Part 0: shared immutable mapper, with the Jackson 2 lines that no longer apply

src/main/java/com/ankurm/jackson3/part0setup/A02SharedMapperConfiguration.java

@@ -0,0 +1,64 @@
+package com.ankurm.jackson3.part0setup;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import tools.jackson.databind.DeserializationFeature;
+import tools.jackson.databind.json.JsonMapper;
+
+import java.time.LocalDate;
+
+/**
+ * Post: Jackson 101 — https://ankurm.com/jackson-java-tutorial/
+ * Section: "The ObjectMapper Lifecycle — The One Rule That Matters Most"
+ *
+ * Build the mapper ONCE at startup and share it. In Jackson 3 this is enforced
+ * by the API rather than by convention: the built mapper has no setters at all.
+ */
+public class A02SharedMapperConfiguration {
+
+    public record Invoice(Long invoiceId, String customerName, LocalDate issuedOn, String note) { }
+
+    /**
+     * The Jackson 3 equivalent of the classic Spring @Bean ObjectMapper.
+     *
+     * Three of the four settings people habitually copy from Jackson 2 tutorials
+     * are unnecessary or wrong in Jackson 3 — the comments say which and why.
+     */
+    static JsonMapper buildSharedMapper() {
+        return JsonMapper.builder()
+                // NOT NEEDED: FAIL_ON_UNKNOWN_PROPERTIES already defaults to false in
+                // Jackson 3. Listed here only because it is the single most-copied line
+                // from Jackson 2 configuration; deleting it changes nothing.
+                .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+
+                // NOT NEEDED: java.time support is built into jackson-databind 3.x and
+                // dates already serialise as ISO-8601 strings. There is no
+                // SerializationFeature.WRITE_DATES_AS_TIMESTAMPS in Jackson 3 — the flag
+                // moved to tools.jackson.databind.cfg.DateTimeFeature and is off by
+                // default. See beyond/Y03DateTimeDefaults for proof.
+
+                // Skip null fields. Jackson 2's setSerializationInclusion(...) and the
+                // builder's serializationInclusion(...) do NOT exist in Jackson 3.
+                // The real API is changeDefaultPropertyInclusion.
+                .changeDefaultPropertyInclusion(
+                        incl -> incl.withValueInclusion(JsonInclude.Include.NON_NULL))
+                .build();
+    }
+
+    public static void main(String[] args) {
+        JsonMapper mapper = buildSharedMapper();
+
+        Invoice invoice = new Invoice(500L, "Alice", LocalDate.of(2026, 4, 9), null);
+        System.out.println("configured : " + mapper.writeValueAsString(invoice));
+
+        JsonMapper plain = JsonMapper.builder().build();
+        System.out.println("defaults   : " + plain.writeValueAsString(invoice));
+
+        // Proof that the mapper is immutable: ObjectMapper in Jackson 3 exposes no
+        // set*() mutators at all, so there is no way to reconfigure a shared instance.
+        long setterCount = java.util.Arrays.stream(
+                        tools.jackson.databind.ObjectMapper.class.getMethods())
+                .filter(m -> m.getName().startsWith("set"))
+                .count();
+        System.out.println("ObjectMapper set*() methods in Jackson 3: " + setterCount);
+    }
+}