Customizing "Not Found" (404) Handling with httprouter

When developing an API using the httprouter library, handling 404 (Not Found) responses is a crucial task. While the documentation mentions the possibility of handling 404s manually, implementing a custom handler can be challenging.

Understanding the NotFound Field

The httprouter.Router struct includes a field named NotFound, which is of type http.Handler. This means that the value for NotFound must implement the ServeHTTP method present in the http.Handler interface.

Creating a Custom "Not Found" Handler

To define your own custom handler, you can create a function with a signature matching the ServeHTTP method:

func MyNotFound(w http.ResponseWriter, r *http.Request) {
    // ... Custom handling logic
}

Converting the Function to a Handler

To convert your function to a value that implements the http.Handler interface, you can use the http.HandlerFunc() helper function:

router := httprouter.New()
router.NotFound = http.HandlerFunc(MyNotFound)

Manual Invocation of Custom Handler

If you wish to call your custom handler manually from within other handlers, provide the handler with a ResponseWriter and a *Request:

func ResourceHandler(w http.ResponseWriter, r *http.Request) {
    // ... Code to determine resource validity

    if !resourceExists {
        MyNotFound(w, r) // Manual invocation of custom handler
        return
    }

    // ... Resource exists, serve it normally
}

Conclusion

By following these steps, you can effectively customize the "Not Found" handling process in your httprouter-based API, ensuring that users receive appropriate responses when attempting to access non-existent resources.