How to add compilation dependency to javadoc class path in gradle

Question

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

compile 'com.example: foo: 0.82.2'

And in my javadoc it refers to the character in this package.

Then, if I run the javadoc task, it will say an error: link not found

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

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

But how can I find out the class path for my dependency other than adding a hard coding point to the gradle cache?