Part 4: custom deserialiser using ctxt.readTree and null-safe path()

src/main/java/com/ankurm/jackson3/part4custom/E02MoneyValueDeserializer.java

@@ -0,0 +1,35 @@
+package com.ankurm.jackson3.part4custom;
+
+import tools.jackson.core.JsonParser;
+import tools.jackson.databind.DeserializationContext;
+import tools.jackson.databind.JsonNode;
+import tools.jackson.databind.deser.std.StdDeserializer;
+
+import java.math.BigDecimal;
+
+/**
+ * Post: Custom Serialisers and Mix-ins — https://ankurm.com/jackson-custom-serializer-mixin/
+ * Section: "Writing a Custom Deserialiser"
+ *
+ * Jackson 3 differences from the post's code:
+ *   1. jsonParser.getCodec().readTree(jsonParser) is gone. Use ctxt.readTree(parser).
+ *   2. No `throws IOException` — JacksonException is unchecked.
+ *   3. path() rather than get(), so a missing field yields a MissingNode instead of
+ *      a NullPointerException. The post's version NPEs on {"currency":"USD"}.
+ *   4. A bare decimalValue() on a MissingNode THROWS in Jackson 3 (it returned
+ *      BigDecimal.ZERO in Jackson 2). Use the defaulting overload.
+ */
+public class E02MoneyValueDeserializer extends StdDeserializer<Money> {
+
+    public E02MoneyValueDeserializer() {
+        super(Money.class);
+    }
+
+    @Override
+    public Money deserialize(JsonParser jsonParser, DeserializationContext ctxt) {
+        JsonNode rootNode = ctxt.readTree(jsonParser);
+        BigDecimal amount = rootNode.path("amount").decimalValue(BigDecimal.ZERO);
+        String currency   = rootNode.path("currency").asString("GBP");  // default when absent
+        return new Money(amount, currency);
+    }
+}