Part 3: @JsonAlias, and why @JsonIgnoreProperties matters less in Jackson 3

src/main/java/com/ankurm/jackson3/part3annotations/D03AliasAndUnknownFields.java

@@ -0,0 +1,55 @@
+package com.ankurm.jackson3.part3annotations;
+
+import com.fasterxml.jackson.annotation.JsonAlias;
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+import tools.jackson.databind.DeserializationFeature;
+import tools.jackson.databind.json.JsonMapper;
+
+/**
+ * Post: Jackson Annotations Cheat Sheet — https://ankurm.com/jackson-annotations-guide/
+ * Sections: "@JsonAlias" and "@JsonIgnoreProperties"
+ *
+ * Correction to the post: it frames @JsonIgnoreProperties(ignoreUnknown = true) as the
+ * per-class alternative to configuring FAIL_ON_UNKNOWN_PROPERTIES globally. In Jackson 3
+ * that feature is already DISABLED by default, so unknown fields are tolerated with no
+ * annotation at all. The annotation now matters mainly when you have deliberately turned
+ * strictness back ON — which is what the `strict` mapper below does.
+ */
+public class D03AliasAndUnknownFields {
+
+    public record SearchQuery(
+            @JsonAlias({ "q", "query", "search_term" }) String searchKeyword) { }
+
+    public record LenientResponse(String status, String message) { }
+
+    @JsonIgnoreProperties(ignoreUnknown = true)
+    public record OptedOutResponse(String status, String message) { }
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+
+        for (String json : new String[] {
+                "{\"q\":\"jackson\"}", "{\"query\":\"jackson\"}", "{\"search_term\":\"jackson\"}" }) {
+            System.out.println("alias " + String.format("%-24s", json)
+                    + " -> " + mapper.readValue(json, SearchQuery.class).searchKeyword());
+        }
+
+        String extra = "{\"status\":\"OK\",\"message\":\"done\",\"undocumentedField\":42}";
+
+        // Default Jackson 3 mapper: unknown fields are already ignored.
+        System.out.println("default mapper : " + mapper.readValue(extra, LenientResponse.class));
+
+        // A mapper with strictness deliberately re-enabled.
+        JsonMapper strict = JsonMapper.builder()
+                .enable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
+                .build();
+        try {
+            strict.readValue(extra, LenientResponse.class);
+            System.out.println("strict mapper  : unreachable");
+        } catch (Exception e) {
+            System.out.println("strict mapper  : " + e.getClass().getSimpleName() + " (as expected)");
+        }
+        // ...but the annotation opts this one class back out of the strictness.
+        System.out.println("strict + anno  : " + strict.readValue(extra, OptedOutResponse.class));
+    }
+}