Use Java Spark after filter to run custom action when a 404 happens

Question

I'll try to keep it short. Here's the problem I'm having while trying to understand Spark filters. I'm trying to create a simple app and one of the things that it should do is to create an error report every time the client is about to see an http error e.g. 404 or 500. Here's how my app looks like:

import static spark.Spark.*;

public class MyApp {
    public static void main(String[] args) {
        get("/hello", (req, res) -> "{\"status\":\"OK\"}");

        after((request, response) -> {
            if (response.raw().getStatus() == 404) {
                // here run the code that will report the error e.g. 
                System.out.println("An error has occurred!!");
            }
        });
    }
}

For some reason, the response parameter has its status attribute set to 0 when I'm checking if it's set to 404. The documentation says "after" filters are evaluated after each request and can read the request and read/modify the response so, I should be able to do it somehow (unless the docs are wrong).

Basically, I'm trying to intercept http errors using an after filter but when I try to check the response I don't get what I expect.

Has anyone an idea what would be a different way of doing the same thing or how to make this work?

Thanks.

Answer 1

I solved this one using wildcard routes. Instead of calling the after method, I added a route for each of the HTTP methods that binds the "*" route.

It's important to have them at the bottom of the your Main method so if no route gets resolved these ones always get triggered.

Here's an example:

import static spark.Spark.*;

public class MyApp {
    public static void main(String[] args) {
        get("/hello", (req, res) -> "{\"status\":\"OK\"}");

        get("*", (request, response) -> {
            System.out.println("404 not found!!");
            // email me the request details ...    
        );
    }
}

Answer 2

The preferred manner to achieve what you are looking for would look like this.

get("/hello", (request, response) -> {
    // look up your resource using resourceId in request/path/query
    // oh dear, resource not found
    throw new NotFoundException(resourceId);
});

exception(NotFoundException.class, (e, request, response) -> {
    response.status(404);
    response.body(String.format("Resource {%s} not found", e.getResourceId()));
    // doReporting here.
});

public class NotFoundException extends Exception {
    private final String resourceId;
    public NotFoundException(String resourceId) {
        this.resourceId = resourceId;
    }

    public String getResourceId() {
        return resourceId;
    }
}