Retrofit2: Modifying request body in OkHttp Interceptor

Since this cannot be written in the comments of the previous answer by @Fabian, I am posting this one as separate answer. This answer deals with both "application/json" as well as form data.

import android.content.Context;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;

import okhttp3.FormBody;
import okhttp3.Interceptor;
import okhttp3.MediaType;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;
import okio.Buffer;

/**
 * Created by debanjan on 16/4/17.
 */

public class TokenInterceptor implements Interceptor {
    private Context context; //This is here because I needed it for some other cause 

    //private static final String TOKEN_IDENTIFIER = "token_id";
    public TokenInterceptor(Context context) {
        this.context = context;
    }

    @Override
    public Response intercept(Chain chain) throws IOException {
        Request request = chain.request();
        RequestBody requestBody = request.body();
        String token = "toku";//whatever or however you get it.
        String subtype = requestBody.contentType().subtype();
        if(subtype.contains("json")){
            requestBody = processApplicationJsonRequestBody(requestBody, token);
        }
        else if(subtype.contains("form")){
            requestBody = processFormDataRequestBody(requestBody, token);
        }
        if(requestBody != null) {
            Request.Builder requestBuilder = request.newBuilder();
            request = requestBuilder
                    .post(requestBody)
                    .build();
        }

        return chain.proceed(request);
    }
    private String bodyToString(final RequestBody request){
        try {
            final RequestBody copy = request;
            final Buffer buffer = new Buffer();
            if(copy != null)
                copy.writeTo(buffer);
            else
                return "";
            return buffer.readUtf8();
        }
        catch (final IOException e) {
            return "did not work";
        }
    }
    private RequestBody processApplicationJsonRequestBody(RequestBody requestBody,String token){
        String customReq = bodyToString(requestBody);
        try {
            JSONObject obj = new JSONObject(customReq);
            obj.put("token", token);
            return RequestBody.create(requestBody.contentType(), obj.toString());
        } catch (JSONException e) {
            e.printStackTrace();
        }
        return null;
    }
    private RequestBody processFormDataRequestBody(RequestBody requestBody, String token){
        RequestBody formBody = new FormBody.Builder()
                .add("token", token)
                .build();
        String postBodyString = bodyToString(requestBody);
        postBodyString += ((postBodyString.length() > 0) ? "&" : "") +  bodyToString(formBody);
        return RequestBody.create(requestBody.contentType(), postBodyString);
    }

}

I using this to add post parameter to the existing ones.

 OkHttpClient client = new OkHttpClient.Builder()
                    .protocols(protocols)
                    .addInterceptor(new Interceptor() {
                        @Override
                        public Response intercept(Chain chain) throws IOException {
                            Request request = chain.request();
                            Request.Builder requestBuilder = request.newBuilder();
RequestBody formBody = new FormEncodingBuilder()
            .add("email", "[email protected]")
            .add("tel", "90301171XX")
            .build();
                            String postBodyString = Utils.bodyToString(request.body());
                            postBodyString += ((postBodyString.length() > 0) ? "&" : "") +  Utils.bodyToString(formBody);
                            request = requestBuilder
                                    .post(RequestBody.create(MediaType.parse("application/x-www-form-urlencoded;charset=UTF-8"), postBodyString))
                                    .build();
                            return chain.proceed(request);
                        }
                    })
                    .build();

public static String bodyToString(final RequestBody request){
        try {
            final RequestBody copy = request;
            final Buffer buffer = new Buffer();
            if(copy != null)
                copy.writeTo(buffer);
            else
                return "";
            return buffer.readUtf8();
        }
        catch (final IOException e) {
            return "did not work";
        }
    }

OkHttp3:

RequestBody formBody = new FormBody.Builder()
                .add("email", "[email protected]")
                .add("tel", "90301171XX")
                .build();

I'll share my Kotlin implementation of @Fabian's answer using Dagger. I wanted origin=app added to the request url for GET requests, and added to the body for form-encoded POST requests

@Provides
@Singleton
fun providesRequestInterceptor() =
        Interceptor {
            val request = it.request()

            it.proceed(when (request.method()) {
                "GET" -> {
                    val url = request.url()
                    request.newBuilder()
                            .url(url.newBuilder()
                                    .addQueryParameter("origin", "app")
                                    .build())
                            .build()
                }
                "POST" -> {
                    val body = request.body()
                    request.newBuilder()
                            .post(RequestBody.create(body?.contentType(),
                                    body.bodyToString() + "&origin=app"))
                            .build()
                }
                else -> request
            })
        }

fun RequestBody?.bodyToString(): String {
    if (this == null) return ""
    val buffer = okio.Buffer()
    writeTo(buffer)
    return buffer.readUtf8()
}

You can edit the request body by below method, Pass the request and the parameter to edit.

private fun editBody(request: Request, parameter: String): RequestBody {
           
  val oldBody = request.body //retrieve the current request body
  val buffer =  Buffer()
  oldBody?.writeTo(buffer)

  val strOldBody = buffer.readUtf8() // String representation of the current request body
  buffer.clear()
  buffer.close()

  val strNewBody = JSONObject(strOldBody).put("parameter", parameter).toString()
  return strNewBody.toRequestBody(request.body?.contentType()) // New request body with the encrypted/modified string of the current request body

}

Now you can request again with updated request body

override fun intercept(chain: Interceptor.Chain): Response {
      val request: Request = chain.request()
      return chain.proceed(requestWithUpdatedParameter(request, "parameter"))
    
}
    
private fun requestWithUpdatedParameter(req: Request, parameter: String): Request {
                val newRequest: Request
                val body = editBody(req, parameter)
                newRequest = req.newBuilder().method(req.method, body).build()
                return newRequest
}