How to add compile dependency to javadoc class path in gradle

Question

In gradle if I my project depends on another package, for example:

compile 'com.example:foo:0.82.2'

And in my javadoc, it references symbol in that package.

Then if I run javadoc task, it will say error: reference not found

I know I should add that package to class path, just like the following way to add android library

classpath += project.files(android.getBootClasspath().join(File.pathSeparator))

But how could I know the classpath for my dependency except add a hard coding path point to the gradle cache?