如何从 java servlet 发送一个数组作为 JSON 以使用 fetch() 响应前端？

Question

I've been working on a react app. In this app, I will be sending the input from the user to the Java servlet on the tomcat server to sort it. After sorting, I'm trying to display it on a label in my react app. I've successfully sent it to the java servlet using fetch method() and sorted it.

This is how my fetch() method looks like:

 const [text, setText] = useState("");
  async function onSubmit() {
 
    var newText = { text: text}; //object
      await fetch(`http://localhost:8080/backend/link`, {
        method: "POST",
        headers: {
          "Content-Type": "application/json",
          "Access-Control-Allow-Origin" : "*",
 "Access-Control-Allow-Credentials" : true,
 "status" : 200
        },
        body: JSON.stringify(newText),
        mode: 'no-cors',
      })
        .then((response) => {
        console.log("response");
        console.log(response.body); //displays null
        })
        .then((data) => {
        console.log(data);
          console.log("Success");
        });
    }

My Java servlet looks like this:

public void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException
{
System.out.println("invoked");
String jsonBody = new BufferedReader(new InputStreamReader(request.getInputStream())).lines().collect(
           Collectors.joining("\n"));
System.out.println(jsonBody);
if (jsonBody == null || jsonBody.trim().length() == 0) {
       return;
   }
   JSONObject jObj;
try {
jObj = new JSONObject(jsonBody);
String lines[] = ((String) jObj.get("text")).split(","); //The words in the input are separated by comma
Arrays.sort(lines);
for (String a : lines)
           System.out.println(a);

response.setContentType("application/json");
   response.setCharacterEncoding("UTF-8");
   
} catch (JSONException e) {
System.out.print("Exception");
}
}

whatever I send in the response object (Using Printwriter), the fetched response's body is null. How can I send the array so that I can get it in the response object of the fetch and then display it in a label?

Please leave your suggestions

Answer 1

I do not see where you write the body in the servlet response.

Perhaps you need something like this:

final PrintWriter writer = response.getWriter();
for (String a : lines)
           writer.println(a);

Note: your client is expecting a JSON object back, so you probably want to write your jObj to the output and not lines of text.

Use google chrome debug to view response headers of your network request. There you can see the body of the response. My guess is that your client-side code is fine and the server is not sending any content in the body.

Answer 2

From your code it's not clear how did you use PrintWriter. You can try something like below and then check the response:

String message = new ArrayList<String>();
PrintWriter writer = response.getWriter();
JSONObject obj = new JSONObject();
obj.put("message",message); 
response.setStatus(200);
writer.append(obj.toString());
writer.close();

Put a string message on JSON object to check whether nothing is being passed or only response is not being passed. Call " response.message " to fetch the message on client side.

Answer 3

I do not see where you write the body in the servlet response.

Perhaps you need something like this:

final PrintWriter writer = response.getWriter();
for (String a : lines)
           writer.println(a);

Use google chrome debug to view response headers of your network request. There you can see the body of the response. My guess is that your client-side code is fine and the server is not sending any content in the body.

Answer 4

const [text, setText] = useState(""); async function onSubmit() {

var newText = { text: text}; //Create a json object here and pass it to body
  await fetch(`http://localhost:8080/backend/link`, {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      "Access-Control-Allow-Origin" : "*",

"Access-Control-Allow-Credentials": true, "status": 200 }, body: newText, //do not need to use any json stringfy method mode: 'no-cors', }).then((response) => { console.log("response"); console.log(response.body); //displays null }).then((data) => { console.log(data); console.log("Success"); }); }

Answer 5

It was because I used "no-cors" mode in the client side which means that JavaScript cannot access the properties of the resulting response. Here's a detailed explanation about it: https://stackoverflow.com/a/43319482/13893049 . I've done several modifications on the client side as well as the server side including changing the content type from application/json to text/plain since json format is not among one of the allowed types.

The only allowed values for the Content-Type header are:

application/x-www-form-urlencoded
multipart/form-data
text/plain

Now my client side code looks like this:

    var newText = { text: text, commands: new_list.join(), reg: reg };

      await fetch(`http://localhost:8080/backend/link`, {
        method: "POST",
        headers: {
          "Content-Type": "text/plain",
          "Origin" : "http://localhost:3000/",
        },
        body: JSON.stringify(newText),
      })
        .then((response) => response.text())
        .then((data) => {
          //console.log("Success");
          //console.log(typeof data);
          setRes(regex(data));
        });
  }

(I've added few variables for my requirement)

From the server side I'm sending back an array in the form of string. I'm answering my own question since someone in the future might find in helpful. Do let me know in the comments if you got any doubts.