Post Method returns response code 400 on Android with Volley Singleton

I've created two custom classes according to what my teacher instructed to do the request on the server, but I'm getting a 400 return and I can not figure out the reason. Searching, I saw that it may be something related to the header, but I do not know how to send it in my custom JSon request.

This is my code from my custom code for JsonObject request:

package com.example.sandrini.pokeagenda;

import com.android.volley.AuthFailureError;
import com.android.volley.Response;
import com.android.volley.RetryPolicy;
import com.android.volley.toolbox.JsonObjectRequest;

import org.json.JSONObject;

import java.util.HashMap;
import java.util.Map;

public class CustomJSONObjectRequest extends JsonObjectRequest {

    public CustomJSONObjectRequest(int method, String url, JSONObject jsonRequest,
                                   Response.Listener<JSONObject> listener,
                                   Response.ErrorListener errorListener) {
        super(method, url, jsonRequest, listener, errorListener);
    }

    @Override
    public Map<String, String> getHeaders() throws AuthFailureError {
        HashMap<String, String> headers = new HashMap<>();
        headers.put("Content-Type", "application/json; charset=utf-8");
        return headers;
    }

    @Override
    public RetryPolicy getRetryPolicy() {
        return super.getRetryPolicy();
    }
}

This is the code for my Request Queue:

package com.example.sandrini.pokeagenda;

import android.content.Context;

import com.android.volley.Cache;
import com.android.volley.Network;
import com.android.volley.RequestQueue;
import com.android.volley.toolbox.BasicNetwork;
import com.android.volley.toolbox.DiskBasedCache;
import com.android.volley.toolbox.HurlStack;

public class CustomVolleyRequestQueue {

private static CustomVolleyRequestQueue mInstance;
private static Context mCtx;
private RequestQueue mRequestQueue;

private CustomVolleyRequestQueue(Context context) {
    //recebe o contexto da aplicação
    //Assim a fila dura o ciclo de vida do app
    mCtx = context;
    mRequestQueue = getRequestQueue();
}

public static synchronized CustomVolleyRequestQueue getmInstance(Context context) {
    if (mInstance == null) {
        mInstance = new CustomVolleyRequestQueue(context);
    }
    //retorna uma instância da fila
    return mInstance;
}

public RequestQueue getRequestQueue() {
    if (mRequestQueue == null) {
        //Cache 10MB
        Cache cache = new DiskBasedCache(mCtx.getCacheDir(), 10 *1024);
        Network network = new BasicNetwork(new HurlStack());
        mRequestQueue = new RequestQueue(cache, network);
        mRequestQueue.start();
    }
    return mRequestQueue;
}
}

This is the Activity code where I am trying to make the request:

package com.example.sandrini.pokeagenda;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.Toast;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonRequest;
import com.google.gson.Gson;

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

import java.util.EmptyStackException;

public class CreateUserActivity extends AppCompatActivity implements Response.Listener, Response.ErrorListener {
    private static final String REQUEST_TAG = "CreateUser";
    Trainer trainer = new Trainer();
    EditText loginCreateInput, emailCreateInput, pwdCreateInput;
    Button createUserButton;
    private RequestQueue mQueue;
    JsonRequest jsonRequest;
    ProgressBar progressBarCreateUser;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_create_user);

        loginCreateInput = (EditText) findViewById(R.id.login_create_input);
        emailCreateInput = (EditText) findViewById(R.id.email_create_input);
        pwdCreateInput = (EditText) findViewById(R.id.pwd_create_input);
        createUserButton = (Button) findViewById(R.id.create_user_button);
        progressBarCreateUser = (ProgressBar) findViewById(R.id.progress_bar_create_user);

        progressBarCreateUser.setVisibility(View.INVISIBLE);
    }

    @Override
    protected void onStart() {
        super.onStart();

        createUserButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (loginCreateInput.getText().toString().equals("")) {
                    Toast.makeText(getApplicationContext(), "Please, set a login", Toast.LENGTH_SHORT).show();
                } else if (emailCreateInput.getText().toString().equals("")) {
                    Toast.makeText(getApplicationContext(), "Please, set an email", Toast.LENGTH_SHORT).show();
                } else if (pwdCreateInput.getText().toString().equals("")) {
                    Toast.makeText(getApplicationContext(), "Please, set a password", Toast.LENGTH_SHORT).show();
                } else {
                    String url = "http://192.168.25.6:8081/PokedexWS/webresources/pokews/poke/insert/trainer";
                    /*trainer.setUsername(loginCreateInput.getText().toString());
                    trainer.setEmail(emailCreateInput.getText().toString());
                    trainer.setPassword(pwdCreateInput.getText().toString());*/
                    trainer.setUsername("gato");
                    trainer.setEmail("gato");
                    trainer.setPassword("gato");

