Recipe 3.5 Invoking a Servlet Without a web.xml Mapping

Problem

You want to request a servlet that does not have a servlet-mapping element in the web.xml deployment descriptor.

Solution

Use an invoker-style URL of the form http://www.mysite.org/mywebapp/servlet/com.jspservletcookbook.MyServlet.

Discussion

Some servlets may not have a path mapping in the web application's deployment descriptor. So how can a user request this servlet? What name and URL do they use?

Tomcat and other servlet containers provide a method for invoking servlets that are not mapped in web.xml. You can use a URL of the following form:

 http://www.mysite.org/mywebapp/servlet/<fully qualified class name of servlet>

A servlet with the class and package name of jspservletcookbook.MyServlet is invoked as http://www.mysite.org/mywebapp/servlet/jspservletcookbook.MyServlet. Ensure that the path segment following the name of your web application is /servlet/ and not /servlets/. If the servlet is stored in the default web application (generally at the top level of the servlet container), the URL for invoking it is http:// www.mysite.org/servlet/jspservletcookbook.MyServlet.

The web.xml file located in <Tomcat_install_directory>/conf includes this definition and mapping for the invoker servlet:

<servlet>
  <servlet-name>invoker</servlet-name>
  <servlet-class>org.apache.catalina.servlets.InvokerServlet</servlet-class>
  <init-param>
    <param-name>debug</param-name>
    <param-value>0</param-value>
  </init-param>
  <load-on-startup>2</load-on-startup>
</servlet>

<servlet-mapping>
  <servlet-name>invoker</servlet-name>
  <url-pattern>/servlet/*</url-pattern>
</servlet-mapping>

The invoker servlet can also be used to invoke the servlets that are registered in web.xml. These URLs look like http://www.mysite.org/cookbook/servlet/<RegisteredServletName>. For instance, imagine you have a servlet element like this:

<servlet>
  <servlet-name>myservlet</servlet-name>
  <servlet-class>jspservletcookbook.MyServlet</servlet-class>
</servlet>

Consider that the web application context path is /cookbook. If the Tomcat invoker servlet is enabled in this application, then this servlet can be invoked with its registered name at http://www.mysite.org/cookbook/servlet/myservlet.

In Tomcat 4.1.x, the invoker servlet mapping may be commented out inside of the <tomcat-installation-directory>/conf/web.xml file. The purpose of thus disabling the invoker is to ensure that servlets can be invoked using only the paths specified by the servlet-mapping elements in web.xml.

If a servlet is requested using the form http://www.mysite.org/myapp/servlet/<fully-qualified-classname>, rather than using the servlet's registered name, any initialization parameters provided for that servlet in the web.xml file are not available. Example 3-7 shows a registered servlet with init parameters.

Example 3-7. A registered servlet with init parameters

<servlet>
  <servlet-name>Weather</servlet-name>
  <servlet-class>home.Weather</servlet-class>
  <init-param>
    <param-name>region</param-name>
    <param-value>New England</param-value>
  </init-param>
</servlet>

Because it is the registered name of the servlet that has the region parameter assigned to it, only a request for that registered name (or a servlet path mapped to that name) triggers the region parameters. Accessing the servlet through its fully qualified name will not result in the region parameter being passed to the Weather servlet.

Recipe 3.5 Invoking a Servlet Without a web.xml Mapping

Problem

Solution

Discussion

Example 3-7. A registered servlet with init parameters

See Also