Support sequenced collections (JDK 21)

pom.xml

@@ -394,6 +394,59 @@
         </plugins>
       </build>
     </profile>
+    <profile>
+      <!-- And different set up for JDK 21 -->
+      <id>java21</id>
+      <activation>
+        <jdk>21</jdk>
+      </activation>
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>org.codehaus.mojo</groupId>
+            <artifactId>build-helper-maven-plugin</artifactId>
+            <executions>
+              <execution>
+                <id>add-test-source</id>
+                <phase>generate-test-sources</phase>
+                <goals>
+                  <goal>add-test-source</goal>
+                </goals>
+                <configuration>
+                  <sources>
+                    <source>src/test-jdk14/java</source>
+                    <source>src/test-jdk17/java</source>
+                    <source>src/test-jdk21/java</source>
+                  </sources>
+                </configuration>
+              </execution>
+            </executions>
+          </plugin>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-compiler-plugin</artifactId>
+            <inherited>true</inherited>
+            <configuration>
+              <!-- Enable Java 21 for all sources so that Intellij picks the right language level -->
+              <source>21</source>
+              <release>21</release>
+              <compilerArgs>
+                <arg>-parameters</arg>
+                <arg>--add-opens=java.base/java.lang=ALL-UNNAMED</arg>
+                <arg>--add-opens=java.base/java.util=ALL-UNNAMED</arg>
+              </compilerArgs>
+            </configuration>
+          </plugin>
+          <plugin>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-surefire-plugin</artifactId>
+            <configuration>
+              <argLine>--add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED</argLine>
+            </configuration>
+          </plugin>
+        </plugins>
+      </build>
+    </profile>
     <profile>
       <id>errorprone</id>
       <build>

src/main/java/com/fasterxml/jackson/databind/deser/BasicDeserializerFactory.java

@@ -2562,6 +2562,10 @@ protected static class ContainerDefaultMappings {
             fallbacks.put(Deque.class.getName(), LinkedList.class);
             fallbacks.put(NavigableSet.class.getName(), TreeSet.class);
 
+            // Sequenced types added in JDK21
+            fallbacks.put("java.util.SequencedCollection", DEFAULT_LIST);
+            fallbacks.put("java.util.SequencedSet", LinkedHashSet.class);
+
             _collectionFallbacks = fallbacks;
         }
 
@@ -2582,6 +2586,9 @@ protected static class ContainerDefaultMappings {
             fallbacks.put(java.util.concurrent.ConcurrentNavigableMap.class.getName(),
                     java.util.concurrent.ConcurrentSkipListMap.class);
 
+            // Sequenced types added in JDK21
+            fallbacks.put("java.util.SequencedMap", LinkedHashMap.class);
+
             _mapFallbacks = fallbacks;
         }
 

src/test-jdk21/java/com/fasterxml/jackson/databind/jdk21/Java21CollectionsTest.java

@@ -0,0 +1,51 @@
+package com.fasterxml.jackson.databind.jdk21;
+
+import java.util.*;
+
+import com.fasterxml.jackson.databind.BaseMapTest;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.json.JsonMapper;
+
+public class Java21CollectionsTest extends BaseMapTest
+{
+    // [databind#4089]
+    record SequencedCollections(
+            SequencedCollection<String> sequencedCollection,
+            SequencedSet<String> sequencedSet,
+            SequencedMap<String, Integer> sequencedMap) {
+    }
+
+    public void testSequencedCollectionTypesDeserialize() throws Exception {
+        String json = """
+                {
+                    "sequencedCollection": ["A", "B"],
+                    "sequencedSet": ["C", "D"],
+                    "sequencedMap": {"A": 1, "B": 2}
+                }
+                """;
+
+        ObjectMapper objectMapper = JsonMapper.builder().build();
+        SequencedCollections value = objectMapper.readValue(json, SequencedCollections.class);
+        assertEquals(ArrayList.class, value.sequencedCollection.getClass());
+        assertEquals(LinkedHashSet.class, value.sequencedSet.getClass());
+        assertEquals(LinkedHashMap.class, value.sequencedMap.getClass());
+    }
+
+    public void testSequencedCollectionTypesRoundTrip() throws Exception {
+        ArrayList<String> arrayList = new ArrayList<>();
+        arrayList.add("A");
+        arrayList.add("B");
+        LinkedHashSet<String> linkedHashSet = new LinkedHashSet<>();
+        linkedHashSet.add("C");
+        linkedHashSet.add("D");
+        LinkedHashMap<String, Integer> linkedHashMap = new LinkedHashMap<>();
+        linkedHashMap.put("A", 1);
+        linkedHashMap.put("B", 2);
+        SequencedCollections input = new SequencedCollections(arrayList, linkedHashSet, linkedHashMap);
+
+        ObjectMapper objectMapper = JsonMapper.builder().build();
+        String json = objectMapper.writeValueAsString(input);
+        SequencedCollections value = objectMapper.readValue(json, SequencedCollections.class);
+        assertEquals(input, value);
+    }
+}