Java Servlet Programming under Ubuntu Linux by Tushar B Kute

Programming Simple Servlet
under Ubuntu GNU/Linux
Tushar B Kute
Nashik Linux Users Group
tushar@tusharkute.com

What can you build with Servlets?
• Search Engines
• E-Commerce Applications
• Shopping Carts
• Product Catalogs
• Intranet Applications
• Groupware Applications:
– bulletin boards
– file sharing

Servlets vs. CGI
• A Servlet does not run
in a separate process.
• A Servlet stays in
memory between
requests.
• A CGI program needs to
be loaded and started
for each CGI request.
• There is only a single
instance of a servlet
which answers all
requests concurrently.
Browser 1
Web
Server
Browser 2
Browser N
Perl 1
Perl 2
Perl N
Browser 1
Web
Server
Browser 2
Browser N
Servlet

• Performance
– The performance of servlets is superior to CGI because there is
no process creation for each client request.
– Each request is handled by the servlet container process.
– After a servlet has completed processing a request, it stays
resident in memory, waiting for another request.
• Portability
– Like other Java technologies, servlet applications are portable.
• Rapid development cycle
– As a Java technology, servlets have access to the rich Java
library that will help speed up the development process.
• Robustness
– Servlets are managed by the Java Virtual Machine.
– Don't need to worry about memory leak or garbage collection,
which helps you write robust applications.
• Widespread acceptance
– Java is a widely accepted technology.
Benefits of Java Servlets

• A servlet is a Java class that can be
loaded dynamically into and run by a special
web server.
• This servlet-aware web server, is known as
servlet container.
• Servlets interact with clients via a
request-response model based on HTTP.
• Therefore, a servlet container must support
HTTP as the protocol for client requests and
server responses.
• A servlet container also can support similar
protocols such as HTTPS (HTTP over SSL) for
secure transactions.
Definitions

Browser HTTP
Server
Static
Content
Servlet
Container
HTTP Request
HTTP Response
Servlet
Servlet Container Architecture

Receive
Request
is servlet
loaded?
is servlet
current?
Send
Response
Process Request
Load Servlet
Yes
Yes
No
No
How Servlets Work

Servlet APIs
• Every servlet must implement javax.servlet.Servlet
interface
• Most servlets implement the interface by extending
one of these classes
–javax.servlet.GenericServlet
–javax.servlet.http.HttpServlet

Generic Servlet & HTTP Servlet
GenericServletGenericServlet
service ( )Server
ClientClient
HTTPServletHTTPServlet
service ( )HTTP
Server
BrowserBrowser
request
response
doGet( )
doPost( )
request
response

Interface javax.servlet.Servlet
• The Servlet interface defines methods
– to initialize a servlet
– to receive and respond to client requests
– to destroy a servlet and its resources
– to get any startup information
– to return basic information about itself, such as its
author, version and copyright.
• Developers need to directly implement
this interface only if their servlets
cannot (or choose not to) inherit
from GenericServlet or HttpServlet.
Life
Cycle
Methods

• void init(ServletConfig config)
– Initializes the servlet.
• void service(ServletRequest req,
ServletResponse res)
– Carries out a single request from the client.
• void destroy()
– Cleans up whatever resources are being held (e.g., memory, file
handles, threads) and makes sure that any persistent state is
synchronized with the servlet's current in-memory state.
• ServletConfig getServletConfig()
– Returns a servlet config object, which contains any initialization
parameters and startup configuration for this servlet.
• String getServletInfo()
– Returns a string containing information about the servlet, such as its
author, version, and copyright.
GenericServlet - Methods

Initialization
init()
Service
service()
doGet()
doPost()
doDelete()
doHead()
doTrace()
doOptions()
Destruction
destroy()
Concurrent
Threads
of Execution
Servlet Life Cycle

HttpServlet - Methods
• void doGet (HttpServletRequest request,
HttpServletResponse response)
–handles GET requests
• void doPost (HttpServletRequest request,
–handles POST requests
• void doPut (HttpServletRequest request,
–handles PUT requests
• void doDelete (HttpServletRequest request,
– handles DELETE requests

Servlet Request Objects
• provides client request information to a servlet.
• the servlet container creates a servlet request object and
passes it as an argument to the servlet's service method.
• the ServletRequest interface define methods to retrieve
data sent as client request:
–parameter name and values
– attributes
– input stream
• HTTPServletRequest extends the ServletRequest
interface to provide request information for HTTP
servlets

HttpServletRequest - Methods
Enumeration getParameterNames()
an Enumeration of String objects, each String
containing the name of a request parameter; or an
empty Enumeration if the request has no
parameters
java.lang.String[] getParameterValues (java.lang.String name)
Returns an array of String objects containing all of
the values the given request parameter has, or
null if the parameter does not exist.
java.lang.String getParameter (java.lang.String name)
Returns the value of a request parameter as a
String, or null if the parameter does not exist.

