Can Spoon deal the .class file

Spoon is a nice job, thanks for your contribution.
I have a `.java` file, and I can derive the FQN with the following code: (e.g., `StringBuffer` in ` StringBuffer tabs = new StringBuffer();` will be derived into `java.lang.StringBuilder`.
```
Launcher launcher = new Launcher();
launcher.getEnvironment().setPrettyPrintingMode(Environment.PRETTY_PRINTING_MODE.FULLYQUALIFIED);
launcher.addInputResource(java_file_path);
CtModel model = launcher.buildModel();

String output = model.getAllTypes()
.stream()
.map(Object::toString)
.collect(Collectors.joining("\n\n", "", "\n"));
return output;
```
But, when I input the **java_file_path** is the **.class** file, the return output is the **Null**. How can I fix it? Thank you. :)