Custom Java annotations to introduce method parameters and execute code

Question

Question

Basically, I want to create a custom annotation for methods. When used, a method parameter and a line of code is added.

The question is: Is this possible and how?

Example

I want to simplify the following method:

@GetMapping("/thing")
public ResponseEntity getThing(@CookieValue("Session-Token") String sessionToken) {
    User user = authenticator.authenticateSessionTokenOrThrow(sessionToken);
    ... // user is successfully authenticated, get the "thing" from the database
}

to

@GetMapping("/thing")
@User
public ResponseEntity getThing() {
    ... // user is successfully authenticated, get the "thing" from the database
}

How can I implement the custom annotation @User so that the above two methods behave in the exact same way? ^{For the sake of the example, please ignore the fact that the above code is for the Spring Boot framework.}

Answer 1

Say you have a class with methods like this

class MyHandlerClass implements Whatever {
    @GetMapping("/thing")
    @User
    public ResponseEntity getThing() {
        ... // user is successfully authenticated, get the "thing" from the database
    }

You can use annotation processing to generate a class like this

class AuthenticatingMyHandlerClass extends MyHandlerClass {

    @GetMapping("/thing")
    public ResponseEntity getThing(@CookieValue("Session-Token") String sessionToken) {
        User user = authenticator.authenticateSessionTokenOrThrow(sessionToken);
        ResponseEntity ret = super.getThing(sessionToken);
        doSomethingWith(ret);
        return ret;
    }

Then you use the generated class instead of the main class for handling requests and will have any code you added as well.