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Servlets & Java Server Pages

发布于 2025-02-22 22:20:07 字数 12538 浏览 0 评论 0 收藏 0

A servlet is a Java Web component that generates dynamic content. Servlets are managed by containers like Jetty or Tomcat; they are classes used to build web applications based on request-response programming model. Java Server Pages (JSP) technology allows us to easily create web content that has both static and dynamic components. JSPs are compiled into servlets by a JSP compiler.

This chapter will show how to set up servlets and Java Server Pages in Jetty.

Processing a POST request

An HTTP POST request sends data from a client to a server. An HTML form sends data to server using the POST method.

In our example a servlet processes the data sent by the client. It retrieves the values and sends it back to the client.

 tree
.
├── build.xml
└── src
  ├── com
  │   └── zetcode
  │     └── MyServlet.java
  └── web
    ├── index.html
    └── WEB-INF
      └── web.xml

5 directories, 4 files

This is how our project directory looks like.

index.html

<!DOCTYPE html>
<html>
<body>

<form id="contact" method="post" action="process.do">
 
<label for="name">Name:</label>
<input type="text" name="name">
<br> 
<label for="age">Age:</label>
<input type="text" name="age">
 
<input type="submit" value="Submit">
 
</form>
 
</body>
</html>

The index.html file contains an HTML form. It has two input tags to get data from the user. The values can be retrieved later from the request parameters whose names match the input's name attributes. The action attribute of the form tag provides a URL pattern that is mapped to a specific servlet to process.

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_3_1.xsd"
     version="3.1">
  
</web-app>

The web.xml is empty. The mapping of the URL to the servlet is created using an annotation. Actually, the file is not needed in our example. However, since not all features can be replaced by annotations, the web.xml file is included in larger projects.

MyServlet.java

package com.zetcode;

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

@WebServlet(urlPatterns = "/process.do")
public class MyServlet extends HttpServlet {

  @Override
  protected void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    response.setContentType("text/plain");
    response.setStatus(HttpServletResponse.SC_OK);
    response.getWriter().format("Name: %s%n", request.getParameter("name"));
    response.getWriter().format("Age: %s%n", request.getParameter("age"));
  }
}

This is the code of our processing servlet.

@WebServlet(urlPatterns = "/process.do")

The WebServlet annotation maps com.zetcode.MyServlet to the /process.do URL pattern.

response.getWriter().format("Name: %s%n", request.getParameter("name"));

We retrieve the name value from the request object using the getParameter() method. The name parameter corresponds to the name attribute of the input tag.

build.xml

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

<project name="ProcessForm" default="compile">
  
  <property name="name" value="myform"/>
  <property environment="env"/>
  <property name="src.dir" value="src"/>
  <property name="web.dir" value="${src.dir}/web"/>
  <property name="build.dir" location="${web.dir}/WEB-INF/classes"/>
  <property name="jetty.lib.dir" location="${env.JETTY_HOME}/lib"/>
  <property name="dist.dir" location="dist"/>
  <property name="deploy.path" location="${env.JETTY_BASE}/webapps"/>
  
  <path id="compile.classpath">
    <fileset dir="${jetty.lib.dir}"/>
  </path>
  
  <target name="init">
    <mkdir dir="${build.dir}"/>
    <mkdir dir="${dist.dir}"/>
  </target>   
  
  <target name="compile" depends="init">
    <javac srcdir="${src.dir}" destdir="${build.dir}" 
         includeantruntime="false">
      <classpath refid="compile.classpath"/>
    </javac>
    <echo>Compilation completed</echo>
  </target>
  
  <target name="archive" depends="compile">
    <war destfile="${dist.dir}/${name}.war" 
       webxml="${web.dir}/WEB-INF/web.xml">
      <fileset dir="${web.dir}"/>
    </war>
    <echo>Archive created</echo>
  </target> 
  
  <target name="clean" depends="init">
    <delete dir="${build.dir}"/>
    <delete dir="${dist.dir}"/>
    <echo>Cleaning completed</echo>
  </target>  
  
  <target name="deploy" depends="archive">
    <copy file="${dist.dir}/${name}.war" overwrite="true" 
        todir="${deploy.path}"/>
    <echo>Archive deployed</echo>
  </target>  
  
</project>

This is the Ant build file.

$ curl --data "name=Robert&age=33" localhost:8080/myform/process.do
Name: Robert
Age: 33

We do a POST request with the curl tool and the servlet responds with these two lines.

Custom 404 error page

This example sets up a custom JSP page for displaying an HTTP 404 error. The 404 or Not Found error message is a HTTP standard response code indicating that the client was able to communicate with a given server, but the server could not find what was requested.

