add event handler

Author: xavdid-stripe

src/main/java/com/stripe/StripeClient.java

@@ -1333,4 +1333,8 @@ public StripeResponse rawRequest(
   public StripeObject deserialize(String rawJson, ApiMode apiMode) throws StripeException {
     return StripeObject.deserializeStripeObject(rawJson, this.getResponseGetter(), apiMode);
   }
+
+  public StripeEventHandler handler(String webhookSecret) {
+    return new StripeEventHandler(webhookSecret, this);
+  }
 }

src/main/java/com/stripe/StripeEventHandler.java

@@ -0,0 +1,59 @@
+package com.stripe;
+
+import com.stripe.exception.SignatureVerificationException;
+import com.stripe.model.v2.core.EventNotification;
+import java.util.HashMap;
+
+public class StripeEventHandler {
+  /** Functional interface for event handlers that can throw StripeException. */
+  @FunctionalInterface
+  public interface EventHandler<T extends EventNotification> {
+    // this is an internal-facing method name that dictates how we call the stored method
+    void process(T event, StripeClient client);
+  }
+
+  private final String webhookSecret;
+  private final StripeClient stripeClient;
+  private final HashMap<String, EventHandler<? extends EventNotification>> registeredHandlers =
+      new HashMap<>();
+
+  public StripeEventHandler(String webhookSecret, StripeClient stripeClient) {
+    this.webhookSecret = webhookSecret;
+    this.stripeClient = stripeClient;
+  }
+
+  public <T extends EventNotification> StripeEventHandler register(
+      Class<T> eventClass, EventHandler<T> handler) {
+    this.registeredHandlers.put(eventClass.getName(), handler);
+    return this;
+  }
+
+  /**
+   * Handle an incoming webhook event notification.
+   *
+   * @param webhookBody the incoming webhook body
+   * @param sigHeader the incoming webhook signature header
+   * @throws SignatureVerificationException if the validation of the webhook signature fails
+   * @throws IllegalArgumentException if no handler is registered for the event type
+   * @throws Exception any other exception that the registered handler might throw
+   */
+  @SuppressWarnings("unchecked")
+  public void handle(String webhookBody, String sigHeader) throws SignatureVerificationException {
+    EventNotification eventNotification =
+        this.stripeClient.parseEventNotification(webhookBody, sigHeader, this.webhookSecret);
+
+    EventHandler<? extends EventNotification> handler =
+        // I don't _love_ getName as a matcher but lets us bucket all the unknown events in a way
+        // that checking .type doesn't.
+        registeredHandlers.get(eventNotification.getClass().getName());
+
+    if (handler == null) {
+      throw new IllegalArgumentException(
+          "No handler registered for event type: " + eventNotification.getType());
+    }
+
+    // TODO: re-bind client context
+    // this is technically unsafe but we control the registration API so should be ok
+    ((EventHandler<EventNotification>) handler).process(eventNotification, this.stripeClient);
+  }
+}