How to Handle JSON Data and Images in Multipart/Form-Data Requests with Gin?

Go: Handling JSON Data and Image in a Form with Gin

Gin is a popular web framework in Go that simplifies handling HTTP requests. This article addresses a specific issue with parsing both JSON data and an image from a multipart/form-data request using Gin's binding mechanisms.

Request Handling Code

The request handler function in Gin receives and handles HTTP requests. In this case, it expects to receive a multipart request containing both JSON data and an image file.

func (h *Handlers) UpdateProfile() gin.HandlerFunc {
    type request struct {
        Username    string `json:"username" binding:"required,min=4,max=20"`
        Description string `json:"description" binding:"required,max=100"`
    }

    return func(c *gin.Context) {
        var updateRequest request

        // Bind JSON data to `updateRequest` struct.
        if err := c.BindJSON(&updateRequest); err != nil {
            // Handle error here...
            return
        }

        // Get the image file from the request.
        avatar, err := c.FormFile("avatar")
        if err != nil {
            // Handle error here...
            return
        }

        // Validate file size and content type.
        if avatar.Size > 3<<20 || !avatar.Header.Get("Content-Type") { // if avatar size more than 3mb
            // Handle error here...
            return
        }

        // Handle image processing and database operations here...
        // Save username, description, and image to a database.

        c.IndentedJSON(http.StatusNoContent, gin.H{"message": "successful update"})
    }
}

Test Case

A unit test is included to verify the handler's functionality. It sets up a mock request with a multipart/form-data body. The request contains JSON data and an image.

func TestUser_UpdateProfile(t *testing.T) {
    type testCase struct {
        name               string
        image              io.Reader
        username           string
        description        string
        expectedStatusCode int
    }

    // Set up mock request with multipart/form-data body.
    // ...

    for _, tc := range testCases {
        // ...

        w := httptest.NewRecorder()
        router.ServeHTTP(w, req)

        assert.Equal(t, tc.expectedStatusCode, w.Result().StatusCode)
    }
}

Error During Test

During the test, an error occurred due to an invalid character in the request body. The error message was "Error #01: invalid character '-' in numeric literal."

Root Cause

Gin's c.BindJSON function is used to parse JSON data from the request body. However, it assumes that the request body starts with valid JSON. In the case of a multipart/form-data request, the body starts with a boundary (--30b24345de...), which is an invalid character for a JSON literal.

Solution

To resolve this issue, we can use Gin's c.ShouldBind function with binding.FormMultipart to explicitly bind to the multipart/form-data body. This allows Gin to properly parse both the JSON data and the image file.

// Bind JSON data and image file to `updateRequest` struct.
if err := c.ShouldBindWith(&updateRequest, binding.FormMultipart); err != nil {
    // Handle error here...
    return
}

Conclusion

This article demonstrates how to handle JSON data and an image file simultaneously in a multipart/form-data request with Gin. It highlights the importance of using the correct binding method (c.ShouldBindWith(..., binding.FormMultipart)) to avoid parsing errors.

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