Jackson 3 series companion code

37 runnable examples covering the eight feature posts on ankurm.com, verified
against Jackson 3.2.1 on Temurin 21.0.5. Every output committed under docs/ was
produced by run-all.sh.

Also documents 11 places where the published snippets do not compile or do not
behave as printed against a real Jackson 3 build - most notably that
writeValueAsString(List<Base>) silently drops the polymorphic type discriminator,
so the post's serialised output cannot be read back.

src/main/java/com/ankurm/jackson3/part4custom/E05ValueSerializerDirect.java

@@ -0,0 +1,43 @@
+package com.ankurm.jackson3.part4custom;
+
+import tools.jackson.core.JsonGenerator;
+import tools.jackson.databind.SerializationContext;
+import tools.jackson.databind.ValueSerializer;
+import tools.jackson.databind.json.JsonMapper;
+import tools.jackson.databind.module.SimpleModule;
+
+/**
+ * BEYOND THE POST — the rename that breaks every custom handler on upgrade.
+ *
+ * Jackson 2's JsonSerializer<T>/JsonDeserializer<T> are gone. The Jackson 3 names
+ * are ValueSerializer<T>/ValueDeserializer<T>. Extending ValueSerializer directly
+ * (rather than StdSerializer) is the leanest form and shows the rename plainly.
+ */
+public class E05ValueSerializerDirect {
+
+    public record UserId(String value) { }
+
+    /** Renders the wrapper as a bare JSON string rather than {"value":"..."}. */
+    static class UserIdSerializer extends ValueSerializer<UserId> {
+        @Override
+        public void serialize(UserId id, JsonGenerator gen, SerializationContext ctxt) {
+            gen.writeString(id.value());
+        }
+    }
+
+    public record Ticket(UserId assignee, String title) { }
+
+    public static void main(String[] args) {
+        SimpleModule module = new SimpleModule("UserIdModule");
+        module.addSerializer(UserId.class, new UserIdSerializer());
+
+        JsonMapper mapper = JsonMapper.builder().addModule(module).build();
+        System.out.println("custom  : "
+                + mapper.writeValueAsString(new Ticket(new UserId("u-42"), "Fix build")));
+        System.out.println("default : "
+                + JsonMapper.builder().build()
+                        .writeValueAsString(new Ticket(new UserId("u-42"), "Fix build")));
+        System.out.println("base class: "
+                + UserIdSerializer.class.getSuperclass().getName());
+    }
+}