To send files using mlutipart/form-data format in Java, we can utilize Apache HttpClient. This approach is commonly used for file uploads to REST APIs.

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.mime.MultipartEntityBuilder;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

import java.io.File;
import java.nio.charset.StandardCharsets;

public class FileUploader {
    
    public static String uploadFile(String apiUrl, File fileToUpload) {
        try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
            HttpPost uploadRequest = new HttpPost(apiUrl);
            
            MultipartEntityBuilder builder = MultipartEntityBuilder.create();
            builder.setCharset(StandardCharsets.UTF_8);
            
            // Add regular form field
            builder.addTextBody("source", "java-upload", 
                ContentType.TEXT_PLAIN.withCharset("UTF-8"));
                
            // Add file part
            builder.addBinaryBody("fileData", fileToUpload, 
                ContentType.APPLICATION_OCTET_STREAM, 
                fileToUpload.getName());
                
            uploadRequest.setEntity(builder.build());
            
            HttpResponse response = httpClient.execute(uploadRequest);
            return EntityUtils.toString(response.getEntity());
            
        } catch (Exception ex) {
            ex.printStackTrace();
            return "Upload failed: " + ex.getMessage();
        }
    }
}

Required Maven dependency:

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.13</version>
</dependency>

Key points:

• Uses MultipartEntityBuilder to construct the multipart request
• Supports both text fields and binary file data
• Handles UTF-8 encoding properly
• Includes proper resource cleanup with try-with-resources