How to Handle 404 Errors with Custom Handlers in httprouter?

Handling 404 Errors with httprouter's Custom Handlers

In an HTTP API built with httprouter, handling 404 (Not Found) errors requires a custom handler. The documentation mentions this possibility, but it doesn't provide explicit instructions on how to create one.

Setting Up a Custom Handler

To handle 404 errors manually, follow these steps:

1. Define a function with the following signature:

   <code class="go">func(http.ResponseWriter, *http.Request)</code>

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2. Convert the function into an http.Handler using the http.HandlerFunc() helper function.

   <code class="go">func MyNotFound(w http.ResponseWriter, r *http.Request) {
       w.Header().Set("Content-Type", "text/plain; charset=utf-8")
       w.WriteHeader(http.StatusNotFound) // StatusNotFound = 404
       w.Write([]byte("My own Not Found handler."))
   
       // or with more detailed message
       w.Write([]byte(" The page you requested could not be found."))
   }</code>

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3. Assign the MyNotFound handler to the NotFound field of the httprouter:

   <code class="go">var router *httprouter.Router = ... // Your router value
   router.NotFound = http.HandlerFunc(MyNotFound)</code>

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Manually Invoking the Custom Handler

In your handlers, you can manually call the MyNotFound handler, if needed, by passing the ResponseWriter and *Request:

<code class="go">func ResourceHandler(w http.ResponseWriter, r *http.Request) {
    exists := ... // Find out if requested resource is valid and available
    if !exists {
        MyNotFound(w, r) // Pass ResponseWriter and Request
        // Or via the Router:
        // router.NotFound(w, r)
        return
    }
    // Resource exists, serve it
    // ...
}</code>

By implementing these steps, you can effectively handle 404 errors in your httprouter-based API and customize the behavior as needed.

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