There is a problem in AntBuilder - output from some tasks is not redirected to Ant, but is stilll printed out to STDERR.
Thus it is impossible to use Ant logging subsystem to redirect output to log file.
For instance, javac warnings are printed directly to console (actually, to STDERR).
Ant itself uses DemuxOutputStream for STDOUT and STDERR redirection.
AntBuilder uses DemuxInputStream for input handling, but it does not redirect output streams.