post请求的时候报错，

java.lang.NullPointerException

at com.imooc.http.ExampleUnitTest.addition_isCorrect(ExampleUnitTest.java:30)

报错行：request.getBody().write("username=nate&userage=21".getBytes());

HelloServlet服务器已经启动。

源代码如下：

public class ExampleUnitTest {
   @Test
   public void addition_isCorrect() throws Exception {
       assertEquals(4, 2 + 2);

       OkHttpClient client = new OkHttpClient();
       //get请求
       //OkHttpRequest request = new OkHttpRequest(client, HttpMethod.GET,"http://www.imooc.com");

       //post请求，需要启动web服务器
       OkHttpRequest request = new OkHttpRequest(client, HttpMethod.POST,"http://localhost:8080/web/HelloServlet");
       request.getBody().write("username=nate&userage=21".getBytes());

       HttpResponse response = request.execute();

       String content = null;
       BufferedReader reader = new BufferedReader(new InputStreamReader(response.getBody()));

       while((content=reader.readLine())!=null){
           System.out.println(content);
       }
       response.close();

   }
}

请问一下这个是啥问题？