$ tree
.
├── build.xml
└── src
  ├── com
  │   └── zetcode
  │     └── MyServlet.java
  └── web
    ├── error404.jsp
    └── WEB-INF
      └── web.xml

5 directories, 4 files

These are the contents of the project directory.

error404.jsp

<%@ page language="java" isErrorPage="true" contentType="text/html; 
  charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<title>Error page</title>
  <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
  <button onclick="history.back()">Back to Previous Page</button>
  <h1>404 Page Not Found.</h1>
  <p><b>Error code:</b> ${pageContext.errorData.statusCode}</p>
  <p><b>Request URI:</b> 
  ${pageContext.request.scheme}://${header.host}${pageContext.errorData.requestURI}
  </p>
</body>
</html>

This JSP page will be displayed if Jetty cannot find a requested page.

MyServlet.java

package com.zetcode;

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

public class MyServlet extends HttpServlet {

  @Override
  protected void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    response.setContentType("text/plain");
    response.setStatus(HttpServletResponse.SC_OK);
    response.getWriter().println("MyServlet called");
  }
}

A simple servlet is created. The servlet sends a plain message back to the client.

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_3_1.xsd"
     version="3.1">
  
  <display-name>MyServlet</display-name>
  
  <servlet id="jsp">
    <servlet-name>jsp</servlet-name>
    <servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
    <init-param>
      <param-name>fork</param-name>
      <param-value>false</param-value>
    </init-param>
    <init-param>
      <param-name>keepgenerated</param-name>
      <param-value>true</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>  
  
  <servlet>
    <servlet-name>MyServlet</servlet-name>
    <servlet-class>com.zetcode.MyServlet</servlet-class>
  </servlet>
  
  <servlet-mapping>
    <servlet-name>MyServlet</servlet-name>
    <url-pattern>/myservlet.do</url-pattern>
  </servlet-mapping>
  
  <servlet-mapping>
    <servlet-name>jsp</servlet-name>
    <url-pattern>*.jsp</url-pattern>
  </servlet-mapping>  
  
  <error-page>
    <error-code>404</error-code>
    <location>/error404.jsp</location>
  </error-page>  
  
</web-app>

The web.xml file registers a JspServlet which handles JSP pages, registers our MyServlet, and maps the error404.jsp file to the 404 error.

build.xml

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

<project name="CustomErrorPage" default="compile">
  
  <property name="name" value="customerror"/>
  <property environment="env"/>
  <property name="src.dir" value="src"/>
  <property name="web.dir" value="${src.dir}/web"/>
  <property name="build.dir" location="${web.dir}/WEB-INF/classes"/>
  <property name="lib.dir" location="${env.JETTY_HOME}/lib"/>
  <property name="dist.dir" location="dist"/>
  <property name="deploy.path" location="${env.JETTY_BASE}/webapps"/>
  
  <path id="compile.classpath">
    <fileset dir="${lib.dir}"/>
  </path>
  
  <target name="init">
    <mkdir dir="${build.dir}"/>
    <mkdir dir="${dist.dir}"/>
  </target>   
  
  <target name="compile" depends="init">
    <javac srcdir="${src.dir}" destdir="${build.dir}" 
         includeantruntime="false">
      <classpath refid="compile.classpath"/>
    </javac>
    <echo>Compilation completed</echo>
  </target>
  
  <target name="archive" depends="compile">
    <war destfile="${dist.dir}/${name}.war" 
       webxml="${web.dir}/WEB-INF/web.xml">
      <fileset dir="${web.dir}"/>
    </war>
    <echo>Archive created</echo>
  </target> 
  
  <target name="clean" depends="init">
    <delete dir="${build.dir}"/>
    <delete dir="${dist.dir}"/>
    <echo>Cleaning completed</echo>
  </target>  
  
  <target name="deploy" depends="archive">
    <copy file="${dist.dir}/${name}.war" overwrite="true" 
        todir="${deploy.path}"/>
    <echo>Archive deployed</echo>
  </target>  
  
</project>

This is the Ant build file.

$ java -jar $JETTY_HOME/start.jar --add-to-start=http,deploy,jsp,jstl,annotations

These modules must be enabled in the Jetty base.

$ java -jar $JETTY_HOME/start.jar
$ curl localhost:8080/customerror/myservlet.do
MyServlet called

We start Jetty and make a valid request.

$ curl localhost:8080/customerror/servlet.do
<!DOCTYPE html>
<html>
<head>
<title>Error page</title>
  <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
  <button onclick="history.back()">Back to Previous Page</button>
  <h1>404 Page Not Found.</h1>
  <p><b>Error code:</b> 404</p>
  <p><b>Request URI:</b> http://localhost:8080/customerror/servlet.do</p>
</body>
</html>

Trying to reach a non-existing resource causes Jetty to send our custom 404 error page.

In this chapter of the Jetty tutorial, we have worked with Java servlets and JSPs.

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