Question

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import org.json.JSONObject;

  // Extend HttpServlet class
  public class Test extends HttpServlet {
  JSONObject json = new JSONObject();

  public void init() throws ServletException
  {
     json.put("city", "Mumbai");
     json.put("country", "India");
  }

  public void doGet(HttpServletRequest request,
                HttpServletResponse response)
       throws ServletException, IOException
  {
       response.setContentType("application/json");
       String output = json.toString();
  }

  public void destroy()
  {
      //do nothing
  }
}

嗨，我按照在线教程，创建了这个在Apache Tomcat上运行的servlet类。当我运行课程时，我得到一个没有json内容的空白屏幕，我错过了教程或评论部分没有留下的内容，谢谢？

Answer 1

您需要在响应流中编写JSON对象的内容。这是你如何做到的：

public void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    response.setContentType("application/json");
    String output = json.toString();
    PrintWriter writer = response.getWriter();
    writer.write(output);
    writer.close();
}

此外，在Servlet中声明非最终对象是不好的做法，除非它们由应用程序服务器（如EJB或DataSources）管理。我不确定你正在关注哪个教程但是它有一些问题：

我们在Servlet 3.1上，servlet可以使用@WebServlet进行修饰，而不是使用web.xml中的配置。
由于并发问题，您应该避免在Servlet中声明非final字段。当通过对servlet的GET或POST调用更新字段并且多个（2个或更多）客户端同时对servlet执行相同的调用时，可以注意到这一点，这将以servlet的客户端的奇怪结果结束
如果您想/需要获取要在Servlet中使用的数据，您应该在尽可能窄的范围内获取它。对于Servlet，这意味着您应该使用doXXX方法之一获取数据。

以下是您的代码示例的样子：

@WebServlet("/path")
public class Test extends HttpServlet {
    //avoid declaring it here unless it's final
    //JSONObject json = new JSONObject();

    public void init() throws ServletException {
    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

        //check that now JSONObject was moved as local variable
        //for this method and it's initialized with the result
        //of a method called retrieveData
        //in this way, you gain several things:
        //1. Reusability: code won't be duplicated along multiple classes
        //   clients of this service
        //2. Maintainability: centralize how and from where you will
        //   retrieve the data.
        JSONObject json = retrieveData();

        response.setContentType("application/json");
        String output = json.toString();
        PrintWriter writer = response.getWriter();
        writer.write(output);
        writer.close();
    }

    //this method will be in charge to return the data you want/need
    //for learning purposes, you're hardcoding the data
    //in real world applications, you have to retrieve the data
    //from a datasource like a file or a database
    private JSONObject retrieveData() {
        //Initializing the object to return
        JSONObject json = new JSONObject();
        //filling the object with data
        //again, you're hardcoding it for learning/testing purposes
        //this method can be modified to obtain data from
        //an external resource
        json.put("city", "Mumbai");
        json.put("country", "India");
        //returning the object
        return json;
    }

    public void destroy() {
        //do nothing
    }
}

Java Servlet上显示的JSON输出

1 个答案: