2. Handling HTTP Requests with Gin – Gin Handler Functions

2.2 Gin Handler Functions

    In the Gin framework, a handler function is a function used to process incoming requests, responsible for executing business logic and returning responses to the client. Handler functions are typically defined as a function that takes <span>*gin.Context</span> as a parameter.<span>*gin.Context</span> provides methods to access request information (such as URL, HTTP headers, request body, etc.) and to send responses back to the client. An example is shown below:

package main

import (
    "errors"
    "net/http"
    "strconv"
    "github.com/gin-gonic/gin"
)

func MockQueryUserByIdFromDB(id int) (int, error) {
    if id == 123 {
        return id, nil
    }
    return 0, errors.New("query user error")
}

// Handler function
func getUserById(ctx *gin.Context) {
    requestId, _ := strconv.Atoi(ctx.Param("id"))
    queryId, err := MockQueryUserByIdFromDB(requestId)
    if err != nil {
        ctx.JSON(http.StatusNotFound, gin.H{
            "error": "user not found",
        })
        return
    }
    ctx.JSON(http.StatusOK, queryId)
}

func main() {
    r := gin.Default()
    r.GET("/user/:id", getUserById)
    // Default listening on 0.0.0.0:8080
    r.Run()
}

    In the above code, getUserById() is a handler function that accesses the id parameter from the URL using *gin.Context and retrieves user information based on the id. If the user exists, it returns the information in JSON format; otherwise, it returns an error message indicating that the user does not exist.