JAVA_HOME does not point to the JDK

You installed java...

apt-get install default-jre

But not the JDK...

apt-get install default-jdk

This is by design. You cannot use ant's java.home (which is a java.lang.System property) interchangeably with how JAVA_HOME is set in the OS environment. You are probably trying to assert the location of the Java compiler with a fundamentally different value from a different property layer -- i.e. java.home (from Ant's Java internals) points to the Java Runtime Environment at <any_installed_java_pointed_to_by_ant>/jre while JDK_HOME (from the OS environment) is usually set to <DOWNLOADED_AND_INSTALLED_JAVA_DEVELOPMENT_KIT>.

See my question and answer here for more details: Where does Ant set its 'java.home' (and is it wrong) and is it supposed to append '/jre'?

The solution is to access the system environment property within Ant by using ${env.JAVA_HOME}. Specify which java to use explicitly in the Javac Task by setting the executable property to the javac path and the fork property to yes (see Ant's Javac Task Documentation). That way, it doesn't matter what Java environment Ant is running inside, the compiler is always clearly specified!

I know this question is old but the accepted answer does not work anymore and since this is the fist link on google search i'll tell how i solved this problem.

for eclipse using ubuntu:

go to Window->Preferences->Ant->Runtime->Select Ant_Home_Entries and click on add external jars then find in file explorer where your jdk is (default is in /usr/lib/jvm/) and in the lib folder of your jdk you will find the tool.jar. select this one and click apply.

try to build your project and things should work!

note: i hadn't used ant for a long time but needed it for ycsb couchbase workload generator (http://www.couchbase.com/wiki/display/couchbase/Load+Generator+Setup) if anyone is/was stuck on this.

Make JAVA_HOME variable point to a jdk installation, not jre.

You are referencing the runtime environment, not the development kit - it can't find the compiler because its not there.

From the line you posted, which states you have open-jdk you can just remove the jre at end:

export JAVA_HOME='/usr/lib/jvm/java-6-openjdk/'