Sun Java System Web Server 6.1 SP6 NSAPI Programmer's Guide

Creating and Using Custom Filters

Custom filters are defined in shared libraries that are loaded and called by the server.

ProcedureTo create a custom filter

  1. Write the Source Code using the NSAPI functions.

  2. Compile and Link the source code to create a shared library ( .so, .sl, or .dll) file.

  3. Load and Initialize the Filter by editing the magnus.conf file.

  4. Instruct the Server to Insert the Filter by editing the obj.conf file to insert your custom filter(s) at the appropriate time.

  5. Restart the Server .

  6. Test the Filter by accessing your server from a browser with a URL that triggers your filter.

    These steps are described in greater detail in the following sections.

Write the Source Code

Write your custom filter methods using NSAPI functions. For a summary of the NSAPI functions specific to filter development, see Overview of NSAPI Functions for Filter Development Filter Methods for the filter method prototypes.

The filter must be created by a call to filter_create. Typically, each plug-in defines an nsapi_module_init function that is used to call filter_create and perform any other initialization tasks. See nsapi_module_init and filter_create for more information.

Filter methods are invoked whenever the server or an SAF calls certain NSAPI functions such as net_write or filter_insert. As a result, filter methods can be invoked from any thread and should only block using NSAPI functions (for example, crit_enter and net_read). If a filter method blocks using other functions (for example, the Windows WaitForMultipleObjects and ReadFile functions), the server may hang. Also, shared objects that define filters should be loaded with the NativeThread="no" flag, as described in Load and Initialize the Filter

If a filter method must block using a non-NSAPI function, KernelThreads 1 should be set in magnus.conf. For more information about KernelThreads, see the description in the chapter "Syntax and Use of magnus.conf" in the Sun Java System Web Server 6.1 SP6 Administrator’s Configuration File Reference.

Keep the following in mind when writing your filter:

Compile and Link

Filters are compiled and linked in the same way as SAFs. See Compile and Link

Load and Initialize the Filter

For each shared library (plug-in) containing custom SAFs to be loaded into the Sun Java System Web Server, add an Init directive that invokes the load-modules SAF to magnus.conf. The syntax for a directive that loads a filter plug-in is:

Init fn=load-modules shlib=[path]sharedlibname NativeThread="no"

Instruct the Server to Insert the Filter

Add an Input or Output directive to obj.conf to instruct the server to insert your filter into the filter stack. The format of the directive is as follows:

Directive fn=insert-filter filter="filter-name" [name1="value1"]...[nameN="valueN"]

<Object name="default">
Output fn=insert-filter
       from="Old String"
       to="New String"

Restart the Server

For the server to load your plug-in, you must restart the server. A restart is required for all plug-ins that implement SAFs and/or filters.

Test the Filter

Test your SAF by accessing your server from a browser. You should disable caching in your browser so that the server is sure to be accessed. In Netscape Navigator, you can hold the shift key while clicking the Reload button to ensure that the cache is not used. (Note that the shift-reload trick does not always force the client to fetch images from source if the images are already in the cache.) Examine the access and error logs to help with debugging.