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Servlets by Example
Joe Howse
7 June 2011
What is a servlet?
● A servlet is a Java application that receives HTTP requests as input and
generates HTTP responses as output.
○ As the name implies, it runs on a server.
○ Dependency: A "servlet container" such as Jetty or Apache Tomcat.
● A servlet is not similar to an applet.
○ An applet's access to server-side files is unprivileged and slow
(requiring downloads) because the applet itself runs client-side.
● Let's consider a step-by-step approach to setting up a simple servlet:
1. Get and configure Jetty.
2. Create the necessary directory tree for the servlet.
3. Write a webpage as a front-end.
4. Write a config file.
5. Write, compile and deploy the Java code.
6. Test it!
Setting up Jetty
● Follow the instructions at:
○ http://docs.codehaus.org/display/JETTY/Quick+Start
○ At the download step, you want the latest stable version from:
■ http://download.eclipse.org/jetty/
● On Dalhousie's bluenose server, adapt the instructions as follows:
○ $JETTY_HOME should be some web-accessible folder.
■ ex. ~/public_html/jetty
○ You should choose an unused port number between 4001 and 4030.
■ I am using 4020 right now, so choose something else.
○ Your test servlet can be accessed at:
■ http://bluenose.cs.dal.ca:####/
■ (Here, #### should be replaced by your port number.)
Directory tree
$JETTY_HOME/webapps/MyServlet/
index.html
WEB-INF/
web.xml
classes/MyServlet/
*.class
*.java
build.sh
lib/
*.jar
● A servlet must include:
○ statically linked code (.class)
○ configuration (.xml)
● Optionally, it may include:
○ dynamically linked code (.jar)
○ front-end website scripts (.html, .
php, etc.) and content (.jpg, etc.)
● Just for convenience, we are putting our
source code (.java) and build script
(build.sh) inside the servlet directory
tree too.
○ These are not used at run-time.
index.html
<html><body>
<form method="get"
action="MyServlet">
<input type="text"
id="query"
name="query"
size="50"/>
<input type="submit"
value="Submit"/>
</form>
</body></html>
● Our webpage is an input
form that sends to our
servlet a get request with a
field called query.
● Our servlet is called
MyServlet.
web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<servlet>
<servlet-name>MyServlet</servlet-name>
<servlet-class>MyServlet.MyServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>MyServlet</servlet-name>
<url-pattern>/MyServlet</url-pattern>
</servlet-mapping>
<session-config>
<session-timeout>30</session-timeout>
</session-config>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
</welcome-file-list>
</web-app>
● Our web.xml config file specifies the servlet name, paths, etc.
MyServlet.java (1/2)
package MyServlet;
import
import
import
import
import
import
import
java.io.IOException;
java.io.PrintWriter;
javax.servlet.ServletException;
javax.servlet.http.HttpServlet;
javax.servlet.http.HttpServletRequest;
javax.servlet.http.HttpServletResponse;
javax.servlet.ServletConfig;
public class MyServlet extends HttpServlet {
// Initialize servlet and any back-end model it uses.
@Override public void init(ServletConfig config) throws ServletException {
super.init();
// Initialize back-end model here.
}
// Respond to GET request.
@Override protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
String query = request.getParameter("query");
out.println("<html><body>Your query was \"" + query + "\"!</body></html>");
}
// Continued on next slide. ...
MyServlet.java (2/2)
// ... Continued from previous slide.
// Respond to POST request.
@Override protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response); // Here, POST is handled the same as GET.
}
// Return description of servlet.
@Override public String getServletInfo() {
return "MyServlet is a minimal example, echoing back a query string.";
}
}
● Our MyServlet.java source file defines the mapping from requests to
responses.
● Our simple example could be extended to:
○ Handle errors.
○ Initialize and use an instance of another class as its back-end.
build.sh
#!/bin/sh
# Configuration variable.
# The path to Jetty.
JETTY_HOME=~/public_html/jetty
# Build command.
javac -cp $JETTY_HOME/lib/servlet-api-2.5.jar MyServlet.java
● Our build script compiles MyServlet.java and links it with the required
library in Jetty.
● Do not forget to set executable permission for the script.
○ chmod +x build.sh
Testing MyServlet
● Run the build script!
○ ./build.sh
● Navigate to $JETTY_HOME and run:
○ java -jar start.jar
● Direct your Web browser to:
○ http://bluenose.cs.dal.ca:####/MyServlet
■ (Here, #### should be replaced by your port number.)
Summing up
● Writing and deploying a minimal servlet involves just a few steps.
● The complexity is not much greater than scripting solutions, such as:
○ PHP, CherryPy (Python), Ruby on Rails (Ruby)
● A servlet can integrate seamlessly with other back-end Java code.
