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servlet Is a technology to realize dynamic pages , It's a group. Tomcat For programmers API, Help group programmers develop a simple and efficient web app. Without concern Socket,HTTP Form of agreement , Multithreading concurrency and other technical details , To reduce the web app The development threshold of , Improved development efficiency rate .

Servlet The main work is

Allow programs to register a class ,, stay Tomcat Receive a specific HTTP On request ,, Execute some code in this class .

The helper resolves HTTP request , hold HTTP The request parses from a string into a HttpRequest object .

Help program ape construct HTTP Respond to . The program only needs to be assigned to HttpResponse Fill in some properties of the object ,Servlet It will be installed automatically HTTP A protocol is constructed HTTP Response string ,, And pass Socket Write back to the client .

2. first Servlet Program

2.1 Create project

Use IDEA Create a Maven project .

1) menu -> file -> New projects -> Maven

2) Select the directory where the project will be stored

3) Project creation completed

2.2 Introduce dependencies

Maven After the project is created , It will automatically generate a pom.xml file . We need to be in pom.xml Introduction in Servlet API Rely on the jar package . This API No JDK Built in , It's a third party Tomcat Provided .Maven Central warehouse address ：

https://mvnrepository.com/ , It's going on inside Servlet API Copy , Copy the code from step 5 below to pom.xml Inside .

<dependencies>
        <!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>3.1.0</version>
            <scope>provided</scope>
        </dependency>

    </dependencies>

When it was first copied , It is marked in red , That's because of our IDEA Not downloaded yet .jar package . This requires us to manually click IDEA On the top right of Maven Refresh icon for , Just wait for the download .

2.3 Create directory

When the project is created , IDEA Will help us automatically create some directories . Form like

however , These directories are not enough , We need to manually add some necessary directories .

1）webapp

2) establish web.xml

And then in webapp Create a... Inside the directory WEB-INF Catalog , And create a web.xml file

3) To write web.xml

Go to web.xml Copy the following code in

<!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>
</web-app>

, If in IDEA in , The following code appears in red , Do the following .

2.4 Write code

stay java Create a class in the directory HelloServlet, The code is as follows

import javax.servlet.annotation.WebServlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // This line of code needs to be commented out , Cannot reference a parent class's  doGet
        //super.doGet(req, resp);

        // This is to let the server print in its own console 
        System.out.println("hello world");
        // You can also print on the page  hello world  character string , Put it in  http  Responsive  body  in .  The browser will put  body  The content of 
        // Show on page 
        // If nothing is set in the response object ,  This is the time when   Blank page 
        resp.getWriter().write("hello world" + System.currentTimeMillis());
    }
}

In the above code ：

- Create a class HelloServlet , Inherited from HttpServlet

- Add... Above this class @WebServlet("/hello") annotation , Express Tomcat Of the requests received , Path is /hello Will only call HelloServlet The code for this class . ( This path does not contain Context Path)

- rewrite doGet Method . doGet There are two parameters to , Respectively represent the received HTTP request And the... To be constructed HTTP Respond to .. this individual Methods in Tomcat received GET Trigger on request

- HttpServletRequest Express HTTP request . Tomcat according to HTTP The format of the request is character string Format request to Become a HttpServletRequest object . Later, I want to get the information in the request ( Method , url, header, body etc. ) All are Get... Through this object

- HttpServletResponse Express HTTP Respond to . Construct the response object in the code ( Construct the status code of the response , header,

body etc. )

-resp.getWriter() Will get a stream object , Through this stream object, you can write some data , The written data will be Construct into a HTTP Responsive body part , Tomcat Will turn the entire response into a string , adopt socket Write back to browse device .

Be careful , The class you want to write Can be Tomcat call , You have to meet the following conditions

a) The created class needs to inherit from HttpServlet
b) This class needs to use @WebServlet The annotation is associated with the previous HTTP The path of
c) This class needs to implement doXXX Method

2.5 packaged applications

Current code , It can't run alone ( No, main Method ) Current code , It can't run alone ( No, Main Method )
Need to put the current code , pack , And then deploy to Tomcat. On , from Tomcat To make the call , Need to put the current code , pack , And then deploy to Tomcat. On , from Tomcat To make the call .

