Argument extractors

Injecting stuff into your controller - method scope

Ninja can inject a lot of stuff into your methods by default. For instance variable parts of routes via @PathParam. Or query / form parameters via @Param and @Params.

Argument extractors allow you to use the very same mechanism (annotations of method parameters) to inject inject arbitrary things into the method of a controller.

This allows you to process a request, extract things (like a user currently logged in), and provide this via an annotation.

Multibinder<ParamParser> parsersBinder = Multibinder.newSetBinder(binder(), ParamParser.class);
parsersBinder.addBinding().to(ParamParsers.EmptyStringParamParser.class);

Multibinder<ParamParser> parsersBinder = Multibinder.newSetBinder(binder(), ParamParser.class);
parsersBinder.addBinding().to(MyParamParser.class);

How to write an argument extractor

Let's write an argument extractor that lets us inject the user currently logged in into our method via an annotation. At the end our controller should look like:

package controllers;

@Singleton
public class ApplicationController {

    public Result index(
            @LoggedInUser String loggedInUser) {

        //do something with the parameters...
    }

}

The @LoggedInUser is a so called argument extractor and allow you to extract arbitrary things out of the request and re-package them into anything you want. @LoggedInUser for instance determines the user and injects the username into the field annotated with @LoggedInUser.

Argument extractors consist of two things: A marker interface and an implementation.

First the marker interface:

@WithArgumentExtractor(LoggedInUserExtractor.class)
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.PARAMETER})
public @interface LoggedInUser {}

… and the argument extractor itself:

public class LoggedInUserExtractor implements ArgumentExtractor<String> {

    @Override
    public String extract(Context context) {
        
        // if we got no session we break:
        if (context.getSession() != null) {
            
            String username = context.getSession().get("username");
            
            return username;
            
        }
        
        return null;
    }

    @Override
    public Class getExtractedType() {
        return String.class;
    }

    @Override
    public String getFieldName() {
        return null;
    }
}

As you can see the interface of the argument extractor references @WithArgumentExtractor(LoggedInUserExtractor.class). LoggedInUserExtractor itself has full access to the context (and the incoming request) and will return the object annotated initially by @LoggedInUser. In our example this was @LoggedInUser String loggedInUser.

public class LoggedInUserExtractor implements ArgumentExtractor<String> {

    @Inject
    public LoggedInUserExtractor(Context context) {}

    @Inject
    UserDao userDao;
    
    ...
    
}