Bài giảng Web technologies and e-Services - Bài 7, Phần 2: Java servlet - Trường Đại học Bách khoa Hà Nội
Java Servlet
Objectives
• Explain the nature of a servlet and its operation
• Use the appropriate servlet methods in a web
application
• Code the extraction of environment entries within a
servlet
• Handle HTML forms within a servlet
• Explain the significance of web application state
• Explain the purpose and operation of HTTP cookies
and their role in state management
• Develop java web application with MVC model
Free Servlet and JSP Engines (Servlet/JSP
Containers)
• Apache Tomcat
• IDE: NetBeans, Eclipse
• Some Tutorials:
• Creating Servlet in Netbeans:
• Java Servlet Example:
Compiling and Invoking Servlets
• Put your servlet classes in proper location
• Locations vary from server to server. E.g.,
• tomcat_install_dir/webapps/ROOT/WEB-INF/classes
• Invoke your servlets (HTTP request)
• Custom URL-to-servlet mapping (via web.xml)
Purposes of Web Applications (A single
WAR file)
• Organization
• Related files grouped together in a single directory
hierarchy.
• HTML files, JSP pages, servlets, beans, images, etc.
• Portability
• Most servers support Web apps.
• Can redeploy on new server by moving a single file.
• Separation
• Each Web app has its own:
• ServletContext, Class loader
• Sessions, URL prefix, Directory structure
Structure of a Web Application
• JSP and regular Web content (HTML, style sheets,
images, etc.):
• Main directory or a subdirectory thereof.
• Servlets:
• WEB-INF/classes (if servlet is unpackaged – i.e. in default
package)
• A subdirectory thereof that matches the package name.
• JAR files:
• WEB-INF/lib.
• web.xml:
• WEB-INF
• Tag Library Descriptor files:
• WEB-INF or subdirectory thereof
• Files in WEB-INF not directly accessible to outside clients
Example Structure
Java Servlets
• A servlet is a Java program that is invoked by a web
server in response to a request
Server Platform
Client
Web
Server
Web Application
Servlet
Java Servlets
• Together with web pages and other components,
servlets constitute part of a web application
• Servlets can
• create dynamic (HTML) content in response to a request
• handle user input, such as from HTML forms
• access databases, files, and other system resources
• perform any computation required by an application
Java Servlets
• Servlets are hosted by a servlet container, such as Apache
Tomcat*
The web server
Server Platform
handles the HTTP
transaction details
Web
Server
The servlet container
provides a Java
Virtual Machine
for servlet execution
Servlet
Container
*Apache Tomcat can be both a web server and a servlet container
Environment For Developing and Testing
Servlets
• Compile:
• Need Servlet.jar. Available in Tomcat package
• Setup testing environment
• Install and start Tomcat web server
• Place compiled servlet at appropriate location
Servlet Operation
Servlet Methods
• Servlets have three principal methods
.init()
invoked once, when the servlet is loaded by the servlet
container (upon the first client request)
.service(HttpServletRequest req,
HttpServletResponse res)
invoked for each HTTP request
parameters encapsulate the HTTP request and response
.destroy()
invoked when the servlet is unloaded
(when the servlet container is shut down)
Servlet Methods
• The default .service() method simply invokes method-
specific methods
• depending upon the HTTP request method
.doGet()
.doPost()
.doHead()
… etc.
.service()
Methods
Methods
HTTP Requests
Comments
doGet
doPost
GET, HEAD
POST
Usually overridden
Usually overridden
doPut
PUT
Usually not overridden
Almost never overridden
Almost never overridden
doOptions
doTrace
OPTIONS
TRACE
Servlet Example 1
• This servlet will say "Hello!" (in HTML)
package servlet;
import javax.servlet.http.*;
public class HelloServlet extends HttpServlet {
public void service(HttpServletRequest req,
HttpServletResponse res) throws IOException {
PrintWriter htmlOut = res.getWriter();
res.setContentType("text/html");
htmlOut.println("<html><head><title>" +
"Servlet Example Output</title></head><body>" +
"<p>Hello!</p>" + "</body></html>");
htmlOut.close();
}
}
Servlet Example 2
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HelloWorld extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
PrintWriter out = response.getWriter();
out.println("Hello World");
}
}
Servlet Configuration
• The web application configuration file, web.xml,
identifies servlets and defines a mapping from
requests to servlets
An identifying name for the servlet (appears twice)
<servlet>
<servlet-name>HelloServlet</servlet-name>
<servlet-class>servlet.HelloServlet</servlet-
class>
The servlet's package
and class names
</servlet>
<servlet-mapping>
<servlet-name>HelloServlet</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
The pathname used to invoke the servlet
(relative to the web application URL)
Environment Entries
• Servlets can obtain configuration information at run-
time from the configuration file (web.xml)
• a file name, a database password, etc.
• in web.xml:
<env-entry-description>password</env-entry-
description>
<env-entry>
<env-entry-name>UserId</env-entry-name>
<env-entry-value>Xy87!fx9*</env-entry-value>
<env-entry-type>java.lang.String</env-entry-type>
</env-entry>
Environment Entries
• in the init() method of the servlet:
try {
Context envCtx = (Context)
(new InitialContext()).lookup("java:comp/env");
password = (String) envCtx.lookup("password");
} catch (NamingException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
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