Ant: Class not found: javac1.8

This error is known to happen with Apache Ant versions less than 1.9.0, which aren't compatible with Java 8.
Upgrade!
Worked for me.

Thanks to Kristian.

Building with Ant from Eclipse: "Specified VM install not found"

After an update of the JDK and successive removal of the old version, when I tried to build with Ant from Eclipse I got:

Specified VM install not found: type Standard VM, name jdk1.7.0_55

Yeah, right, OK, but where do I update the JDK that's used by Ant??
Here:
Run -> External Tools -> External Tools Configurations -> JRE

Thanks to Michael Borgwardt

Ant unable to locate javadoc.exe

The situation: you're launching an Ant build file from within Eclipse on Windoz and the file has a Javadoc task. But Eclipse can't find the executable.

What you need to do is add the JDK bin directory to the PATH environment variable.
There is nothing to configure in Eclipse.