HttpServletRequest - Methods
Cookie[] getCookies()
Returns an array containing all of the Cookie objects
the client sent with this request.
java.lang.String getMethod()
Returns the name of the HTTP method with whichthi
request was made, for example, GET, POST, or PUT.
java.lang.String getQueryString()
Returns the query string that is contained in the
request URL after the path.
HttpSession getSession()
Returns the current session associated with this
request, or if the request does not have a session,
creates one.

Servlet Response Objects
• Defines an object to assist a servlet in
sending a response to the client.
• The servlet container creates a
ServletResponse object and passes it as
an argument to the servlet's service
method.

HttpServletResponse - Methods
java.io.PrintWriter getWriter()
Returns a PrintWriter object that can send
character text to the client
void setContentType (java.lang.String type)
Sets the content type of the response being sent
to the client. The content type may include the
type of character encoding used, for example,
text/html; charset=ISO-8859-4
int getBufferSize()
Returns the actual buffer size used for the
response

• Create a directory structure under Tomcat for your
application.
• Write the servlet source code.
• Compile your source code.
• deploy the servlet
• Run Tomcat
• Call your servlet from a web browser
Steps to Running a Servlet

Installation of Apache Tomcat7

• Servlets implement the
javax.servlet.Servlet interface.
• Because most servlets extend web servers
that use the HTTP protocol to interact
with clients, the most common way to
develop servlets is by specializing the
javax.servlet.http.HttpServlet class.
• The HttpServlet class implements the
Servlet interface by extending the
GenericServlet base class, and provides a
framework for handling the HTTP protocol.
• Its service() method supports standard
HTTP requests by dispatching each request
to a method designed to handle it.
Write the Servlet Code

Servlet Example
1: import java.io.*;
2: import javax.servlet.*;
3: import javax.servlet.http.*;
4:
5: public class MyServlet extends HttpServlet
6: {
7: protected void doGet(HttpServletRequest req,
8: HttpServletResponse res)
9: {
10: res.setContentType("text/html");
11: PrintWriter out = res.getWriter();
12: out.println( "<HTML><HEAD><TITLE> Hello You!” +
13: “</Title></HEAD>” +
14: “<Body> HelloWorld!!!</BODY></HTML>“ );
14: out.close();
16: }
17: }

An Example of Servlet (I)
Lines 1 to 3 import some packages which
contain many classes which are used by
the Servlet (almost every Servlet needs
classes from these packages).
The Servlet class is declared in line 5. Our
Servlet extends javax.servlet.http.HttpServlet,
the standard base class for HTTP Servlets.
In lines 7 through 16 HttpServlet's doGet
method is getting overridden

An Example of Servlet (II)
In line 12 we request a PrintWriter object to
write text to the response message.
In line 11 we use a method of the
HttpServletResponse object to set the content
type of the response that we are going to
send. All response headers must be set
before a PrintWriter or ServletOutputStream is
requested to write body data to the
response.
In lines 13 and 14 we use the PrintWriter to
write the text of type text/html (as specified
through the content type).

An Example of Servlet (III)
The PrintWriter gets closed in line 15 when
we are finished writing to it.
In lines 18 through 21 we override the
getServletInfo() method which is supposed to
return information about the Servlet, e.g.
the Servlet name, version, author and
copyright notice. This is not required for
the function of the HelloClientServlet but can
provide valuable information to the user of
a Servlet who sees the returned text in the
administration tool of the Web Server.

• Compile the Servlet class
•
•
• The resulting MyServlet.class file should go under
/var/lib/tomcat7/webapps/ROOT/WEB-INF/classes
Compile the Servlet

• In the Servlet container each
application is represented by a servlet
context
• each servlet context is identified by a
unique path prefix called context path
• The remaining path is used in the
selected context to find the specific
Servlet to run, following the rules
specified in the deployment descriptor.
Deploy the Servlet

• The deployment descriptor is a XML file
called web.xml that resides in the WEB-INF
directory within an application.
<web-app xmlns=http://java.sun.com/xml/ns/j2ee……>
<display-name>test</display-name>
<description>test example</description>
<servlet>
<servlet-name>Testing</servlet-name>
<servlet-class>TestingServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Testing</servlet-name>
<url-pattern>/servlet/TestingServlet</url-pattern>
</servlet-mapping>
</web-app>
Deployment Descriptor

Start / Restart the Servlet Web Engine, Tomcat7

• To execute your Servlet, type the following URL in
the Browser’s address field:
• http://localhost/MyServlet
Run the Servlet

• By default, servlets written by
specializing the HttpServlet class can
have multiple threads concurrently
running its service() method.
• If you would like to have only a single
thread running a service method at a
time, then your servlet should also
implement the SingleThreadModel
interface.
– This does not involve writing any extra methods, merely
declaring that the servlet implements the interface.
Running service() on a single thread

public class SurveyServlet extends HttpServlet
implements SingleThreadModel
{
/* typical servlet code, with no threading concerns
* in the service method. No extra code for the
* SingleThreadModel interface. */
...
}
Example

Thank You
http://snashlug.wordpress.com

Java Servlet Programming under Ubuntu Linux by Tushar B Kute

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