Servlets provide a component-based platform-independent
method for building web-based applications, and are generally used to
generate dynamic web content. To put it simply, a servlet receives a
request from a client (the web browser, for instance), processes the
request, and returns the response to the client. For example, the response
can be a queried from a database that is displayed using HTML.
For more information about Servlets, refer to the Useful
References section later in this document.
What is a Servlet Request Redirection?
At times, outdated/old URLs need to be redirected to
new ones so that the user does not see an error when the URL is accessed.
In such situations, the URL redirection option allows us to forward
the request of that URL to the new location.
There are several instances in which the URL, through
which the application is accessed, needs to be changed:
When the document moves to a new URL
When the application is enhanced to include
more pages, like an error page or extra information pages
When the user accesses a servlet, differing
from the order of the transaction, for example, when the user goes
to the final order page before filling out the requisite order details
When the servlet requests are distributed
to other servers for load balancing.
In all such cases, the application can make use of different URL
redirection techniques that suit the application's behavior.
How can a Servlet Request be Redirected to another
URL?
URL redirection can be achieved by using:
The forward method of the javax.servlet.RequestDispatcher interface
The sendRedirect method of the javax.servlet.http.HttpServletResponse interface
The header properties of a few popular browsers
Prerequisites
To work your way through this HowTo, it is necessary
to have a basic understanding of the fundamental concepts of servlets,
and how to develop and deploy them to a web server.
Software Requirements
Oracle10g
JDeveloper 9.0.3 or later ( JDeveloper is Oracle's Visual Java Development
Tool and can be downloaded from here)
OR
- Oracle9i AS Container for
J2EE(OC4J) 9.0.3 or later (Download from here
)
As mentioned earlier, there are three different techniques
of URL redirection:
Redirection using javax.servlet.RequestDispatcher
interface methods
Redirection using sendRedirect method of the
javax.servlet.http.HttpServletResponse
interface
Redirection by setting the HTTP Response Header's
Refresh field
Next we'll look at each of these techniques in some
detail.
Redirection using javax.servlet.RequestDispatcher
Interface
The javax.servlet.RequestDispatcher
object can be obtained from:
A javax.servlet.ServletContext
interface - a string containing the relative path to the new resource
can be passed to the RequestDispatcher object
to redirect the original request to a new one
A javax.servlet.ServletRequest
interface - a string containing the relative
path to the new resource can be passed to the RequestDispatcher object to redirect the
original request to a new one
The getNamedDispatcher
method of the javax.servlet.ServletContext
interface - a string containing the name of the new resource can
be passed to the NamedDispatcher object
to redirect the original request to a new one
Let's now look at each of these methods
in some detail.
From a javax.servlet.ServletContext
Interface
......
public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ........ // Redirect the request to a new location - redirected.jsp page. RequestDispatcher rd = getServletContext().getRequestDispatcher("/redirected.jsp"); rd.forward(request, response); } ......
From a javax.servlet.ServletRequest
Interface
......
public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ........ // Redirect the request to a new location - redirected.jsp page. RequestDispatcher rd =request.getRequestDispatcher("/redirected.jsp"); rd.forward(request, response); } ......
From the getNamedDispatcher
Method of the javax.servlet.ServletContext
Interface
The new location, Servlets/JSP needs to be registered within
the web application deployment descriptor, that is, in the web.xml file.
......
public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ........ // Redirect the request to a new location - redirected // 'redirected' is the class generated for the JSP file called redirected.jsp RequestDispatcher rd =getServletContext().getNamedDispatcher("redirected"); rd.forward(request, response); } ...... ......
Redirection using
the sendRedirect Method of the javax.servlet.http.HttpServletResponse Interface In this method, the browser redirects the
request to the URL specified. The resource can be a totally different
URL or a JSP/Servlet file. Further, the method can accept absolute
or relative URLs.
......
public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ........ // Redirect the request to a new location - http://www.oracle.com response.sendRedirect("http://www.oracle.com");
} ......
