Writing the Servlet

• Servlets must subclass the java.servlet.GenericServlet class and override the service(ServletRequest, ServletResponse) method.

• Alternatively, the http-specific servlets can subclass the sun.server.http.HttpServlet class and override the service(HttpServletRequest, HttpServletResponse) method.

• init and init(ServletStub) methods are provided in GenericServlet class. If you are implementing the java.servlet.Servlet interface directly, the init method must be implemented.

Configuring the Server

• The servlet.properties file must be edited to map the servlet name to a class. This class must be locatable in the CLASSPATH environment of the server.

  For example, to associate the class mypackage.FooServlet with the name foo, add the following line to servlet.properties:

  foo.code=mypackage.FooServlet

• The rules.properties must be edited to map the requested URL into a servlet name.

  Continuing with the example above, to map the request URL http://<host>/foo.html to the servlet named foo, add the following line to rules.properties:

  /foo.html=foo

• Servlets may dynamically loaded into the server at runtime through the admin tool, which can be found at http://<server-host-name>/admin/admin.html. Note: Servlets loaded in this manner will not be loaded automatically on server restart. You must edit the configuration files to make persistant changes.

Invoking a Servlet

• Servlets can then be invoked by either requesting the html file that is mapped to a servlet name via the rules.properties file or by requesting the servlet by name after prefixing it with "servlet/"

  For example, the servlet foo mentioned above could be accessed by either of the following URLs:

  http://<host>/foo.html
  http://<host>/servlet/foo

• Server side include files (files with a .shtml extension), are parsed by the server, which invokes any servlets referenced in the HTML. The results are returned from the servlet are embedded into the HTML file, which is the returned to the requestor.

  The syntax for the servlet tag is:

  <servlet name=ServletName code=ServletCode.class 
  initParam1=initArg1 initParam2=initArg2 ...>
  <param name=param1 value=val1>
  <param name=param2 value=val2>
  .
  .
  .
  </servlet>
  

  The server first trys to invoke the servlet with the name referenced by the name field. If this fails or if no name is provided it attempts to load the servlet based on the code field, passing any remaining fields within the <servlet> tag to the servlet as initialization arguments. If the server loads the servlet and a name was specified, the server keeps the servlet loaded so that the next time the servlet is accessed it is not reloaded. If no name is specified, the server reloads the servlet each time it is accessed.

  Once the server has a handle to the servlet, either by referencing it from the name or loading it, it calls service on the servlet. The name value pairs specified in the html file with the <param> tag are accessible to the servlet using the standard getParameter and getParameters methods on the ServletRequest object passed to the servlet in service.

  Everything the servlet writes to ServletResponse.getOutputStream() gets written out to the client as part of the document the client requested. The servlet's output appears in the document at the location where the servlet tag was embedded.

Servlet Examples