code: private File findSourceFile (final String filename)
    {
      final Collection <File> sourceRoots = this.nonGeneratedSourceRoots;
      for (final File sourceRoot : sourceRoots)
      {
        final File sourceFile = new File (sourceRoot, filename);
        if (sourceFile.exists ())
        {
          return sourceFile;
        }
      }
      return null;
    }
docs: Determines whether the specified source file is already present in any of the
  compile source roots registered with the current Maven project.
  
  @param filename
  The source filename to check, relative to a source root, must not be
  <code>null</code>.
  @return The (absolute) path to the existing source file if any,
  <code>null</code> otherwise.