how to use okhttp to upload a file?

Here is a basic function that uses okhttp to upload a file and some arbitrary field (it literally simulates a regular HTML form submission)

Change the mime type to match your file (here I am assuming .csv) or make it a parameter to the function if you are going to upload different file types

  public static Boolean uploadFile(String serverURL, File file) {
    try {

        RequestBody requestBody = new MultipartBody.Builder().setType(MultipartBody.FORM)
                .addFormDataPart("file", file.getName(),
                        RequestBody.create(MediaType.parse("text/csv"), file))
                .addFormDataPart("some-field", "some-value")
                .build();

        Request request = new Request.Builder()
                .url(serverURL)
                .post(requestBody)
                .build();

        client.newCall(request).enqueue(new Callback() {

            @Override
            public void onFailure(final Call call, final IOException e) {
                // Handle the error
            }

            @Override
            public void onResponse(final Call call, final Response response) throws IOException {
                if (!response.isSuccessful()) {
                    // Handle the error
                }
                // Upload successful
            }
        });

        return true;
    } catch (Exception ex) {
        // Handle the error
    }
    return false;
}

Note: because it is async call, the boolean return type does not indicate successful upload but only that the request was submitted to okhttp queue.

Here's an answer that works with OkHttp 3.2.0:

public void upload(String url, File file) throws IOException {
    OkHttpClient client = new OkHttpClient();
    RequestBody formBody = new MultipartBody.Builder()
        .setType(MultipartBody.FORM)
        .addFormDataPart("file", file.getName(),
            RequestBody.create(MediaType.parse("text/plain"), file))
        .addFormDataPart("other_field", "other_field_value")
        .build();
    Request request = new Request.Builder().url(url).post(formBody).build();
    Response response = client.newCall(request).execute();
}

Note: this answer is for okhttp 1.x/2.x. For 3.x, see this other answer.

The class Multipart from mimecraft encapsulates the whole HTTP body and can handle regular fields like so:

Multipart m = new Multipart.Builder()
        .type(Multipart.Type.FORM)
        .addPart(new Part.Builder()
                .body("value")
                .contentDisposition("form-data; name=\"non_file_field\"")
                .build())
        .addPart(new Part.Builder()
                .contentType("text/csv")
                .body(aFile)
                .contentDisposition("form-data; name=\"file_field\"; filename=\"file1\"")
                .build())
        .build();

Take a look at examples of multipart/form-data encoding to get a sense of how you need to construct the parts.

Once you have a Multipart object, all that's left to do is specify the right Content-Type header and pass on the body bytes to the request.

Since you seem to be working with the v2.0 of the OkHttp API, which I don't have experience with, this is just guess code:

// You'll probably need to change the MediaType to use the Content-Type
// from the multipart object
Request.Body body =  Request.Body.create(
        MediaType.parse(m.getHeaders().get("Content-Type")),
        out.toByteArray());

For OkHttp 1.5.4, here is a stripped down code I'm using which is adapted from a sample snippet:

OkHttpClient client = new OkHttpClient();
OutputStream out = null;
try {
    URL url = new URL("http://www.example.com");
    HttpURLConnection connection = client.open(url);
    for (Map.Entry<String, String> entry : multipart.getHeaders().entrySet()) {
        connection.addRequestProperty(entry.getKey(), entry.getValue());
    }
    connection.setRequestMethod("POST");
    // Write the request.
    out = connection.getOutputStream();
    multipart.writeBodyTo(out);
    out.close();

    // Read the response.
    if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
        throw new IOException("Unexpected HTTP response: "
                + connection.getResponseCode() + " " + connection.getResponseMessage());
    }
} finally {
    // Clean up.
    try {
        if (out != null) out.close();
    } catch (Exception e) {
    }
}