I\'m using Ant 1.7.0 and installed java 1.6 which is in JAVA_HOME.
I want to build a project using java 1.5, so I\'ve exported JAVA_HOME to be my java 1.5 directory.
Set your JAVA_HOME environment variable with the required java version (in your case java 1.5), then in build.xml use executable="${JAVA_HOME}/bin/javac" inside tag .
JAVA_HOME
build.xml
executable="${JAVA_HOME}/bin/javac"
example: