A required library class, Doclet, is not loaded. It's usually found in tools.jar in the JDK so make sure to add it (tools.jar) to your project's build path.
A few things cause this exception:
1) Check if you have all jars and if they're in the correct path.
2) Your classpath might be broken, you can define it in the command line with
java -cp yourClassPath or at your IDE if you're using one.