Ant depend task

I am writing ruby helper class that will be used in Rakefile to build complex java application. Everything seem to be quite simple, I just copied what has been done by Matt Foemmel in his JRake. The ruby helper class has a method that check if java class is upto date by comparing access time of the class file and the java source file. I believe that Ant javac task does the same.
Using the ruby helper file, my java application most of time get build correctly but sometime, few class files are not upto date. It take me a while to figure out what is going wrong.
The problem is when a java class depends on other, if other class changes then the java file shall be recompiled even though it is not modified. Looking at Ant documentation, I found that Ant solve this problem using depend task.
In order to solve this problem in ruby, I have to do the same as Ant depend task does, read our class file, extract all class references verify if these class references are changed.