Bài giảng Web technologies and e-Services - Bài 7, Phần 1: Web development with Java - Trường Đại học Bách khoa Hà Nội
Web development with
Java
1
Outline
1. Servlet
2. JSP – Java Server Page
3. Java Beans
4. ORM (Object Relational Mapping)
2
Free Servlet and JSP Engines (Servlet/JSP Containers)
v Apache Tomcat
v IDE: NetBeans, Eclipse
v Some Tutorials:
§ Creating Servlet in Netbeans:
§ Creating Java Servlets With NetBeans:
§ Developing JSPs and Servlets with Netbeans:
Compiling and Invoking Servlets
v Put your servlet classes in proper location
§ Locations vary from server to server. E.g.,
• tomcat_install_dir/webapps/ROOT/WEB-INF/classes
v Invoke your servlets (HTTP request)
§ Custom URL-to-servlet mapping (via web.xml)
Java Servlets
v 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
v Together with web pages and other components,
servlets constitute part of a web application
v 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
v 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
Servlet
Container
for servlet execution
*Apache Tomcat can be both a web server and a servlet container
Environment For Developing and Testing Servlets
v Compile:
§ Need Servlet.jar. Available in Tomcat package
v Setup testing environment
§ Install and start Tomcat web server
§ Place compiled servlet at appropriate location
Servlet Operation
Servlet Methods
v 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
v The default .service() method simply invokes
method-specific methods
§ depending upon the HTTP request method
.doGet()
.doPost()
.doHead()
… etc.
.service()
HTTP Servlet
Methods of HttpServlet and HTTP requests
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
All methods take two arguments: an HttpServletRequestobject and an
HttpServletResponseobject.
Return a BAD_REQUEST (400) error by default.
Servlet Example 1
v 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
• This servlet also will say "Hello World" (not in
HTML)
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
v 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>
</servlet>
The servlet's package
and class names
<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
v Servlets can obtain configuration information at
run-time from the configuration file (web.xml)
§ a file name, a database password, etc.
v 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
v 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();
}
Handling HTML Forms
v An HTML form can be sent to a servlet for
processing
v The action attribute of the form must match the
servlet URL mapping
<form method="post" action="hello" />
<servlet-mapping>
<servlet-name>HelloServlet</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
Simple Form Servlet
<form action="hello" method="post" >
<p>User Id:<input type="text" name="userid" /></p>
<p><input type="submit" value="Say Hello" /></p>
</form>
public class HelloServlet extends HttpServlet {
public void doPost(HttpServletRequest req,
HttpServletResponse res) throws IOException {
PrintWriter out = res.getWriter();
res.setContentType("text/html");
String userId = req.getParameter("userid");
out.println("<html><head><title>Hello</title></head>"
+ "<body><p>Hello, " + userId
+ "!</p></body></html>");
out.close();
}
State Management
• session: a series of transaction between user and
application
• session state: the short-term memory that the application
needs in order to maintain the session
• e.g., shopping cart, user-id
• cookie: a small file stored by the client at the instruction
of the server
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