Before packing , First in pom.xml Add a <packaging>war</packaging> , Because we packed war package , If not , Will default to jar package .

jar and war All are java The compressed package format used by the publisher .war It's for Tomcat Specialized in . There will not only be some .class, You can also include configuration files , And the second 3 Tripartite jar , also html, Css, js....
Add location as follows

Use maven package . open maven window ( Generally in IDEA On the right you can see Maven window , If you can't see it , Can pass menu -> View -> Tool Window -> Maven open

And then unfold Lifecycle , double-click package You can pack it .

When the console appears BUILD SUCCESS, And the directory list appears servlet_hello.war It indicates that the packaging is successful

2.6 The deployment process

After the open hold war Package copy to Tomcat Of webapps Under the table of contents . Then start Tomcat , Tomcat Will automatically put war Packet decompression .

2.7 Verification procedure

Enter the URL of the direct deployment through the browser , You can get a response http://127.0.0.1:8080/servlet_hello/hello, as follows

Be careful : URL Medium PATH Divided into two parts , among servlet_hello by Context Path, hello by Servlet Path

2.8 summary

These seven steps , This is for a new project , After the project is created , Subsequent modification code , The first three steps need not be repeated . Directly from 4—7 Just operate .

3. Easier deployment

Copy manually war Package to Tomcat The process is quite troublesome . We have a more convenient way . Here we use IDEA Medium Smart Tomcat The plug-in does the job .

3.1 install Smart Tomcat plug-in unit

menu -> file -> Settings -> choice Plugins, choice Marketplace, Search for "tomcat", Click on "Install".

After installation , Will prompt " restart IDEA"

3.2 To configure Smart Tomcat plug-in unit

1) Click on the "Add Configuration. 2) Click on ＋ Number ,3) Choose the one on the left "Smart Tomcat"

4) stay Name Fill in a name in this column ( I could just write ).5） choice Tomcat Installation path for

Click the green triangle , IDEA Will automatically compile , Deploy

start-up Tomcat The process of , The font is red , It is not an error that makes it red .

Use Smart Tomcat When deployed , We found that Tomcat Of webapps There is no copy inside war package ,

I didn't see the decompressed content .Smart Tomcat Quite so in Tomcat When starting, it directly refers to... In the project webapp and target Catalog .

4. Common error problems

4.1 appear 404

404 Indicates that the resource accessed by the user does not exist . The approximate rate is URL The path of is not written correctly .

At present , The right is http://127.0.0.1:8080/servlet_hello/hello

1） Write less. Context Path namely servlet_hello

2) Write less. Servlet Path namely hello

3)Servlet Path It's written and URL No match

modify @WebServlet Path to annotation , The correct here is as follows

Error is as follows

4）web.xml Write the wrong

4.2 appear 405

405 Indicates the corresponding HTTP The request method is not implemented .

1） It didn't come true doGet Method .

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
// Be annotated out , So it didn't come true doGet  Method 
  //@Override
    //protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { }
}

4.3 appear 500

To be Servlet The exception thrown in the code results in .

Error instance : Modify the code

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServlet Requestreq, HttpServlet Responseresp) throws Servlet Exception, IOException {
        String s = null;
        resp.getWriter().write(s.length());
    }
}

4.4 appear " Blank page "

Error instance : Modify the code , Comment out resp.getWritter().write() operation .


@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // This line of code needs to be commented out , Cannot reference a parent class's  doGet
        //super.doGet(req, resp);

        // This is to let the server print in its own console 
        System.out.println("hello world");
        // You can also print on the page  hello world  character string , Put it in  http  Responsive  body  in .  The browser will put  body  The content of 
        // Show on page 
        // If nothing is set in the response object ,  This is the time when   Blank page 
        //resp.getWriter().write("hello world" + System.currentTimeMillis());
    }
}

4.5 appear " Can't access this website "

It's usually Tomcat The startup failed .

Error instance : Servlet Path Write the wrong .

// hold 
@WebServlet("/hello")
// It has been written. , Just missing one  /
@WebServlet("hello")

5. summary

Beginners Servlet, There will be many such problems . We don't just have to learn Servlet The basic way of writing code , Also learn Troubleshoot the wrong .

4xx The status code of indicates that the path does not exist , Often need to check URL Whether it is right , And the... Set in the code Context Path as well as

Servlet Path Is it consistent .

5xx The status code of indicates that the server has an error , You often need to observe the content and content of the page prompt Tomcat Your own journal , Observe if

There is an error .

A connection failure often means Tomcat Not started correctly , Also need to observe Tomcat Whether there is an error prompt in your own log .

This situation of blank pages requires us to use the packet capture tool to analyze HTTP The specific interaction process of request response .

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