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Complete solution of servlet: inheritance relationship, life cycle, container, request forwarding and redirection, etc

2022-07-02 09:08:00 【Programmer Qiqi】

One 、Servlet summary

1、Servlet name

Servlet = Server + applet

Server： The server applet： Applet Servlet： Server side applet

2、Servlet stay Web Role in application

① Examples in life

② Corresponding Web application

③ Specific details

④Servlet Play a role Throughout Web Application ,Servlet Mainly responsible for processing requests 、 Coordination and dispatching function . We can Servlet be called Web In application 『 controller 』

Two 、Servlet HelloWorld

1、HelloWorld analysis

① The goal is Click the hyperlink on the page , from Servlet Process this request , And return a response string ：Hello,I am Servlet! .

② Ideas

2、 Specific operation

① Create dynamic Web Module 《 Step on the pit + Demining new edition IDEA2021.1 Create a configuration Javaweb Project and deploy in Tomcat Containers 》

② Create front page hyperlinks begin.html The code for is as follows ：

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
  <a href="hello"> Please click on me </a>
</body>
</html>
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③ establish HelloServlet Of Java class

package com.yeman.Servlets;

import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import java.io.IOException;
import java.io.PrintWriter;

/**
 * @Author: Yeman
 * @Date: 2022-02-11-18:15
 * @Description:
 */
public class Hello extends HttpServlet {
    @Override
    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        System.out.println(" Yes Hello This Servlet！");
        
        //  Returns the response string 
        // 1、 Gets a character stream object that can return response data 
        PrintWriter writer = res.getWriter();

        // 2、 Write data to the character stream object 
        writer.write("Hello,I am Servlet!");
        
    }
}
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④ To configure Hello Servlet Profile location ：WEB-INF/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 In itself  -->
    <servlet>
    	 <!--  to Servlet Set a short name  -->
        <servlet-name>Hello</servlet-name>
        <!--  To configure Servlet Full class name of  -->
        <servlet-class>com.yeman.Servlets.Hello</servlet-class>
    </servlet>
    <!--  take Servlet Associated with the access address  -->
    <servlet-mapping>
        <servlet-name>Hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>
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⑤ test

⑥ Summary demand ： Click the hyperlink on the browser to access Java Program .

3、 Combing concepts

① Native Tomcat

②IDEA Medium Tomcat example

③IDEA Medium Web engineering

④ according to Web Project generated war package

⑤Web Resources in the project [1] Static resources

HTML file
CSS file
JavaScript file
Picture file

[2] Dynamic resources

Servlet

⑥ The address of the access resource [1] Static resources

/Web apply name / The path of the static resource itself

[2] Dynamic resources

/Web apply name / Virtual path

⑦Web apply name

⑧ The overall logical structure

3、 ... and 、Servlet Inheritance relationships

1、 Inheritance relationships

javax.servlet.Servlet Interface ----->
javax.servlet.GenericServlet abstract class ----->javax.servlet.http.HttpServlet Abstract subclass

2、 Related methods

①javax.servlet.Servlet Interface ：

void init(config) - Initialization method
void service(request,response) - The service method
void destory() - Destruction method

②
javax.servlet.GenericServlet abstract class ：

void service(request,response) - Still abstract

③
javax.servlet.http.HttpServlet Abstract subclass ：

void service(request,response) - It's not abstract

String method = req.getMethod(); How to get the request Various if Judge , Depending on the way of request , Decide to call different do Method ：

if (method.equals("GET")) {
                this.doGet(req,resp);
            } else if (method.equals("HEAD")) {
                this.doHead(req, resp);
            } else if (method.equals("POST")) {
                this.doPost(req, resp);
            } else if (method.equals("PUT")) {
            ......
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stay HttpServlet In this abstract class ,do The methods are almost the same ：

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
	String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_get_not_supported");
    if (protocol.endsWith("1.1")) {
    	resp.sendError(405, msg);
    } else {
        resp.sendError(400, msg);
    }
}
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3、 Summary

① Inheritance relationships ： HttpServlet -> GenericServlet -> Servlet

②Servlet The core approach in ：init(),service(),destroy()

③ The service method ： When a request comes ,service Method will automatically respond （ It's actually tomcat Container called ）.

④ stay HttpServlet The method of the request will be analyzed in ： What is the get、post、head still delete etc. , Then decide which one to call do Opening method .

