Servlets intro

Servlets

Static Pages

request

response

Web
browser
Web server

Dynamic Pages

request request
Servlet
Servlet
JSP
JSP
response response
ASP
ASP
Web
browser
Web server

What is a Servlet?

Servlets are Java programs that can be run
dynamically in a Web Server
Servlets are a server-side technology
A Servlet is an intermediating layer between
an HTTP request of a client and the Web
server

What do Servlets do?

• Read data sent by the user (e.g., form data)
• Look up other information about request in the HTTP
request (e.g. authentication data, cookies, etc.)
• Generate the results (may do this by talking to a database,
file system, etc.)
• Format the results in a document (e.g., make it into HTML)
• Set the appropriate HTTP response parameters (e.g.
cookies, content-type, etc.)
• Send the document to the user

Supporting Servlets
The Web server must support Servlets (since it must run the
Servlets):
Apache Tomcat
Sun’s Java System Web Server and Java System
Application Server
IBM's WebSphere Application Server
Allaire Jrun – an engine that can be added to IIS, PWS,
old Apache Web servers etc…
Oracle Application Server
BEA WebLogic

The Servlet Interface
Java provides the interface Servlet
Specific Servlets implement this interface
Whenever the Web server is asked to invoke a
specific Servlet, it activates the method service() of
an instance of this Servlet

MyServlet
(HTTP)
response
service(request,response)
(HTTP)
request

Servlet Hierarchy

service(ServletRequest,
Servlet ServletResponse)

Generic Servlet
doGet(HttpServletRequest ,
HttpServletResponse)
HttpServlet doPost(HttpServletRequest
HttpServletResponse)
doPut
YourOwnServlet
doTrace
…

Class HttpServlet

Class HttpServlet handles requests and
responses of HTTP protocol
The service() method of HttpServlet checks
the request method and calls the appropriate
HttpServlet method:
doGet, doPost, doPut, doDelete, doTrace,
doOptions or doHead
This class is abstract

Creating a Servlet
Extend the class HTTPServlet
Implement doGet or doPost (or both)
Both methods get:
HttpServletRequest: methods for getting form
(query) data, HTTP request headers, etc.
HttpServletResponse: methods for setting HTTP
status codes, HTTP response headers, and get an
output stream used for sending data to the client
Usually implement doPost by calling doGet, or
vice-versa

Returning HTML
By default a text response is generated
(text/plain)
In order to generate HTML
Tell the browser you are sending HTML, by
setting the Content-Type header (text/html)
Modify the printed text to create a legal HTML
page
You should set all headers before writing the
document content.

import java.io.*;
import javax.servlet.*; import javax.servlet.http.*;

public class HelloWorld extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse
res) throws ServletException, IOException {
res.setContentType("text/html");
PrintWriter out = res.getWriter();

out.println("<HTML><HEAD><TITLE>Hello
World</TITLE></HEAD>");
out.println(“<BODY><H1>Hello World
</H1></BODY></HTML>");
out.close();
}
}

Configuring the Server
After you code your servlet, you need to compile it
to generate class files.
When your Servlet classes are ready, you have to
configure the Web server to recognize it.
This includes:
Telling the Server that the Servlet exists
Placing the Servlet class file in a place known to the
server
Telling the server which URL should be mapped to
the Servlet
These details are server specific.

The Big Picture
J2EE Web Component (WAR file)
J2EE Enterprise Application (EAR file)

.class
.class
.html
.jpeg .jar

.jsp

Getting Information
From the Request

Getting HTTP Data

Values of the HTTP request can be accessed
through the HttpServletRequest object
Get the value of the header hdr using
getHeader("hdr") of the request argument
Get all header names: getHeaderNames()
Methods for specific request information:
getCookies, getContentLength, getContentType,
getMethod, getProtocol, etc.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

public class ShowRequestHeaders extends HttpServlet {

public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html");
PrintWriter out = response.getWriter();
String title = "Servlet Example:
Showing Request Headers";

out.println("<HTML><HEAD><TITLE>" + title +
"</TITLE></HEAD>" +
"<BODY BGCOLOR="#AACCAA" TEXT="#990000">n" +
"<H1 ALIGN=CENTER>" + title + "</H1>n" +
"<B>Request Method: </B>" +
request.getMethod() + "<BR>n" +
"<B>Request URI: </B>" +
request.getRequestURI() + "<BR>n" +
"<B>Request Protocol: </B>" +
request.getProtocol() + "<BR><BR>n" +
"<TABLE BORDER=1 ALIGN=CENTER>n" +
"<TR BGCOLOR="#88AA88">n" +
"<TH>Header Name<TH>Header Value");

