Solution for no output from digestBuilder in Eclipse siteOptimizer

The article Update Site Optimization provides instructions for creating optimized update sites in Eclipse to speed up downloads. While following those instructions, you may run into a problem that the digestBuilder component seems to run (displaying lines such as "Extracting locales from feature jar" and "Processing... feature jar"), but no output is ever written to the specified digestOutputDir. Alternatively, digest.zip might be created, but missing some features.

Solution: make sure that each of your feature JAR files contains a file named feature.properties (even an empty one). As of Eclipse 3.4, features without this file are skipped altogether by the digestBuilder.

Further reading: Debugging Eclipse helper applications and the tutorial on how to optimize an Eclipse update site for P2.