Redirection by Setting
the HTTP Response Header's Refresh Field The header must be set before sending the
response body. This method is useful if it's needed to display a message
before redirecting the client to a different URL. The delay in seconds
can be set in the header properties that actually redirects the request
to go to the new URL in specified number of seconds. Following is
the code snippet:
res.setHeader("Refresh", wait in seconds
+ "; URL=" + new location);
Note that this header is not officially part
of HTTP 1.1. This extension, however, is supported by both Netscape
and Internet Explorer.
......
public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ........ public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // set the content type response.setContentType("text/html");
PrintWriter out = response.getWriter();
// Instruct the browser to wait 5 seconds before redirecting to a new URL // Here two ways are shown. It can be directed to a different URL altogether // or can also be directed to a servlet/JSP within the application. String mesg = "Hi!";
// output a message that notifies the user that the // browser is going to be redirected to a new page out.println("<HTML>"); out.println("<BODY>"); out.println("The page you requested is moved to a different location. "); out.println("Your browser will automatically take you<BR>"); out.println("to the new location in 5 seconds.<BR>"); out.println("If the browser does not take you to the new location, "); out.println("or you don't want to wait then,"); out.println(" <a href=\"../redirected.jsp?param1="+mesg+"\">Click Here</a><BR>"); out.println("</BODY>"); out.println("</HTML>"); } ......
Running the Sample Servlet Application
This section talks about how to get the servlets
up and running. The application downloadable Jar file has a set of three
servlet applications that demonstrate different redirection techniques.
Running the Servlet
Sample Application in OC4J (EAR Deployment):
The sample application ServletURLRedirection.jar
can be downloaded from here.
The Jar file extracts all the sample application files to a folder called
ServletURLRedirection. Go through the
following steps to deploy and run the servlet in OC4J.
Add servlet.jar
to the CLASSPATH; this file is present under <OC4J_HOME>/j2ee/home/lib folder. Note: <OC4J_HOME>
is the folder where the OC4J server is installed; for example, d:\oc4j
From the ServletURLRedirection
folder, compile the servlet files: Servlet1.java,
UsingHeaderRedirection.java, UsingRequestDispatcher.java, UsingSendRedirect.java
using the following command: javac src/oracle/otnsamples/servlets/*.java
-d ./WEB-INF/classes
- This creates the classes for the servlet files under the WEB-INF/classes/oracle/otnsamples/servlets folder.
Next, create a WAR file containing all the
servlet classes, JSPs, HTML files and the web.xml
file using the JAR command: jar -cvfM redirection.war WEB-INF/* web/*.jsp
web/*.html
Create a EAR file containg the
application.xml file and WAR file
using the JAR command again. jar -cvfM redirection.ear redirection.war
META-INF
Start the OC4J server.
Copy the EAR file: redirection.ear
to the <OC4J_HOME>/j2ee/home/applications
folder.
Deploy the servlet applications using the
command below:
java -jar <OC4J_HOME>/j2ee/home/admin.jar ormi://<server>:<rmiport>
admin <admin_password> -deploy -file <Sample_Name>.ear
-deploymentName <earDeploymentName>
Note: <server> is the IP Address of
the system where the OC4J instance is running. <rmiport> is the RMI port number
of the OC4J instance; default is 23791 <admin_password> is the password
of the OC4J installation <Sample_Name>.ear is the EAR
file created, that is, redirection.ear <earDeploymentName> is the deployment
name for the application, that is, redirection
Bind the web application to a context root
using the following command:
Here localhost is your
machine where OC4J instance is running. 8888 is the default port of
OC4J.
Each of the servlet thus invoked demonstrate
a different redirection technique.
Running the Servlet Sample
Application from Oracle JDeveloper9i:
The sample ServletURLRedirection.jar
application can be downloaded from here. The Jar file extracts all the
sample application files to ServletURLRedirectionfolder.
Go through the following steps to run the servlet from JDeveloper.
From JDeveloper, open the ServletURLRedirection.jws workspace file. This
will open the corresponding project and source files of the application.
Compile the ServletURLRedirection.jpr
project.
Click on UsingRequestDispatcher.java
and run the servlet. Similarly, you can run the other servlets - UsingSendRedirect and UsingHeaderRedirection
Each of the servlet thus invoked demonstrate
a different redirection technique.
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