Added xml support for event listener and made minor cleanup

spring-data-mongodb/src/main/java/org/baeldung/event/FieldCallback.java

@@ -0,0 +1,22 @@
+package org.baeldung.event;
+
+import java.lang.reflect.Field;
+
+import org.springframework.data.annotation.Id;
+import org.springframework.util.ReflectionUtils;
+
+public class FieldCallback implements ReflectionUtils.FieldCallback {
+    private boolean idFound;
+
+    public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
+        ReflectionUtils.makeAccessible(field);
+
+        if (field.isAnnotationPresent(Id.class)) {
+            idFound = true;
+        }
+    }
+
+    public boolean isIdFound() {
+        return idFound;
+    }
+}