                    try {
                        JSONObject jsonObject;
                        Gson gson = new Gson();
                        String trainerToGson = gson.toJson(trainer);
                        jsonObject = new JSONObject(trainerToGson);
                        //String trainerToGson = "{\"username\":"+ "\"" + trainer.getUsername()+ "\"" + "," +
                        //"\"email\":"+ "\"" + trainer.getEmail() + "\"" + "," +
                        //"\"username\":"+ "\"" + trainer.getPassword()+ "\"" + "}";
                        //createUserButton.setText(trainerToGson);


                        mQueue = CustomVolleyRequestQueue.getmInstance(CreateUserActivity.this.getApplicationContext()).getRequestQueue();
                        jsonRequest = new CustomJSONObjectRequest(Request.Method.POST, url, jsonObject,
                                CreateUserActivity.this, CreateUserActivity.this);
                        jsonRequest.getHeaders();
                        mQueue.add(jsonRequest);
                        progressBarCreateUser.setVisibility(View.VISIBLE);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        });

    }

    @Override
    protected void onStop() {
        super.onStop();
        if(mQueue != null) {
            mQueue.cancelAll(REQUEST_TAG);
        }
    }



    @Override
    public void onErrorResponse(VolleyError error) {
        progressBarCreateUser.setVisibility(View.INVISIBLE);
        Toast.makeText(getApplicationContext(), "Register failed", Toast.LENGTH_LONG).show();
        Log.e("erro400", error.getMessage().toString());
        Log.e("erro400", error.getCause().toString());
        Log.e("erro400", error.getStackTrace().toString());
    }

    @Override
    public void onResponse(Object trainerObject) {
        progressBarCreateUser.setVisibility(View.INVISIBLE);
        Toast.makeText(getApplicationContext(), "Registered successfully", Toast.LENGTH_LONG).show();
                Intent intent = new Intent(CreateUserActivity.this, WelcomeTrainerActivity.class);
                startActivity(intent);
                finish();
    }
}

This is my Web Service:

package ws;

import com.google.gson.Gson;
import dao.PokemonDAO;
import dao.TrainerDAO;
import java.util.ArrayList;
import javax.ws.rs.core.Context;
import javax.ws.rs.core.UriInfo;
import javax.ws.rs.Produces;
import javax.ws.rs.Consumes;
import javax.ws.rs.DELETE;
import javax.ws.rs.GET;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.PUT;
import javax.ws.rs.PathParam;
import javax.ws.rs.core.MediaType;
import model.Pokemon;
import model.Trainer;

/**
 * REST Web Service
 *
 * @author sandrini
 */
@Path("pokews")
public class PokedexWebService {

    PokemonDAO pokemonDAO = new PokemonDAO();
    TrainerDAO trainerDAO = new TrainerDAO();

    @Context
    private UriInfo context;

    /**
     * Creates a new instance of PokedexWebService
     */
    public PokedexWebService() {
    }

    /**
     * Retrieves representation of an instance of teste.GenericResource
     * @return an instance of java.lang.String
     */
    @GET
    @Produces(MediaType.APPLICATION_XML)
    public String getXml() {
        //TODO return proper representation object
        throw new UnsupportedOperationException();
    }

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    @Path("poke/list")
    public ArrayList<Pokemon> pokeList() {
        return pokemonDAO.listPokemon();
    }

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    @Path("poke/fetch/{id}")
    public Pokemon pokeFetch(@PathParam("id") int id) {
        return pokemonDAO.fetchPokemon(id);
    }

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    @Path("poke/isFavorite/{id}")
    public String pokeIsFavorite(@PathParam("id") int id) {
        return pokemonDAO.isFavorite(id);
    }

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    @Path("poke/login/{login}/{pwd}")
    public Trainer pokelogin(@PathParam("login") String login, @PathParam("pwd") String pwd) {
        return trainerDAO.authUser(login, pwd);
    }

    @DELETE
    @Path("poke/delete/{id}")
    public boolean delete(@PathParam("id")int id) {
        return pokemonDAO.deletePokemon(id);
    }

    @POST
    @Consumes(MediaType.APPLICATION_JSON)
    @Path("poke/insert")
    public boolean pokeInsert(String content) {
        Gson gson = new Gson();
        Pokemon pokemon = (Pokemon) gson.fromJson(content, Pokemon.class);
        return pokemonDAO.insertPokemon(pokemon);
    }

