Design Overview

In order to combine Java's threaded processing with Apache's forked processing mode, mod_gcj loads and runs Java code in a separate server process that hosts the GCJ runtime.

The GCJ server process is created by forking when the module is first activated, and then exec()ing an external program called mod-gcj-server located in the apache bin directory. The reason to put the GCJ server code into a separate executable is so that we can implement the module in plain C rather than C++ and get rid of any GCJ dependencies in the module itself.

The GCJ module is written in C, while the GCJ server consists of some C++ mostly for Unix domain socket communication, and is otherwise mostly written in Java. It is multithreaded using GCJ java threads.

The fact that the GCJ server is executing in a separate process is largely invisible to users/administrators:

• The server is started/restarted/stopped automatically by the Apache module

• All configuration is done from Apache configuration files using standard Apache directives

• Java handlers can decide to decline a request, allowing it to be handled by other Apache modules.

In the future it may be possible to interact with other Apache modules from Java code, implement Apache hooks in Java etc.

Credits

After coming up with this design blueprint, I found out that mod_mono uses pretty much the same approach to implement ASP.NET for Apache. Thanks go to the authors of mod_mono, Daniel Lopez Ridruejo and Gonzalo Paniagua Javuer, for allowing me to use their work to get started with mod_gcj. This has allowed me to get some useful code much faster than it would have been otherwise.