⑤ stay HttpServlet Medium do The default method is 405 The implementation style of , Subclasses are required to implement the corresponding methods , Otherwise, it will report by default 405 error . therefore , We're building Servlet when , The request method needs to be considered , To decide which one to rewrite do Method .

Four 、Servlet Life cycle

1、 Life cycle

The process from birth to death is the life cycle . Corresponding Servlet Three methods in init(),service(),destroy().

By default ： The first time you receive a request , This Servlet Instantiate （ Call constructor ）、 initialization （ call init()）、 And then service （ call service()）. Start with the second request , Every time it's a service . When the container is closed , All of them servlet The instance will be destroyed , Call destroy method .

Servlet example Tomcat Only one will be created , All requests are answered by this instance . By default , On the first request ,Tomcat To instantiate , initialization , Then serve . The advantage of this is that it can improve the startup speed of the system , The drawback is that the first request takes a long time . So come to the conclusion ： If you need to improve the startup speed of the system , The current default is OK , If you need to improve response speed , It should be set up Servlet Initialization time of .

2、 Initialization time

We can go through <load-on-startup> To set up servlet Order of start up , The smaller the number is. , Start more forward , minimum value 0. It can be instantiated and initialized when the system starts , Improve post response speed .

modify web.xml in Servlet Configuration of ：

<!--  To configure Servlet In itself  -->
<servlet>
    <!--  The full class name is too long , to Servlet Set a short name  -->
    <servlet-name>HelloServlet</servlet-name>

    <!--  To configure Servlet Full class name of  -->
    <servlet-class>com.yeman.servlet.HelloServlet</servlet-class>

    <!--  To configure Servlet Startup sequence  -->
    <load-on-startup>1</load-on-startup>
</servlet>
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3、Servlet Containers

① Containers Among various technologies developed and used , There are often many objects placed in containers .

② Functions provided by the container The container manages the entire lifecycle of the internal objects . Objects can only work properly in containers , Get full support from the container .

Create objects
initialization
Work
clear

③ The container itself is also an object characteristic 1： Often very large objects characteristic 2： Usual singleton

④ A typical Servlet Examples of container products Tomcat、jetty、jboss、Weblogic、WebSphere、glassfish

⑤Servlet In the container is ： The singleton 、 Thread unsafe .

Single case ： All requests are answered by the same instance Thread unsafe ： A thread needs to make logical judgment according to the value of a member variable in this instance , But at some point in the middle , Another thread changed the value of the member variable , This causes the execution path of the first thread to change .

Be careful ： Try not to be in servlet Define member variables in , If you have to define member variables , Then don't go ：① Change the value of the member variable ② Make some logical judgments according to the value of member variables .

4、 Summary

5、 ... and 、ServletConfig and ServletContext

1、ServletConfig Interface

① An overview of the interface

② Interface method

③ Examples of initialization parameters

<!--  To configure Servlet In itself  -->
<servlet>
    <!--  The full class name is too long , to Servlet Set a short name  -->
    <servlet-name>HelloServlet</servlet-name>

    <!--  To configure Servlet Full class name of  -->
    <servlet-class>com.yeman.servlet.HelloServlet</servlet-class>

    <!--  Configure initialization parameters  -->
    <init-param>
        <param-name>goodMan</param-name>
        <param-value>me</param-value>
    </init-param>

    <!--  To configure Servlet Startup sequence  -->
    <load-on-startup>1</load-on-startup>
</servlet>
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④ Experience

public class HelloServlet implements Servlet {

    //  Declare a member variable , For reception init() Method passed in servletConfig object 
    private ServletConfig servletConfig;

    public HelloServlet(){
        System.out.println("HelloServlet objects creating ！");
    }

    @Override
    public void init(ServletConfig servletConfig) throws ServletException {

        System.out.println("HelloServlet Object initialization ");

        //  take Tomcat call init() Method servletConfig Object is assigned to a member variable 
        this.servletConfig = servletConfig;

    }

    @Override
    public ServletConfig getServletConfig() {

        //  Returns the member variable servletConfig, Easy to use 
        return this.servletConfig;
    }

    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {

        //  Console printing , Prove that this method was called 
        System.out.println(" I am a HelloServlet, I did ！");

        //  Returns the response string 
        // 1、 Gets a character stream object that can return response data 
        PrintWriter writer = servletResponse.getWriter();

        // 2、 Write data to the character stream object 
        writer.write("Hello,I am Servlet!");