    @POST
    @Consumes(MediaType.APPLICATION_JSON)
    @Path("poke/insert/trainer")
    public boolean trainerInsert(String content) {
        Gson gson = new Gson();
        Trainer trainer = (Trainer) gson.fromJson(content, Trainer.class);
        return trainerDAO.insertTrainer(trainer);
    }

    @PUT
    @Consumes(MediaType.APPLICATION_JSON)
    @Path("poke/update")
    public boolean pokeUpdate(String content) {
        Gson gson = new Gson();
        Pokemon pokemon = (Pokemon) gson.fromJson(content, Pokemon.class);
        return pokemonDAO.updatePokemon(pokemon);
    }

    /**
     * PUT method for updating or creating an instance of GenericResource
     * @param content representation for the resource
     */
    @PUT
    @Consumes(MediaType.APPLICATION_XML)
    public void putXml(String content) {
    }


}

I've got a java code to test the WS and it works, I just can not get the android:

package acessorest;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.InputStreamReader;
import java.lang.ProcessBuilder.Redirect.Type;
import java.net.HttpURLConnection;
import java.net.URL;
import javax.net.ssl.HttpsURLConnection;

/**
 *
 * @author marcelosiedler
 */
public class HttpExemplo {

    private final String USER_AGENT = "Mozilla/5.0";

    public static void main(String[] args) throws Exception {

        HttpExemplo http = new HttpExemplo();
        Gson gson = new Gson();  
                Pokemon pokemon = new Pokemon();
                java.lang.reflect.Type pokemonType = new TypeToken<Pokemon>() {
                }.getType();

                String chamadaWS = "http://192.168.25.6:8081/PokedexWS/webresources/pokews/poke/insert/trainer";
                Trainer trainer = new Trainer();

                trainer.setUsername("quadrado");
                trainer.setEmail("quadrado");
                trainer.setPassword("quadrado");

                //System.out.println(gson.toJson(trainer));
                String trainerToGson = gson.toJson(trainer);

                /*String trainerToGson = "{\"username\":"+ "\"" + trainer.getUsername()+ "\"" + "," +
                        "\"email\":"+ "\"" + trainer.getEmail() + "\"" + "," +
                        "\"password\":"+ "\"" + trainer.getPassword()+ "\"" + "}";*/


                //System.out.println(trainerToGson);
                //String retorn = http.sendGet(chamadaWS, "DELETE");
                //pokemon = gson.fromJson(json, pokemonType);
                //System.out.println(retorn);
                http.sendPost(chamadaWS,trainerToGson, "POST");




    }

    // HTTP GET request
    private String sendGet(String url, String method) throws Exception {

        URL obj = new URL(url);
        HttpURLConnection con = (HttpURLConnection) obj.openConnection();

        // optional default is GET
        con.setRequestMethod(method);

        //add request header
        con.setRequestProperty("User-Agent", USER_AGENT);

        int responseCode = con.getResponseCode();
        System.out.println("Response Code : " + responseCode);

        BufferedReader in = new BufferedReader(
                new InputStreamReader(con.getInputStream()));
        String inputLine;
        StringBuffer response = new StringBuffer();

        while ((inputLine = in.readLine()) != null) {
            response.append(inputLine);
        }
        in.close();

        //print result
        System.out.println(response.toString());

                return response.toString();

    }

    // HTTP POST request
    private void sendPost(String url, String parameters, String method) throws Exception {

        URL obj = new URL(url);
        HttpURLConnection con = (HttpURLConnection) obj.openConnection();

        //add reuqest header
        con.setRequestMethod(method);
                con.setRequestProperty("Content-Type", "application/json");
        con.setRequestProperty("User-Agent", USER_AGENT);
        con.setRequestProperty("Accept-Language", "en-US,en;q=0.5");

        String urlParameters = parameters;

        // Send post request
        con.setDoOutput(true);
        DataOutputStream wr = new DataOutputStream(con.getOutputStream());
        wr.writeBytes(urlParameters);
        wr.flush();
        wr.close();

        int responseCode = con.getResponseCode();
        System.out.println("\nSending 'POST' request to URL : " + url);
        System.out.println("Post parameters : " + urlParameters);
        System.out.println("Response Code : " + responseCode);

        BufferedReader in = new BufferedReader(
                new InputStreamReader(con.getInputStream()));
        String inputLine;
        StringBuffer response = new StringBuffer();

        while ((inputLine = in.readLine()) != null) {
            response.append(inputLine);
        }
        in.close();

        //print result
        System.out.println(response.toString());

    }

}