How to access/read the init-params defined in the web.xml from Servlet/JSP
How To Access/Read The Init-Params Defined In The web.xml
From A Servlet/JSP
Date: 27/01/2004
This Document will help you understand:
• How to add init parameters to web.xml that
can be accessed in a servlet/JSP
• How to access/read init parameters defined in web.xml from a servlet/JSP
• How to run the sample servlet/JSP application
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.
JavaServer Pages (JSPs) on the other hand is an extension
of the Java Servlet technology. When executed, JSPs get compiled to servlets.
JavaServer Pages technology separates the user interface from content generation
enabling designers to change the overall page layout without altering the
underlying dynamic content.
What are servlet initialization parameters?
Initialization Parameters are name/value pair that specify
the initial values of the servlet/JSP variables. They can be used to customize
and control the behavior of a servlet or a JSP. For example, you can define
a parameter for a JNDI name of a data source or a parameter that can be initialized
to count the hits of the page.
The init parameters can be defined in the application's
Web Application Deployment Descriptor file - web.xml.
A servlet container uses a servlet configuration object ServletConfig
and, getInitParameter()
method to read the init parameters from the web.xml,
and passes the information to a servlet during initialization.
In case of the JSP, they can be read from jspInit()
method or directly in a Java scriplet using the implicit object: ServletConfig.
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
Oracle JDeveloper 10g or later ( JDeveloper is Oracle's Visual Java Development Tool and can be
downloaded from here)
OR
Oracle Application Server Containers for J2EE(OC4J)
9.0.3 or later. ( Download from here
)
Init parameters are added between the <init-param></init-param>
tags under the <servlet></servlet>
tags in the web.xml file. For example, the
following web.xml entries show the init parameters
for the servlet - ReadInitParametersServlet,
and the JSP - ReadInitParamJSP.
The init parameters defined for the example servlet are "emailHost"
and "webMaster". The init parameters defined for the example
JSP are "sys" and "master".
web.xml for Servlet/JSP
<web-app>
<!-- Init parameters for the servlet ReadInitParams --> <servlet>
<servlet-name>ReadInitParams</servlet-name>
<servlet-class>oracle.otnsamples.servlets.ReadInitParametersServlet</servlet-class>
<display-name>Read Initialization Parameters of the Servlet</display-name>
<init-param>
<param-name>emailHost</param-name>
<param-value>151.68.167.201</param-value>
</init-param>
<init-param>
<param-name>webMaster</param-name>
<param-value>Savitha Rajeev</param-value>
</init-param></servlet>
<!-- Init parameters for the JSP readInitParamJSP --> <servlet> <servlet-name>readInitParamJSP</servlet-name> <jsp-file>/web/readInitParamJSP.jsp</jsp-file> <init-param> <param-name>sys</param-name> <param-value>151.68.167.201</param-value> </init-param> <init-param> <param-name>master</param-name> <param-value>OTN Group IDC</param-value> </init-param> </servlet> <servlet-mapping> <servlet-name>readInitParamJSP</servlet-name> <url-pattern>/web/readInitParamJSP.jsp</url-pattern> </servlet-mapping>
</web-app>
Reading the Init Parameters
from the web.xml file
The ServletConfig
object provides a handle to the initialization parameters defined in web.xml.
ServletConfig's method: getInitParameter()
is used to retrieve the init parameters. The following code snippet illustrates
how to read the init parameters from the init()
and the service() methods of a servlet.
or
public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String host = getServletConfig().getInitParameter("emailHost"); String master = getServletConfig().getInitParameter("webMaster"); }
To retrieve all the init parameters, use Enumeration
and ServletConfig's getInitParmeterNames().
For example,
public void init(ServletConfig config) throws ServletException {
Enumeration params = config.getInitParameterNames();
// Print the init parameter names and values to the console
while (params.hasMoreElements()) {
String name = (String) params.nextElement();
System.out.println("Parameter Name: " + name +
" Value: " + config.getInitParameter(name));
}
}
Reading the init parameters from the web.xml file in
a JSP
The implicit object - ServletConfig
can be read either in the jspInit() method or
directly to retrieve the values of init parameters. Following is the code
snippet to retrieve the values from readInitParamJSP.jsp
readInitParamJSP.jsp
<%@ page language="java"%> <html> <head><title>Read Init params from a JSP</title></head> <body> <%! String emailHost = null; String webMaster = null; public void jspInit() { ServletConfig config = getServletConfig(); emailHost = config.getInitParameter("emailHost"); webMaster = config.getInitParameter("webMaster"); } %> <table border="1"> <tr><td>email server</td><td><%=emailHost%></td></tr> <tr><td>WebMaster</td><td><%=webMaster%></td></tr> </table>
<% // It can also be read directly from the implicit object - config %> <%=config.getInitParameter("sys")%><br><br> <%=config.getInitParameter("master")%> </body> </html>
Running the Sample Servlet Application
This section talks about how to get the servlets up
and running. The application downloadable Jar file demonstrates the way the
to access/read the init parameters defined in web.xml, from a servlet as well
as a JSP.
Running the Servlet sample application
in OC4J (EAR Deployment):
The sample application ReadInitParams.jar
can be downloaded from here. The Jar file extracts all the sample application
files to a folder called ReadInitParams. Go through
the following steps to deploy and run the servlet/JSP 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 ReadInitParams
folder, compile the servlet file ReadInitParametersServlet.java. javac src/oracle/otnsamples/servlets/*.java -d ./WEB-INF/classes
- This creates the ReadInitParametersServlet.class
file 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 initparams.war WEB-INF/* web/*.jsp
Create a EAR file containg the application.xml
file and WAR file using the JAR command again. jar -cvfM initparams.ear initparams.war META-INF
Start the OC4J server.
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, initparams.ear <earDeploymentName> is the deployment
name for the application, that is, initparams
Bind the web application to a context root using the
following command:
Wherelocalhost
is your machine where OC4J instance is running. 8888 is the default port
of OC4J.
The URLs will invoke the Servlet/JSP that display the
init parameters and its values in a HTML table.
Running the Servlet Sample Application
from Oracle JDeveloper 10g:
The sample application ReadInitParams.jar
can be downloaded from here. The Jar file extracts all the sample application
files to a folder called ReadInitParams. Go through
the following steps to run the servlet from JDeveloper.
From JDeveloper, open the ReadInitParameterServlet.jws workspace file. This
will open the corresponding project and source files of the application.
Compile the ReadInitParameterServlet.jpr.
project.
Click on the ReadInitParameterServlet.java
and run the file.
This will execute the servlet that displays the init
parameters and their values in a HTML table.
Click on the readInitParamJSP.jsp
and run the file.
This will execute the JSP that displays the init parameters
and their values in a HTML table.
What is the difference between the initialization parameters
under the <servlet></servlet> tags,
under the <context-param></context-param>
and under the <filter></filter> tags
in web.xml?
Context parameters are accessible to any servlet or JSP
in that web-app. The common examples are email server hostname, webmasters
address, some resource required by many parts of the system etc. The init-params
of a servlet are specific to that servlet, example: a value that the servlet
uses like an upload directory name, hit counter parameter etc..
In case of the init parameters under <filter></filter> tags,
as the name suggests, it is specific to that filter and is accessible only
in that filter.
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