        // ============= Split line ===============
        //  test ServletConfig Use of objects 
        // 1. obtain ServletConfig object ： stay init() Method 
        System.out.println("servletConfig = " + servletConfig.getClass().getName());

        // 2. adopt servletConfig Object acquisition ServletContext object 
        ServletContext servletContext = this.servletConfig.getServletContext();
        System.out.println("servletContext = " + servletContext.getClass().getName());

        // 3. adopt servletConfig Object to get initialization parameters 
        Enumeration<String> enumeration = this.servletConfig.getInitParameterNames();
        while (enumeration.hasMoreElements()) {
            String name = enumeration.nextElement();
            System.out.println("name = " + name);

            String value = this.servletConfig.getInitParameter(name);
            System.out.println("value = " + value);
        }
    }

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

    @Override
    public void destroy() {
        System.out.println("HelloServlet The object is about to be destroyed , Now perform the cleanup operation ");
    }
}
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⑤Servlet Standards and JDBC Standard comparison ： In a broad sense Servlet：javax.servlet A set of interface definitions under a package Web Development standards . Follow this set of standards , Different Servlet Containers provide different implementations . narrow sense Servlet：javax.servlet.Servlet Interface and its implementation class , That is, the specific Servlet.

It also embodies the idea of interface oriented programming , It also embodies the idea of decoupling ： As long as the interface doesn't change , Any changes to the underlying method , Will not affect the upper level methods .

2、ServletContext Interface

① brief introduction

On behalf of the entire Web application , It's a singleton .

Typical functions ：

Get the real path of a resource ：getRealPath()
Get the entire Web Application level initialization parameters ：getInitParameter()
As Web Domain object of application scope

In the data ：setAttribute() Take out the data ：getAttribute(

② Experience [1] To configure Web Application level initialization parameters

  <!--  To configure Web Application initialization parameters  -->
    <context-param>
        <param-name>handsomeMan</param-name>
        <param-value>alsoMe</param-value>
    </context-param>
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[2] To obtain parameters

String handsomeMan = servletContext.getInitParameter("handsomeMan");
System.out.println("handsomeMan = " + handsomeMan);
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6、 ... and 、 Request forwarding and redirection

1、 The relay

Send a request to Servlet, The baton passed to Servlet In the hands of . And in most cases ,Servlet Can't do everything alone , The baton needs to be passed on , At this point we need to request 『 forward 』 or 『 Redirect 』.

2、 forward

① The essence ： Hand over

② Complete definition ： During the processing of the request ,Servlet Completed their own tasks , The request needs to be forwarded to the next Servlet To continue processing .

③ Code

request.getRequestDispatcher("/fruit/apple/red/big.html").forward(request, response);
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④ analogy

The key ： Because the core part of the forwarding operation is completed on the server side , So the browser is not aware of , The browser only sends the request once in the whole process .

3、 Redirect

① The essence ： A special response

② Complete definition ： During the processing of the request ,Servlet Completed their own tasks , Then tell the browser in a response way ： To complete this task, you need to access the next resource ”.

③ Code

response.sendRedirect("/app/fruit/apple/red/big.html");
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The key ： Because the core part of the redirection operation is completed on the browser side , So the browser sends two requests during the whole process .

4、 contrast

5、 Application scenarios

It's easy to judge ： If you can use forwarding, use forwarding first , If forwarding fails , Then use redirection .

Need to pass through the same request Object to carry data to the target resource ： Only forward .

If you want to go to the next resource , Browser refresh accesses the second resource ： You can only use redirection .

7、 ... and 、 Get request parameters

1、 The concept of request parameters

The browser sends a request to the server at the same time , Carried parameter data .

2、 The browser sends the basic form of request parameters

URL Request parameters attached after address

/orange/CharacterServlet?username=Tom

Forms
Ajax request

3、 Server encapsulation of request parameters

In general , The server encapsulates the request parameters as Map<String, String[]>.

key ： The name of the request parameter value ： An array of request parameter values

4、 Method to get request parameters

5、 test

①HTML Code

<!--  Form for testing request parameters  -->
<form action="/orange/ParamServlet" method="post">

    <!--  Single line text box  -->
    <!-- input Label fit type="text" Property to generate a single line text box  -->
    <!-- name Property defines the name of the request parameter  -->
    <!--  If set value attribute , Then this value is the default value of the single line text box  -->
     Individuality signature ：<input type="text" name="signal" value=" The default value for a single line text box " /><br/>

