[json] Spring MVC Multipart Request with JSON

This is how I implemented Spring MVC Multipart Request with JSON Data.

Multipart Request with JSON Data (also called Mixed Multipart):

Based on RESTful service in Spring 4.0.2 Release, HTTP request with the first part as XML or JSON formatted data and the second part as a file can be achieved with @RequestPart. Below is the sample implementation.

Java Snippet:

Rest service in Controller will have mixed @RequestPart and MultipartFile to serve such Multipart + JSON request.

@RequestMapping(value = "/executesampleservice", method = RequestMethod.POST,
    consumes = {"multipart/form-data"})
@ResponseBody
public boolean executeSampleService(
        @RequestPart("properties") @Valid ConnectionProperties properties,
        @RequestPart("file") @Valid @NotNull @NotBlank MultipartFile file) {
    return projectService.executeSampleService(properties, file);
}

Front End (JavaScript) Snippet:

1. Create a FormData object.

2. Append the file to the FormData object using one of the below steps.

   1. If the file has been uploaded using an input element of type "file", then append it to the FormData object. formData.append("file", document.forms[formName].file.files[0]);
   2. Directly append the file to the FormData object. formData.append("file", myFile, "myfile.txt"); OR formData.append("file", myBob, "myfile.txt");

3. Create a blob with the stringified JSON data and append it to the FormData object. This causes the Content-type of the second part in the multipart request to be "application/json" instead of the file type.

4. Send the request to the server.

5. Request Details:
   Content-Type: undefined. This causes the browser to set the Content-Type to multipart/form-data and fill the boundary correctly. Manually setting Content-Type to multipart/form-data will fail to fill in the boundary parameter of the request.

Javascript Code:

formData = new FormData();

formData.append("file", document.forms[formName].file.files[0]);
formData.append('properties', new Blob([JSON.stringify({
                "name": "root",
                "password": "root"                    
            })], {
                type: "application/json"
            }));

Request Details:

method: "POST",
headers: {
         "Content-Type": undefined
  },
data: formData

Request Payload:

Accept:application/json, text/plain, */*
Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryEBoJzS3HQ4PgE1QB

------WebKitFormBoundaryvijcWI2ZrZQ8xEBN
Content-Disposition: form-data; name="file"; filename="myfile.txt"
Content-Type: application/txt


------WebKitFormBoundaryvijcWI2ZrZQ8xEBN
Content-Disposition: form-data; name="properties"; filename="blob"
Content-Type: application/json


------WebKitFormBoundaryvijcWI2ZrZQ8xEBN--