public static String getFileExtension(String resource)
    {
        // E.g uf foo.bar.java.stg, it is the . before "java"
        int secondToTheLastDot = resource.lastIndexOf('.', resource.length() - 5);
        if (secondToTheLastDot == -1)
        {
            throw new IllegalArgumentException("The resource must be named like: 'foo.type.stg' " +
                    "where '.type' will be the file extension of the output files.");
        }
        String extension = resource.substring(secondToTheLastDot, resource.length() - 4);
        // to protected against resources like "foo..stg"
        if (extension.length() < 2)
        {
            throw new IllegalArgumentException("The resource must be named like: 'foo.type.stg' " +
                    "where '.type' will be the file extension of the output files.");
        }

        return extension;
    }
Get the file extension of the provided stg resource.
protostuff-compiler/src/main/java/io/protostuff/compiler/PluginProtoCompiler.java