How can I upload files and parameters to an HTTP server using pure Java without relying on third-party libraries?

HTTP Server File Uploads Using Java

Introduction

This article discusses uploading files to an HTTP server from a Java client. We aim to provide a simple and free solution that combines parameters and files within a POST request.

Multipart/Form-Data Encoding

For mixed POST content (binary and character data), multipart/form-data encoding is typically used. The data is divided into sections, each with its own header and body.

Java Solution

The following code demonstrates how to upload files and parameters using a direct HTTP connection without third-party libraries:

<code class="java">// Parameters
String url = "http://example.com/upload";
String param = "value";

// File paths
File textFile = new File("/path/to/file.txt");
File binaryFile = new File("/path/to/file.bin");

// Generate unique boundary value
String boundary = Long.toHexString(System.currentTimeMillis());

// Create connection
URLConnection connection = new URL(url).openConnection();
connection.setDoOutput(true);
connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);

// OutputStream for writing data
try (OutputStream output = connection.getOutputStream();
     PrintWriter writer = new PrintWriter(new OutputStreamWriter(output, charset), true)) {

    // Write parameter
    writer.append("--" + boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"param\"").append(CRLF);
    writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF);
    writer.append(CRLF).append(param).append(CRLF).flush();

    // Write text file
    writer.append("--" + boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"textFile\"; filename=\"" + textFile.getName() + "\"").append(CRLF);
    writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF); // Ensure file is saved using this charset
    writer.append(CRLF).flush();
    Files.copy(textFile.toPath(), output);
    output.flush(); 
    writer.append(CRLF).flush();

    // Write binary file
    writer.append("--" + boundary).append(CRLF);
    writer.append("Content-Disposition: form-data; name=\"binaryFile\"; filename=\"" + binaryFile.getName() + "\"").append(CRLF);
    writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(binaryFile.getName())).append(CRLF);
    writer.append("Content-Transfer-Encoding: binary").append(CRLF);
    writer.append(CRLF).flush();
    Files.copy(binaryFile.toPath(), output);
    output.flush(); 
    writer.append(CRLF).flush();

    // End multipart/form-data
    writer.append("--" + boundary + "--").append(CRLF).flush();
}

// Get HTTP response code
int responseCode = ((HttpURLConnection) connection).getResponseCode();
System.out.println(responseCode);</code>

Additional Notes

• Files.copy is introduced in Java 7; use FileUtils.copyFile for older versions.
• Apache Commons HttpComponents Client is a popular third-party library that simplifies this process.
• Apache Commons FileUpload is used on the server side to process file uploads.

See Also

• [Using java.net.URLConnection to fire and handle HTTP requests](link to resource)

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