Keycloak custom authentication SPI

Extending the server
1

I am working on keycloak authentication SPI in keycloak 15.0.1. I have used https://github.com/thomasdarimont/keycloak-extension-playground . I am working on creating two-way communication between ftl and java. I have faced an issue sending a response from Java to ftl. But ftl sends only a generated response. I want to alter the response and send it to the ftl. From ftl it gives a base64 encoded string to the action method. But the action method gives a generated response. How can I customize the response?

async function send_data_to_java() {
                // Convert `dataUrl` to base64
                let dataUrl = "Jisananam12289";  // Replace with actual data URL if needed
                let base64DataUrl = btoa(dataUrl);  // Encodes to base64 format

                // Prepare face authentication data
                let faceAuthData = { faceImage: base64DataUrl };
                let loginActionUrl = document.getElementById("kc-u2f-login-form").action;

                console.log("Face auth data:", faceAuthData);

                // Convert faceAuthData to URL-encoded form data
                let params = new URLSearchParams(faceAuthData).toString();

                try {
                    // Send POST request with encoded data
                    let faceAuthResponse = await fetch(loginActionUrl, {
                        method: "post",
                        redirect: 'manual',
                        credentials: "include",
                        body: params,
                        headers: {
                            'Content-Type': 'application/x-www-form-urlencoded'
                        }
                    });
                    console.log("Response:", faceAuthResponse);
                } catch (error) {
                    console.error("Error:", error);
                }
            }

This is the backend code in Java.


    @Override
    public void action(AuthenticationFlowContext context) {
        MultivaluedMap<String, String> formParameters = context.getHttpRequest().getDecodedFormParameters();
        String biometricImage = formParameters.getFirst("faceImage");
        HttpURLConnection connection = null;
        try {
            // Setup connection
            URL url = new URL(API_ENDPOINT);
            connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("POST");
            connection.setConnectTimeout(CONNECT_TIMEOUT);
            connection.setReadTimeout(READ_TIMEOUT);

            // Setup request headers
            connection.setRequestProperty("Content-Type", "application/json");
            connection.setDoOutput(true);

            // Write request body
            try (OutputStream outputStream = connection.getOutputStream()) {
                String jsonRequest = "{\"type\":\"slap_left\"}";
                outputStream.write(jsonRequest.getBytes("UTF-8"));
            }

            // Handle response
            System.out.println("hello");

            int responseCode = connection.getResponseCode();
            System.out.println(responseCode);
            String response = readResponse(connection);
            log.info("Response Body: " + response);


            String successResponse = "{\"status\":\"success\",\"message\":\"Authentication successful!\"}";
            context.challenge(Response.status(200).entity(response).type(MediaType.APPLICATION_JSON).build());
            context.success();
            return;

        } catch (Exception e) {
            log.error("Error during fingerprint capture", e);
        } finally {
            if (connection != null) {
                connection.disconnect();
            }
        }
    }