Enumeration headerNames = request.getHeaderNames();

while(headerNames.hasMoreElements()) {
String headerName =(String)headerNames.nextElement();
out.println("<TR><TD>" + headerName);
out.println("<TD>“ + request.getHeader(headerName));
}
out.println("</TABLE>n</BODY></HTML>");
}

public void doPost(HttpServletRequest request,
doGet(request, response);
}
}

User Input in HTML
Using HTML forms, we can pass
parameters to web applications
<form action=… method=…> …</form>
comprises a single form
• action: the address of the application to which
the form data is sent
• method: the HTTP method to use when
passing parameters to the application (e.g.
GET or POST)

GET Example
<HTML>
<FORM method="GET"
action="http://www.google.com/search">
<INPUT name="q" type="text">
<INPUT type="submit">
<INPUT type="reset">
</FORM>
</HTML>

http://www.google.com/search?q=servlets

POST Example
<HTML> <FORM method=“POST“
action="http://www.google.com/search">
<INPUT name=“q" type="text">
<INPUT type="submit">
<INPUT type="reset">
</FORM> </HTML>
POST /search HTTP/1.1
Host: www.google.com
…
Content-type: application/x-www-form-urlencoded
Content-length: 10
<empty-line>
q=servlets

Getting the Parameter Values

To get the value of a parameter named x:
req.getParameter("x")
where req is the service request argument
If there can be multiple values for the
parameter:
req.getParameterValues("x")
To get parameter names:
req.getParameterNames()

<HTML>
<HEAD>
<TITLE>Sending Parameters</TITLE>
</HEAD>
<BODY BGCOLOR="#CC90E0">
<H1 ALIGN="LEFT">Please enter the parameters</H1>

<FORM ACTION=“SetColors”
METHOD=“GET”>
<TABLE>
<TR><TD>Background color:</TD>
<TD><INPUT TYPE="TEXT" NAME="bgcolor"></TD></TR>
<TR><TD>Font color:</TD>
<TD><INPUT TYPE="TEXT" NAME="fgcolor"></TD></TR>
<TR><TD>Font size:</TD>
<TD><INPUT TYPE="TEXT" NAME="size"></TD></TR>
</TABLE> <BR>
<INPUT TYPE="SUBMIT" VALUE="Show Page">
</FORM>
</BODY>
</HTML>

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class SetColors extends HttpServlet {
public void doGet(HttpServletRequest request,

response.setContentType("text/html");
PrintWriter out = response.getWriter();
String bg = request.getParameter("bgcolor");
String fg = request.getParameter("fgcolor");
String size = request.getParameter("size");

out.println("<HTML><HEAD><TITLE>Set Colors Example" +
"</TITLE></HEAD>");
out.println("<BODY text='" + fg +
"' bgcolor='" + bg + "'>");
out.println("<H1>Set Colors Example</H1>");
out.println("<FONT size='" + size + "'>");
out.println("You requested a background color " +
bg + "<P>");
out.println("You requested a font color " +
fg + "<P>");
out.println("You requested a font size " +
size + "<P>");
out.println("</FONT></BODY></HTML>");
}
}

Handling Post

You don't have to do anything different to
read POST data instead of GET data!!
<FORM ACTION=“SetColors”
METHOD=“POST”> …

public void doPost(HttpServletRequest request,

doGet(request, response);
}

Creating the
Response of the
Servlet

Setting the Response Status

Use the following HttpServletResponse
methods to set the response status:
- setStatus(int sc)
Use when there is no error, like 201 (created)
- sendError(sc), sendError(sc, message)
Use in erroneous situations, like 400 (bad request)
The server may return a formatted message
- sendRedirect(String location)
Redirect to the new location

Setting the Response Status

Class HTTPServletResponse has static
integer variables for popular status codes
for example:
SC_OK(200), SC_NOT_MODIFIED(304),
SC_UNAUTHORIZED(401),
SC_BAD_REQUEST(400)
Status code 200 (OK) is the default

Setting Response Headers
Use the following HTTPServletResponse
methods to set the response headers:
- setHeader(String hdr, String value),
setIntHeader(String hdr, int value)
Override existing header value
- addHeader(String hdr, String value),
addIntHeader(String hdr, int value)
The header is added even if another header
with the same title exists

Specific Response Headers
Class HTTPServletResponse provides
setters for some specific headers:
- setContentType
- setContentLength
automatically set if the entire response fits
inside the response buffer
- setDateHeader
- setCharacterEncoding

More Header Methods

• containsHeader(String header)
Check existence of a header in the response
• addCookie(Cookie)
• sendRedirect(String url)
automatically sets the Location header
Do not write information to the response
after sendError or sendRedirect

Reference
Representation and Management of Data on the Internet (67633),
Yehoshua Sagiv, The Hebrew University - Institute of Computer
Science.

Servlets intro

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