Retrofit请求javaweb服务器OnResponse方法返回对象response.body为空并且javaweb服务器没有响应，如何解决？

Question

我是想做一个建议的retrofit请求以及一个javaweb服务器来响应，分别用的是androidstudio和Intellij
下面是代码：
客户端：

public interface Get_Interface {
    @GET("/hello?username=cwl")
    Call<UserName> login();
}

自定义的responsebody

```java
public class UserName {

    private String name;
    public void setName(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }

}

```java
package com.example.apistudy;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class PostRequest extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_post_request1);

    }

    private void request() {
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("http://192.168.0.103:80/apidemo/")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        Get_Interface pi = retrofit.create(Get_Interface.class);

        Call<UserName> call = pi.login();
        call.enqueue(new Callback<UserName>() {
            @Override
            public void onResponse(Call<UserName> call, Response<UserName> response) {
                if (response.isSuccessful())
                {
                    Log.d("success", response.toString());
                }
                else
                {
                    try
                    {
                        Log.d("error1", response.toString());
                    }
                    catch (Exception e)
                    {
                        e.printStackTrace();
                        Log.d("error2", e.toString());
                    }
                }
            }

            @Override
            public void onFailure(Call<UserName> call, Throwable t) {
                System.out.println("小失败");
                System.out.println(t.getMessage());
            }
        });
    }

    public void Get(View view) {
        request();
    }
}

Get方法是button按钮Onclick调用的
接下来是服务端

import com.alibaba.fastjson.JSONObject;
import com.sun.deploy.net.HttpRequest;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Map;

@WebServlet("/hello")
public class helloRequest extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("aaaa");
        req.setCharacterEncoding("UTF-8");
        resp.setContentType("application/json; charset=utf-8");
        String username = req.getParameter("username");
        JSONObject json = new JSONObject();
        try {
            if(username.equals("cwl")){
                json.put("name", "yes");
            }else{
                json.put("name", "no");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally{
            PrintWriter write = resp.getWriter();
            write.write(json.toString());
        }

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

现在的问题就是安卓端运行retrofit返回onResponse方法，但是response.body()是null。response.issucessful()返回false,输出类似


D/error1: retrofit2.Response@8da6216

retrofit请求服务端完全没有反应，但是在安卓虚拟机上网页访问返回正常，电脑网页访问返回json数据也正常。现在项目卡在这里，希望懂的同志可以讲讲是哪个地方出了问题。

Answer 1

PrintWriter write = resp.getWriter();
write.println(json.toString());
write.flush();