How it works…

Once we run the program, an HTTP server will start locally listening on port 8080. Opening http://localhost:8080 in a browser will display Hello World! from the server, as shown in the following screenshot:

Let’s understand what each line in the program means:

• package main: This defines the package name of the program.
• import ( "fmt" "log" "net/http" ): This is a preprocessor command that tells the Go compiler to include all files from fmt, log, and the net/http package.

• const ( CONN_HOST = "localhost" CONN_PORT = "8080" ): We declare constants in the Go program using the const keyword. Here we declared two constants—one is CONN_HOST with localhost as a value and another one is CONN_PORT with 8080 as a value.
• func helloWorld(w http.ResponseWriter, r *http.Request) { fmt.Fprintf(w, "Hello World!") }: This is a Go function that takes ResponseWriter and Request as an input and writes Hello World! on an HTTP response stream.

Next, we declared the main() method from where the program execution begins, as this method does a lot of things. Let’s understand it line by line:

• http.HandleFunc("/", helloWorld): Here, we are registering the helloWorld function with the / URL pattern using HandleFunc of the net/http package, which means helloWorld gets executed, passing (http.ResponseWriter, *http.Request) as a parameter to it whenever we access the HTTP URL with pattern /.
• err := http.ListenAndServe(CONN_HOST+":"+CONN_PORT, nil): Here, we are calling http.ListenAndServe to serve HTTP requests that handle each incoming connection in a separate Goroutine. ListenAndServe accepts two parameters—server address and handler. Here, we are passing the server address as localhost:8080 and handler as nil, which means we are asking the server to use DefaultServeMux as a handler.
• if err != nil { log.Fatal("error starting http server : ", err) return}: Here, we check whether there is a problem starting the server. If there is, then log the error and exit with a status code of 1.