    <!--  Password box  -->
    <!-- input Label fit type="password" Property to generate a password box  -->
    <!--  The content filled in by the user in the password box will not be displayed in clear text  -->
     password ：<input type="password" name="secret" /><br/>

    <!--  Radio buttons  -->
    <!-- input Label fit type="radio" Property to generate a radio box  -->
    <!-- name Property consistent radio Will be recognized by the browser as the same set of radio boxes , Only one... Can be selected in the same group  -->
    <!--  After submitting the form , What is really sent to the server is name Properties and value The value of the property  -->
    <!--  Use checked="checked" Property settings are selected by default  -->
     Please choose your favorite season ：
    <input type="radio" name="season" value="spring" /> spring 
    <input type="radio" name="season" value="summer" checked="checked" /> summer 
    <input type="radio" name="season" value="autumn" /> autumn 
    <input type="radio" name="season" value="winter" /> winter 

    <br/><br/>

     Your favorite animal is ：
    <input type="radio" name="animal" value="tiger" /> Land rover 
    <input type="radio" name="animal" value="horse" checked="checked" /> BMW 
    <input type="radio" name="animal" value="cheetah" /> jaguar 

    <br/>

    <!--  Checkbox  -->
    <!-- input Labels and type="checkbox" Cooperate to generate multiple selection boxes  -->
    <!--  After multiple selection boxes are selected by the user and the form is submitted, a 『 A name carries multiple values 』 The situation of  -->
     Your favorite team is ：
    <input type="checkbox" name="team" value="Brazil"/> Brazil 
    <input type="checkbox" name="team" value="German" checked="checked"/> Germany 
    <input type="checkbox" name="team" value="France"/> The French 
    <input type="checkbox" name="team" value="China" checked="checked"/> China 
    <input type="checkbox" name="team" value="Italian"/> Italy 

    <br/>

    <!--  The drop-down list  -->
    <!--  Use select Label definition drop-down list , stay select In label settings name attribute  -->
     Your favorite sport is ：
    <select name="sport">
        <!--  Use option Property defines the list item of the drop-down list  -->
        <!--  Use option Labeled value Property to set the value submitted to the server , stay option The value set to the user in the label body of the label  -->
        <option value="swimming"> swimming </option>
        <option value="running"> running </option>

        <!--  Use option Labeled selected="selected" Property setting this list item is selected by default  -->
        <option value="shooting" selected="selected"> Shooting </option>
        <option value="skating"> skating </option>
    </select>

    <br/>

    <br/><br/>

    <!--  Form hidden fields  -->
    <!-- input Labels and type="hidden" Cooperate to generate form hidden fields  -->
    <!--  Form hidden fields will not be displayed on the page , Used to save data to be submitted to the server but do not want users to see  -->
    <input type="hidden" name="userId" value="234654745" />

    <!--  Multi-line text box  -->
     Self introduction. ：<textarea name="desc"> The default value for multiline text boxes </textarea>

    <br/>

    <!--  General button  -->
    <button type="button"> General button </button>

    <!--  Reset button  -->
    <button type="reset"> Reset button </button>

    <!--  Form submit button  -->
    <button type="submit"> Submit button </button>
</form>
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②Java Code

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    //  Get the... That contains all the request parameters Map
    Map<String, String[]> parameterMap = request.getParameterMap();

    //  Iterate through the... That contains all the request parameters Map
    Set<String> keySet = parameterMap.keySet();

    for (String key : keySet) {

        String[] values = parameterMap.get(key);

        System.out.println(key + "=" + Arrays.asList(values));
    }

    System.out.println("---------------------------");

    //  Obtain the specified request parameter value according to the request parameter name 
    // getParameter() Method ： Get the request parameters of the radio box 
    String season = request.getParameter("season");
    System.out.println("season = " + season);

    // getParameter() Method ： Get the request parameters of multiple selection boxes 
    //  Only the first of multiple values can be obtained 
    String team = request.getParameter("team");
    System.out.println("team = " + team);

    // getParameterValues() Method ： Take the request parameters of the radio box 
    String[] seasons = request.getParameterValues("season");
    System.out.println("Arrays.asList(seasons) = " + Arrays.asList(seasons));

    // getParameterValues() Method ： Take the request parameters of multiple selection boxes 
    String[] teams = request.getParameterValues("team");
    System.out.println("Arrays.asList(teams) = " + Arrays.asList(teams));

}

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