在Volley库中，Header未设置为Content type text / plain

Question

import android.app.ProgressDialog;
import android.content.Context;
import android.util.Log;
import android.widget.Toast;

import com.android.volley.AuthFailureError;
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.StringRequest;
import com.android.volley.toolbox.Volley;
import com.google.gson.Gson;

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

import java.io.UnsupportedEncodingException;
import java.util.HashMap;
import java.util.Map;


public class LoginService {


    public interface VolleyCallback{
        void onSuccess(String result);
    }

    // method to call login webservice
    public void loginWebservice(final Context context, final ProgressDialog Dialog, final String username, final String password, final VolleyCallback callback) {

        Dialog.setMessage("Please wait..");
        Dialog.show();
        String POST_Login_URL = "http://example.com";

        StringRequest stringRequest = new StringRequest(Request.Method.POST, POST_Login_URL,
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {
                        Log.d("response", response);
                        Dialog.dismiss();

                        try {
                            JSONObject myJson = new JSONObject(response);

                            String authenticated = myJson.optString("status");
                            if (authenticated.equals("true")) {
                                callback.onSuccess(authenticated);

                            } else {
                                Toast.makeText(context, "Invalid Username/Password", Toast.LENGTH_LONG).show();
                            }
                        } catch (JSONException e) {
                            e.printStackTrace();
                        }

                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        Dialog.dismiss();
                        Log.d("errorString", error.toString());
                        Toast.makeText(context, error.toString(), Toast.LENGTH_LONG).show();

                    }
                }) {

            @Override
            public String getBodyContentType() {
                return "text/plain; charset=" + getParamsEncoding();


            }

            @Override
            public Map<String, String> getHeaders() throws AuthFailureError {
                HashMap<String, String> map = new HashMap<String, String>();
                map.put("Content-Type", "text/plain; charset="+ getParamsEncoding());

                return map;
            }

            @Override
            public byte[] getBody() throws AuthFailureError {

                try {
                    Map<String, String> params = new HashMap<String, String>();

                    params.put("username", username);
                    params.put("password", password);
                    String json = convert(params);
                    if (json.length() < 3)
                        return ("{}").getBytes();
                    // log(json);
                    return json.getBytes(getParamsEncoding());
                } catch (UnsupportedEncodingException e) {
                    Log.e("TAG", "getBody(): request has no json");
                    e.printStackTrace();
                }
                return new byte[0];
            }

        };
        RequestQueue requestQueue = Volley.newRequestQueue(context);
        requestQueue.add(stringRequest);


    }

    public static String convert(Map<String, String> map) {
        Gson gson = new Gson();
        String json = gson.toJson(map);
        return json;
    }
}

在上面的代码中，我无法将标题内容类型application / json更新为text / plain。我重写了getBodyContentType（）方法和getHeaders（）方法但是当我调试代码时，content-type显示了application / json，即我从服务器获得了500个错误代码。 Web api工作正常。因为我使用自己的网络代码检查了api。