Basic Go Code Example: Simple HTTP Server

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This example demonstrates a basic HTTP server written in Go. The server listens on a specified port and responds with a simple message when accessed.

Code

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// Package main defines the entry point of the application.
package main

import (
	"fmt"      // For formatted I/O
	"net/http" // For creating an HTTP server
)

// handler function processes incoming HTTP requests.
func handler(w http.ResponseWriter, r *http.Request) {
	// Send a response back to the client.
	fmt.Fprintf(w, "Hello, World! You've requested: %s\n", r.URL.Path)
}

// main function is the entry point of the application.
func main() {
	// Register the handler function for the root URL path ("/").
	http.HandleFunc("/", handler)

	// Start the HTTP server on port 8080.
	fmt.Println("Server is listening on http://localhost:8080")
	err := http.ListenAndServe(":8080", nil)
	if err != nil {
		// Log any error if the server fails to start.
		fmt.Println("Error starting server:", err)
	}
}

Explanation

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1. Import Statements

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• fmt: Provides functions for formatted I/O, such as printing to the console or formatting text for HTTP responses.
• net/http: Contains functions and types for building HTTP servers and making HTTP requests.

2. The handler Function

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• This is where the server processes incoming HTTP requests.
• The parameters are:
  • w http.ResponseWriter: Used to send a response back to the client.
  • r *http.Request: Represents the client’s HTTP request.
• The fmt.Fprintf function sends a response message to the client, including the requested URL path.

3. The main Function

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• Registers the handler function with the root URL path ("/").
• Calls http.ListenAndServe to start the server:
  • The first argument specifies the port (:8080).
  • The second argument is nil, which means the DefaultServeMux is used.

4. Running the Server

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1. Save the code in a file named main.go.
2. Open a terminal and navigate to the file’s directory.
3. Run the following command to start the server:

   go run main.go
   

4. Open your browser and go to http://localhost:8080. You should see:

   Hello, World! You've requested: /
   

Key Concepts

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• Go’s net/http Package: Provides everything needed for creating robust HTTP servers.
• Handler Functions: Functions that process incoming HTTP requests and generate responses.
• ListenAndServe: Starts the HTTP server and blocks until the program is terminated.