Fix #5475: add support for @JsonDeserializeAs (#5484)

cowtowncoder · web-flow · commit c8abad974fcf · 2025-12-09T14:05:08.000-08:00
diff --git a/release-notes/VERSION-2.x b/release-notes/VERSION-2.x
@@ -30,6 +30,8 @@ Project: jackson-databind
  (contributed by Hélios G)
 #5429: Formatting and Parsing of Large ISO-8601 Dates is inconsistent
  (reported by @DavTurns)
+#5475: Support `@JsonDeserializeAs` annotation
+ (implemented by @cowtowncoder, w/ Claude code)
 
 2.20.2 (not yet released)
 
diff --git a/src/main/java/com/fasterxml/jackson/databind/introspect/JacksonAnnotationIntrospector.java b/src/main/java/com/fasterxml/jackson/databind/introspect/JacksonAnnotationIntrospector.java
@@ -50,7 +50,8 @@ public class JacksonAnnotationIntrospector
     @SuppressWarnings("unchecked")
     private final static Class<? extends Annotation>[] ANNOTATIONS_TO_INFER_DESER = (Class<? extends Annotation>[])
             new Class<?>[] {
-        JsonDeserialize.class,
+        JsonDeserialize.class, // databind-specific
+        JsonDeserializeAs.class, // since 2.21 alias (and eventual replacement) for `@JsonDeserialize.as`
         JsonView.class,
         JsonFormat.class,
         JsonTypeInfo.class,
@@ -1295,9 +1296,14 @@ public JavaType refineDeserializationType(final MapperConfig<?> config,
         final TypeFactory tf = config.getTypeFactory();
 
         final JsonDeserialize jsonDeser = _findAnnotation(a, JsonDeserialize.class);
+        final JsonDeserializeAs jsonDeserAs = _findAnnotation(a, JsonDeserializeAs.class);
 
