For a Java servlet to be accessible from a browser, you must tell the
servlet container what servlets to deploy, and what URL's to map the servlets
to. This is done in the web.xml file of your Java web application.
Here is a list of the topics covered in this blog about the web.xml servlet
configuration:
- Configuring
and Mapping a Servlet
- Servlet
Init Parameters
- Servlet
Load-on-Startup
- Context
Parameters
Configuring and Mapping a Servlet
To configure a servlet in the web.xml file, you write this:
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app
PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd">
<web-app>
<servlet>
<servlet-name>controlServlet</servlet-name>
<servlet-class>com.jenkov.butterfly.ControlServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>controlServlet</servlet-name>
<url-pattern>*.html</url-pattern>
</servlet-mapping>
</web-app>
First you configure the servlet. This is done using the <servlet> element. Here you give
the servlet a name, and writes the class name of the servlet.
Second, you map the servlet to a URL or URL pattern. This is done in the <servlet-mapping> element. In the
above example, all URL's ending in .html
are sent to the servlet.
Other possible servlet URL mappings are:
/myServlet
/myServlet.do
/myServlet*
The * is a wild card, meaning any text. As you can see, you can either map a
servlet to a single, specific URL, or to a pattern of URL's, using a wild card
(*). What you will use depends on what the servlet does.
Servlet Init Parameters
You can pass parameters to a servlet from the web.xml file. The init
parameters of a servlet can only be accessed by that servlet. Here is how you
configure them in the web.xml file:
<servlet>
<servlet-name>controlServlet</servlet-name>
<servlet-class>com.jenkov.butterfly.ControlServlet</servlet-class>
<init-param>
<param-name>myParam</param-name>
<param-value>paramValue</param-value>
</init-param>
</servlet>
Here is how you read the init parameters from inside your servlet - in the
servlets init() method:
public class SimpleServlet extends GenericServlet {
protected String myParam = null;
public void init(ServletConfig servletConfig) throws ServletException{
this.myParam = servletConfig.getInitParameter("myParam");
}
public void service(ServletRequest request, ServletResponse response)
throws ServletException, IOException {
response.getWriter().write("<html><body>myParam = " +
this.myParam + "</body></html>");
}
}
A servlets init() method
is called when the servlet container loads the servlet for the first time. No
one can access the servlet until the servlet has been loaded, and the init() method has been called successfully.
Servlet Load-on-Startup
very useful and detailed explanation.
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