Java Web Server 1.1 - Loading and Invoking Servlets

Loading and Invoking Servlets

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Loading Servlets

Servlets can be loaded from any one of these places:

From a directory that is on the CLASSPATH. The CLASSPATH of the Java^TM Web Server^TM includes <service_root>/classes/ which is where the system classes reside.
From the <service_root>/servlets/ directory. This is not in the server's classpath. A classloader is used to create servlets from this directory. New servlets can be added - existing servlets can be recompiled and the server will notice these changes.
From a remote location. For this a codebase like http://nine.eng/classes/foo/ is required in addition to the servlet's class name.

Loading Remote Servlets

Remote servlets can be loaded by:

configuring the Administration Tool to setup automatic loading of remote servlets
setting up server side include tags in .shtml files
defining a filter chain configuration

NOTE: Remote servlets can have security implications. For more information, see Servlet Sandbox.

Identifying Servlets

Servlets are identified by <servletName> which is either:

a virtual name that is assigned to the servlet by using the Adding Servlets section of the Administration Tool.
by its own class name if the servlet is dropped into the /servlets/ directory and invoked by its class name.

Invoking Servlets

A servlet invoker is a servlet that invokes the "service" method on a named servlet. If the servlet is not loaded in the server, then the invoker first loads the servlet (either from local disk or from the network) and then invokes the "service" method. Also like applets, local servlets in the server can be identified by just the class name. In other words, if a servlet name is not absolute, it is treated as local.

Servlets can be invoked by a client in the following ways:

The client can ask for a document that is served by the servlet.
The server gets a request for a document, looks up the configuration parameters, finds out that the document is not a static document residing on the disk, but a live document generated by a servlet object, and forwards the request to the servlet which services the request by generating the output. This method is the same as used by traditional web servers for invoking CGI scripts.
The client (browser) can invoke the servlet directly using a URL, once it has been mapped using the Servlet Aliases section of the administration GUI.
The client invokes the servlet with a URL of the form:
http://server_host_name/servlet/<servlet URL>

where the <servlet URL> is a regular URL that points to the location of the servlet. The host where the servlet resides might be different from the host where the server is running. In this case the servlet class will be dynamically downloaded to the server, instantiated, and then run. The reason this invocation is useful is that it does not need any special understanding of servlets on the part of the browser.
The servlet can be invoked through server side include tags.
Any file ending with .shtml is a server parsed file. In .shtml, if a servlet tag or server-insert tag is present, then the server will run the servlet and insert the output generated by the servlet in the place of the insert tag.
The servlet can be invoked by placing it in the servlets/ directory.
Servlets can be dropped in to the servlets/ directory relative to the root of the server. Servlets placed in this directory can be invoked using their class names. (For example, if HelloWorld.class was placed in the server_root/servlets directory):
http://server_host_name:8080/servlet/HelloWorld

A class_file.initArgs file can be placed in the same directory for passing init args to the servlet in question. For example, to pass two initial arguments A=B and C=D to the HelloWorld servlet, a HelloWorld.initArgs file must be created in the extensions directory whose syntax would be:
```
	A=B
	C=D
```
When Servlets are recompiled in this directory, the new version will be automatically loaded by the server.
The servlet can be invoked by using it in a filter chain.
This happens when a servlet is configured to be invoked when a particular mime type is set as the response. See the Filters and Servlet Chaining section for more on this topic.

NOTE: <servletName> is any valid servlet identifier as described in the previous section.

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