How do you return a JSON object from a Java Servlet

How do you return a JSON object form a Java servlet.

Previously when doing AJAX with a servlet I have returned a string. Is there a JSON object type that needs to be used, or do you just return a String that looks like a JSON object e.g.

String objectToReturn = "{ key1: 'value1', key2: 'value2' }";

Write the JSON object to the response object's output stream.

You should also set the content type as follows, which will specify what you are returning:

response.setContentType("application/json");
// Get the printwriter object from response to write the required json object to the output stream		
PrintWriter out = response.getWriter();
// Assuming your json object is **jsonObject**, perform the following, it will return your json object	
out.print(jsonObject);
out.flush();

First convert the JSON object to String. Then just write it out to the response writer along with content type of application/json and character encoding of UTF-8.

Here's an example assuming you're using Google Gson to convert a Java object to a JSON string:

protected void doXxx(HttpServletRequest request, HttpServletResponse response) {
    // ...

    String json = new Gson().toJson(someObject);
    response.setContentType("application/json");
    response.setCharacterEncoding("UTF-8");
    response.getWriter().write(json);
}

That's all.

See also:

• https://stackoverflow.com/questions/4112686/how-to-use-servlets-and-ajax/

• https://stackoverflow.com/questions/477816/what-is-the-correct-json-content-type

I do exactly what you suggest (return a String).

You might consider setting the MIME type to indicate you're returning JSON, though (according to this other stackoverflow post it's "application/json").

> How do you return a JSON object from a Java Servlet

response.setContentType("application/json");
response.setCharacterEncoding("utf-8");
PrintWriter out = response.getWriter();

  //create Json Object
  JsonObject json = new JsonObject();

	// put some value pairs into the JSON object .
	json.addProperty("Mobile", 9999988888);
	json.addProperty("Name", "ManojSarnaik");

    // finally output the json string		
	out.print(json.toString());

I used Jackson to convert Java Object to JSON string and send as follows.

PrintWriter out = response.getWriter();
ObjectMapper objectMapper= new ObjectMapper();
String jsonString = objectMapper.writeValueAsString(MyObject);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
out.print(jsonString);
out.flush();

Just write a string to the output stream. You might set the MIME-type to text/javascript (edit: application/json is apparently officialer) if you're feeling helpful. (There's a small but nonzero chance that it'll keep something from messing it up someday, and it's a good practice.)

Gson is very usefull for this. easier even. here is my example:

public class Bean {
private String nombre="juan";
private String apellido="machado";
private List<InnerBean> datosCriticos;

class InnerBean
{
    private int edad=12;
   
}
public Bean() {
    datosCriticos = new ArrayList<>();
    datosCriticos.add(new InnerBean());
}

}

    Bean bean = new Bean();
    Gson gson = new Gson();
    String json =gson.toJson(bean);

>out.print(json);

> {"nombre":"juan","apellido":"machado","datosCriticos":[{"edad":12}]}

Have to say people if yours vars are empty when using gson it wont build the json for you.Just the >{}

There might be a JSON object for Java coding convenience. But at last the data structure will be serialized to string. Setting a proper MIME type would be nice.

I'd suggest JSON Java from json.org.

Depending on the Java version (or JDK, SDK, JRE... i dunno, im new to the Java ecosystem), the JsonObject is abstract. So, this is a new implementation:

import javax.json.Json;
import javax.json.JsonObject;

...

try (PrintWriter out = response.getWriter()) {
    response.setContentType("application/json");       
    response.setCharacterEncoding("UTF-8");
    
    JsonObject json = Json.createObjectBuilder().add("foo", "bar").build();

    out.print(json.toString());
}

response.setContentType("text/json");

//create the JSON string, I suggest using some framework.

String your_string;

out.write(your_string.getBytes("UTF-8"));

You may use bellow like.

If you want use json array:

1. download json-simple-1.1.1.jar & add to your project class path

2. Create A class named Model like bellow

   public class Model {
   
    private String id = "";
    private String name = "";
   
    //getter sertter here
   }
   

3. In sevlet getMethod you can use like bellow

   @Override
   protected void doGet(HttpServletRequest request, HttpServletResponse response)
       throws ServletException, IOException {
   
     //begin get data from databse or other source
     List<Model> list = new ArrayList<>();
     Model model = new Model();
     model.setId("101");
     model.setName("Enamul Haque");
     list.add(model);
   
     Model model1 = new Model();
     model1.setId("102");
     model1.setName("Md Mohsin");
     list.add(model1);
     //End get data from databse or other source
   try {
   
       JSONArray ja = new JSONArray();
       for (Model m : list) {
           JSONObject jSONObject = new JSONObject();
           jSONObject.put("id", m.getId());
           jSONObject.put("name", m.getName());
           ja.add(jSONObject);
       }
       System.out.println(" json ja = " + ja);
       response.addHeader("Access-Control-Allow-Origin", "*");
       response.setContentType("application/json");
       response.setCharacterEncoding("UTF-8");
       response.getWriter().print(ja.toString());
       response.getWriter().flush();
      } catch (Exception e) {
        e.printStackTrace();
     }
   
    }
   

4. Output:

       [{"name":"Enamul Haque","id":"101"},{"name":"Md Mohsin","id":"102"}]
   

I you want json Object just use like:

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    try {

        JSONObject json = new JSONObject();
        json.put("id", "108");
        json.put("name", "Enamul Haque");
        System.out.println(" json JSONObject= " + json);
        response.addHeader("Access-Control-Allow-Origin", "*");
        response.setContentType("application/json");
        response.setCharacterEncoding("UTF-8");
        response.getWriter().print(json.toString());
        response.getWriter().flush();
        // System.out.println("Response Completed... ");
    } catch (Exception e) {
        e.printStackTrace();
    }

}

Above function Output:

{"name":"Enamul Haque","id":"108"}

Full source is given to GitHub: https://github.com/enamul95/ServeletJson.git

Close to BalusC answer in 4 simple lines using Google Gson lib. Add this lines to the servlet method:

User objToSerialize = new User("Bill", "Gates");    
ServletOutputStream outputStream = response.getOutputStream();

response.setContentType("application/json;charset=UTF-8");
outputStream.print(new Gson().toJson(objToSerialize));

Good luck!

By Using Gson you can send json response see below code

You can see this code

@WebServlet(urlPatterns = {"/jsonResponse"})
public class JsonResponse extends HttpServlet {

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    response.setContentType("application/json");
    response.setCharacterEncoding("utf-8");
    Student student = new Student(12, "Ram Kumar", "Male", "1234565678");
    Subject subject1 = new Subject(1, "Computer Fundamentals");
    Subject subject2 = new Subject(2, "Computer Graphics");
    Subject subject3 = new Subject(3, "Data Structures");
    Set subjects = new HashSet();
    subjects.add(subject1);
    subjects.add(subject2);
    subjects.add(subject3);
    student.setSubjects(subjects);
    Address address = new Address(1, "Street 23 NN West ", "Bhilai", "Chhattisgarh", "India");
    student.setAddress(address);
    Gson gson = new Gson();
    String jsonData = gson.toJson(student);
    PrintWriter out = response.getWriter();
    try {
        out.println(jsonData);
    } finally {
        out.close();
    }

  }
}

helpful from json response from servlet in java