get
-classpath classpathlist
Specifies the paths where javadoc will look for referenced classes (.class files) -- these are the documented classes plus any classes referenced by those classes. The classpathlist can contain multiple paths by separating them with a semicolon (;). The Javadoc tool will search in all subdirectories of the specified paths. Follow the instructions in class path documentation for specifying classpathlist.
If -sourcepath is omitted, the Javadoc tool uses -classpath to find the source files as well as class files (for backward compatibility). Therefore, if you want to search for source and class files in separate paths, use both -sourcepath and -classpath.
For example, if you want to document com.mypackage, whose source files reside in the directory C:/user/src/com/mypackage, and if this package relies on a library in C:/user/lib, you would specify:
javadoc -classpath /user/lib -sourcepath /user/src com.mypackage
As with other tools, if you do not specify -classpath, the Javadoc tool uses the CLASSPATH environment variable, if it is set. If both are not set, the Javadoc tool searches for classes from the current directory.
For an in-depth description of how the Javadoc tool uses -classpath to find user classes as it relates to extension classes and bootstrap classes, see How Classes Are Found.