Bug 29755 Attach source does not allow one to select a folder... [build path]
When opening some .class that does not yet have a source attachment eclipse shows a text version of the interface defintion and a button for Attaching source. When pressing this button a window appaer where one can enter the name/location of a folder, zip or jar for the source. There are also a button for choosing it via a file selection dialog and this works fine for zip and jar files - but not for selecting a directory :( Workaround is just to enter the directory manually.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/SourceAttachmentBlock.java
}
		
		if (fFileVariablePath != null) {
			return fFileVariablePath.toFile().isDirectory();
		}
		return false;
	}
	
	private String getResolvedLabelString(String path, boolean osPath) {
		IPath resolvedPath= getResolvedPath(new Path(path));
		if (resolvedPath != null) {
			if (osPath) {
				return resolvedPath.toOSString();
			} else {
				return resolvedPath.toString();
			}
		}
		return ""; 
	}	
	
	private IPath getResolvedPath(IPath path) {
		if (path != null) {
			String varName= path.segment(0);
			if (varName != null) {
				IPath varPath= JavaCore.getClasspathVariable(varName);
				if (varPath != null) {
					return varPath.append(path.removeFirstSegments(1));
				}
			}
		}