Part 2 extra: Jackson 3 introspects permits, so @JsonSubTypes can go

src/main/java/com/ankurm/jackson3/part2modernjava/C04SealedAutoDiscovery.java

@@ -0,0 +1,40 @@
+package com.ankurm.jackson3.part2modernjava;
+
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import com.fasterxml.jackson.annotation.JsonTypeName;
+import tools.jackson.databind.json.JsonMapper;
+
+/**
+ * BEYOND THE POST — the Jackson-3-only shortcut.
+ *
+ * Jackson 3 introspects the `permits` clause of a sealed type, so @JsonSubTypes
+ * can be dropped as long as each permitted type carries @JsonTypeName. That
+ * removes the parallel registry which, in Jackson 2, silently drifts out of sync
+ * with `permits` whenever someone adds a subtype.
+ *
+ * Note there is NO @JsonSubTypes anywhere in this file.
+ */
+public class C04SealedAutoDiscovery {
+
+    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "shapeType")
+    public sealed interface Shape permits Circle, Rectangle, Triangle { }
+
+    @JsonTypeName("circle")    public record Circle(double radius) implements Shape { }
+    @JsonTypeName("rectangle") public record Rectangle(double width, double height) implements Shape { }
+    // Added later. In Jackson 2 this line alone would break deserialisation until
+    // someone remembered to also add it to @JsonSubTypes. Here it just works.
+    @JsonTypeName("triangle")  public record Triangle(double base, double height) implements Shape { }
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+
+        for (Shape original : new Shape[] {
+                new Circle(5.0), new Rectangle(10.0, 4.0), new Triangle(3.0, 6.0) }) {
+
+            String json = mapper.writeValueAsString(original);
+            Shape restored = mapper.readValue(json, Shape.class);
+            System.out.printf("%-24s -> %-52s -> %s%n",
+                    original.getClass().getSimpleName(), json, restored);
+        }
+    }
+}