To configure Servlet Parameter is used context-param Tags can be shared ; And the above ServletConfig In the object , To configure Servlet Parameters can only be used in servlet Internal init-param label , Only Ben Servlet To obtain parameters .

The code is as follows ,web.xml：

<?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_4_0.xsd" version="4.0">
    <!-- share Servlet Is the initialization parameter of -->
    <context-param>
        <param-name>username</param-name>
        <param-value> ah Q teacher </param-value>
    </context-param>
    <context-param>
        <param-name>pwd</param-name>
        <param-value>123</param-value>
    </context-param>
    
    <servlet>
        <servlet-name>ServletConextTest</servlet-name>
        <servlet-class>com.edu.ServletStudy.ServletConextTest</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ServletConextTest</servlet-name>
        <url-pattern>/ServletConextTest</url-pattern>
    </servlet-mapping>
</web-app>

ServletConextTest.java：

package com.edu.ServletStudy;

import javax.servlet.*;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Enumeration;

public class ServletConextTest implements Servlet {
    
    public void init(ServletConfig servletConfig) throws ServletException {
    
        // visit ServletContext
        ServletContext context = servletConfig.getServletContext();
        // Get configuration servlet Keys for all initialization parameters 
        Enumeration<String> paramNames = context.getInitParameterNames();
        // Traverse the output keys and values 
        while (paramNames.hasMoreElements()) {
    
            String name = paramNames.nextElement();
            String value = context.getInitParameter(name);
            System.out.println(name + " = " + value);
        }
    }

    public ServletConfig getServletConfig() {
    
        return null;
    }

    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
    }
    
    public String getServletInfo() {
    
        return null;
    }
    
    public void destroy() {
     }
}

HttpServlet

HttpServlet The purpose of this is to acquire HTTP request , Respond to HTTP result . stay IDEA Choose HttpServlet,“ctrl + Click the left mouse button ” Will jump to HttpServlet Class , At this time, you will find HttpServlet Abstract class GenericServlet Subclasses of . meanwhile ,HttpServlet Class is written doGet()、doPost()、doOptions()、doDelete() Other methods , They are corresponding to HTTP Medium Get、Post、Options、Delete Wait for those who request services .

Here is a brief introduction to commonly used doGet()、doPost() Method ： There are two ways for forms to submit data to the server , namely < form action = “***” method= " get / post " > attribute method Medium get or post. They correspond to each other HttpServlet Of doGet()、doPost() Method .
doGet()： The data transmitted in this way will follow < key , value > The way in URL It shows , Low security .
doPost()： The data transmitted in this way is stored in HTTP In the message body of the protocol , It is transmitted to the server by entity . Package and send the data , High safety .

Servlet The implementation of the

Next , We went further to Servlet The implementation of the , obtain Register.html Parameters of page form submission .
stay web File directory to create Register.html, The code is as follows ：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Servlet Study </title>
    <link rel="stylesheet" type="text/css" href="CSS/Register.css">
</head>
<body>
    <form action="http://localhost:8080/MSW/Register" method="post">
    	<h1>Welcome</h1>
        <label> account number ：</label>
        <input type="text" name="username"/><br>
        <label> password ：</label>
        <input type="password" name="pwd"/><br>
        <label> Gender ：</label>
        <input type="radio" name="sex" value=" male "/> male 
        <input type="radio" name="sex" value=" Woman "/> Woman <br>
        <label> Birthday ：</label>
        <select name="birthday">
            <optgroup label=" Please select the year "> Please select the year </optgroup>
            <option value="2000">2000</option>
            <option value="2001">2001</option>
            <option value="2002">2002</option>
        </select><br>
        <input type="submit" value=" notes   book ">&nbsp;&nbsp;&nbsp;&nbsp;
        <input type="reset" value=" heavy   Set up ">
    </form>
</body>
</html>

stay web/CSS Create under directory Register.css, The code is as follows ：

body{
    
    text-align:center;
    background-color: antiquewhite;
}

stay src establish com.edu.ServletStudy package , To create a RegisterServlet.java, The code is as follows ：

package com.edu.ServletStudy;

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 RegisterServlet extends HttpServlet {
    
    @Override
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
        // Solve the garbled Chinese display of response results 
        response.setContentType("text/html;charset=utf-8");
        request.setCharacterEncoding("utf-8");

		// Data acquisition 
        String username = request.getParameter("username");
        String pwd = request.getParameter("pwd");
        String sex = request.getParameter("sex");
        String birthdayStr = request.getParameter("birthday"); // Get data of string type 
        Integer birthday = Integer.parseInt(birthdayStr); // String to integer 

        // Print on the console 
        System.out.println("username = " + username);
        System.out.println("pwd = " + pwd);
        System.out.println("sex = " + sex);
        System.out.println("birthday = " + birthday);

        // Jump to print on the new page 
        response.getWriter().println(request.getParameter("username"));
        response.getWriter().println(request.getParameter("pwd"));
        response.getWriter().println(request.getParameter("sex"));
        response.getWriter().println(Integer.parseInt(birthdayStr));
    }
}

stay web/WEB-INF In the catalog web.xml, Add the following code ：

<?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_4_0.xsd" version="4.0">
    <!--  First , The user initiates a request ,action=Register;  then Tomcat Medium web.xml Look first url-pattern = /Register, To servlet-mapping Medium servlet-name = RegisterServlet;  Until then servlet Look for a match servlet-mapping Of servlet-name Of servlet;  And find servlet-class Of com.edu.ServletStudy.RegisterServlet;  Finally, because the user sent post request （method = post）, therefore Tomcat perform doPost Method . -->
    <servlet>
        <servlet-name>RegisterServlet</servlet-name>
        <servlet-class>com.edu.ServletStudy.RegisterServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>RegisterServlet</servlet-name>
        <url-pattern>/Register</url-pattern>
    </servlet-mapping>
</web-app>

stay RegisterServlet.java Click on the run , The effect is as shown in the picture ：

Console results ：

Web results ：

Solve the problems encountered in the implementation process

solve Address localhost:1099 is already in use The problem of

stay Intellij idea The running port appears Address localhost:1099 is already in use The problem of （ That is, the port is occupied ）. The solution is ：
1.win+R, Input cmd command , open dos Command line , Input netstat -ano | find “1099”, obtain PID ( That is, the last column of numbers )
2. adopt PID Find the process , Input ：tasklist | find “ That column of figures ”
3. Finally, enter the command to close the process ：taskkill /f /t /im java.exe

Pictured ：

solve Servlet obtain radio The value of is on The problem of

Pictured ：

The main reason for this problem may be There's something wrong with the code , Unrecognized get value .
The right code ：

<input type="radio" name="sex" value=" male "/> male 
<input type="radio" name="sex" value=" Woman "/> Woman <br>

IDEA Medium clean engineering

clean The function clears the cache in the project and reloads the verification .
The steps are shown in the figure ：
1. Click on File—>Invalidate Caches / Restart…

2. Click on Invalidate and Restart, The cache will expire and be rebuilt the next time it starts , Local history will also be erased .

3.IDEA restart