Part 4: SimpleModule on the builder, and the BigDecimal scale trap

src/main/java/com/ankurm/jackson3/part4custom/E03SimpleModuleRegistration.java

@@ -0,0 +1,57 @@
+package com.ankurm.jackson3.part4custom;
+
+import tools.jackson.databind.DeserializationFeature;
+import tools.jackson.databind.json.JsonMapper;
+import tools.jackson.databind.module.SimpleModule;
+
+import java.math.BigDecimal;
+
+/**
+ * Post: Custom Serialisers and Mix-ins — https://ankurm.com/jackson-custom-serializer-mixin/
+ * Section: "Registering the Serialiser and Deserialiser via SimpleModule"
+ *
+ * Jackson 3 differences:
+ *   1. SimpleModule moved to tools.jackson.databind.module.
+ *   2. The Version-taking constructor from the post is gone; pass just a name.
+ *   3. The module is attached with builder.addModule(...), not mapper.registerModule(...),
+ *      because a built mapper is immutable.
+ */
+public class E03SimpleModuleRegistration {
+
+    public static void main(String[] args) {
+        SimpleModule moneyModule = new SimpleModule("MoneyModule");
+        moneyModule.addSerializer(Money.class,   new E01MoneyValueSerializer());
+        moneyModule.addDeserializer(Money.class, new E02MoneyValueDeserializer());
+
+        JsonMapper mapper = JsonMapper.builder()
+                .addModule(moneyModule)
+                .build();
+
+        // Serialise: note the rounding and the upper-casing done by the serialiser.
+        Money price = new Money(new BigDecimal("19.999"), "usd");
+        System.out.println("serialised   : " + mapper.writeValueAsString(price));
+
+        // Deserialise. NOTE: the scale is NOT preserved by default — Jackson parses
+        // 20.00 as a double first, so you get 20.0 and not 20.00. The blog post claims
+        // 20.00; that only holds if you turn on USE_BIG_DECIMAL_FOR_FLOATS, below.
+        Money restored = mapper.readValue("{\"amount\":20.00,\"currency\":\"USD\"}", Money.class);
+        System.out.println("amount       : " + restored.amount() + "  (scale lost)");
+
+        JsonMapper exact = JsonMapper.builder()
+                .addModule(moneyModule)
+                .enable(DeserializationFeature.USE_BIG_DECIMAL_FOR_FLOATS)
+                .build();
+        System.out.println("amount exact : "
+                + exact.readValue("{\"amount\":20.00,\"currency\":\"USD\"}", Money.class).amount()
+                + "  (scale preserved)");
+
+        // The path()-based deserialiser tolerates a missing field; the post's get()
+        // version would throw NullPointerException here.
+        Money partial = mapper.readValue("{\"currency\":\"EUR\"}", Money.class);
+        System.out.println("missing field: " + partial);
+
+        // Without the module the record would serialise structurally instead.
+        JsonMapper plain = JsonMapper.builder().build();
+        System.out.println("no module    : " + plain.writeValueAsString(price));
+    }
+}