How Does Go's `ServeHTTP` Method Enable Custom HTTP Handlers?

Understanding ServeHTTP: How Does This Code Work?

In Go, implementing the ServeHTTP method for a custom type makes that type compatible with the Handler interface, an essential part of creating HTTP handlers. This enables a custom type to handle incoming HTTP requests.

Let's explore the code in question:

package main

import (
    "fmt"
    "net/http"
)

type foo int

func (m foo) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintln(w, "Some text")
}

func main() {
    var bar foo
    http.ListenAndServe(":8080", bar)
}

How does it work?

1. When http.ListenAndServe is invoked with bar as an argument, it creates an HTTP server and registers the custom handler, foo's ServeHTTP method, to handle incoming requests.
2. When a request is received, the server invokes the ServeHTTP method of the registered handler, which is foo's ServeHTTP method in this case.
3. Inside the ServeHTTP method, the HTTP request and response are handled. In this example, it writes "Some text" to the response writer.

How is the ServeHTTP function accessed?

The ServeHTTP function is accessed through the implementation of Handler interface in foo's custom type. By implementing ServeHTTP, foo signals its readiness to handle HTTP requests, and the HTTP package recognizes it as a valid handler.

HandlerFunc

The HandlerFunc type and its associated func syntax is a helper that makes it easy to define anonymous functions as HTTP handlers without creating a custom type as in foo. The HandlerFunc ServeHTTP method simply invokes the anonymous function with the appropriate arguments.

In essence, the code demonstrates how implementing the ServeHTTP method allows custom types to handle HTTP requests, where the server responsible for listening on a network port dispatches incoming requests to these handlers.

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