Webhooks: The Provider Calls You in Java

Some payments finish later, not during the request: the customer confirms in their bank's app a few minutes after checkout, or pays by bank transfer. Your shop can't wait for that. Instead, the provider calls your shop when it happens: it sends a POST to an address you gave it. That address is a webhook. For once, your shop is the server and the provider is the client.

Events: the provider sends an event as JSON: what happened in "type", and the payment it happened to in "data", a map inside the map. @RequestBody String body gives you the body as the exact text that was sent, and Jackson reads it into a map. The same idea for a mail service that tells you an email bounced:

List<Object> bounced = new ArrayList<>();

@PostMapping("/webhooks/mail")
Map<String, Object> mailWebhook(@RequestBody String body) throws Exception {
    Map<String, Object> event = new ObjectMapper().readValue(body, Map.class);
    if (event.get("type").equals("email.bounced")) {
        Map<String, Object> data = (Map<String, Object>) event.get("data");
        bounced.add(data.get("to"));
    }
    return Map.of("received", true);
}

Answer {"received": true} to every event, even the ones you don't act on, like payment.failed: a provider resends an event until it gets a 2xx answer. The path has no id in it, so the order's id comes from the event, in "data":

POST /webhooks/payments  {"type": "payment.failed", "data": {"id": "pay_7", "amount": 21, "order_id": 10}}
                         →  200  {"received": true}
GET /orders/10           →  200  {…, "payment_id": null, "status": "pending", "total": 21}
POST /webhooks/payments  {"type": "payment.succeeded", "data": {"id": "pay_8", "amount": 21, "order_id": 10}}
                         →  200  {"received": true}
GET /orders/10           →  200  {…, "payment_id": "pay_8", "status": "paid", "total": 21}

Your task: add POST /webhooks/payments with a method paymentWebhook. Read the event, and when its type is "payment.succeeded", mark the order paid with the payment's id. Answer {"received": true} for every event.

Assignment
Follow the Coding Tutorial to build the webhook POST /webhooks/payments.

Hint
The payment is event.get("data"), cast to Map<String, Object>. Use the same UPDATE as in payOrder, with the payment's "id" and its "order_id". Compare strings with .equals, not ==.