         // Ok: start by refining the main type itself; common to all types
-        final Class<?> valueClass = (jsonDeser == null) ? null : _classIfExplicit(jsonDeser.as());
+        Class<?> valueClass = (jsonDeser == null) ? null : _classIfExplicit(jsonDeser.as());
+        // 09-Dec-2025, tatu: [databind#5475] Also check @JsonDeserializeAs
+        if (valueClass == null && jsonDeserAs != null) {
+            valueClass = _classIfExplicit(jsonDeserAs.value(), Void.class);
+        }
         if ((valueClass != null) && !type.hasRawClass(valueClass)
                 && !_primitiveAndWrapper(type, valueClass)) {
             try {
@@ -1313,7 +1319,11 @@ public JavaType refineDeserializationType(final MapperConfig<?> config,
         // First, key type (for Maps, Map-like types):
         if (type.isMapLikeType()) {
             JavaType keyType = type.getKeyType();
-            final Class<?> keyClass = (jsonDeser == null) ? null : _classIfExplicit(jsonDeser.keyAs());
+            Class<?> keyClass = (jsonDeser == null) ? null : _classIfExplicit(jsonDeser.keyAs());
+            // 09-Dec-2025, tatu: [databind#5475] Also check @JsonDeserializeAs
+            if (keyClass == null && jsonDeserAs != null) {
+                keyClass = _classIfExplicit(jsonDeserAs.keys(), Void.class);
+            }
             if ((keyClass != null)
                     && !_primitiveAndWrapper(keyType, keyClass)) {
                 try {
@@ -1329,7 +1339,11 @@ public JavaType refineDeserializationType(final MapperConfig<?> config,
         JavaType contentType = type.getContentType();
         if (contentType != null) { // collection[like], map[like], array, reference
             // And then value types for all containers:
-            final Class<?> contentClass = (jsonDeser == null) ? null : _classIfExplicit(jsonDeser.contentAs());
+            Class<?> contentClass = (jsonDeser == null) ? null : _classIfExplicit(jsonDeser.contentAs());
+            // 09-Dec-2025, tatu: [databind#5475] Also check @JsonDeserializeAs
+            if (contentClass == null && jsonDeserAs != null) {
+                contentClass = _classIfExplicit(jsonDeserAs.content(), Void.class);
+            }
             if ((contentClass != null)
                     && !_primitiveAndWrapper(contentType, contentClass)) {
                 try {
diff --git a/src/test/java/com/fasterxml/jackson/databind/deser/JsonDeserializeAsTest.java b/src/test/java/com/fasterxml/jackson/databind/deser/JsonDeserializeAsTest.java
@@ -0,0 +1,280 @@
+package com.fasterxml.jackson.databind.deser;
+
+import java.util.*;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.annotation.JsonDeserializeAs;
+import com.fasterxml.jackson.databind.*;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import static com.fasterxml.jackson.databind.testutil.DatabindTestUtil.newJsonMapper;
+
+/**
+ * Unit tests for new {@link JsonDeserializeAs} annotation.
+ *
+ * @since 2.21
+ */
+public class JsonDeserializeAsTest
+{
+    /*
+    /**********************************************************************
+    /* Annotated root classes for @JsonDeserializeAs
+    /**********************************************************************
+     */
+
+    @JsonDeserializeAs(RootInterfaceImpl.class)
+    interface RootInterface {
+        public String getA();
+    }
+
+    static class RootInterfaceImpl implements RootInterface {
+        public String a;
+
+        public RootInterfaceImpl() { }
+
+        @Override
+        public String getA() { return a; }
+    }
+
+    /*
+    /**********************************************************************
+    /* Annotated helper classes for @JsonDeserializeAs#value
+    /**********************************************************************
+     */
+
+    // Class for testing valid {@link JsonDeserializeAs} annotation
+    // with 'value' parameter to define concrete class to deserialize to
+    static class CollectionHolder
+    {
+        Collection<String> _strings;
+
+        /* Default for 'Collection' would probably be ArrayList or so;
+         * let's try to make it a TreeSet instead.
+         */
+        @JsonDeserializeAs(TreeSet.class)
+        public void setStrings(Collection<String> s)
+        {
+            _strings = s;
+        }
+    }
+
+    // Another class for testing valid {@link JsonDeserializeAs} annotation
+    // with 'value' parameter to define concrete class to deserialize to
+    static class MapHolder
+    {
+        // Let's also coerce numbers into Strings here
+        Map<String,String> _data;
+
+        /* Default for 'Map' would be HashMap,
+         * let's try to make it a TreeMap instead.
+         */
+        @JsonDeserializeAs(TreeMap.class)
+        public void setStrings(Map<String,String> s)
+        {
+            _data = s;
+        }
+    }
+
+    // Another class for testing valid {@link JsonDeserializeAs} annotation
+    // with 'value' parameter, but with array
+    static class ArrayHolder
+    {
+        String[] _strings;
+
+        @JsonDeserializeAs(String[].class)
+        public void setStrings(Object[] o)
+        {
+            // should be passed instances of proper type, as per annotation
+            _strings = (String[]) o;
+        }
+    }
+
+    /*
+    /**********************************************************************
+    /* Annotated helper classes for @JsonDeserializeAs#keys
+    /**********************************************************************
+     */
+
+    static class StringWrapper
+    {
+        final String _string;
+
+        public StringWrapper(String s) { _string = s; }
+    }
+
+    static class MapKeyHolder
+    {
+        Map<Object, String> _map;
+
+        @JsonDeserializeAs(keys=StringWrapper.class)
+        public void setMap(Map<Object,String> m)
+        {
+            // type should be ok, but no need to cast here (won't matter)
+            _map = m;
+        }
+    }
+
+    /*
+    /**********************************************************************
+    /* Annotated helper classes for @JsonDeserializeAs#content
+    /**********************************************************************
+     */
+
+    static class ListContentHolder
+    {
+        List<?> _list;
+
+        @JsonDeserializeAs(content=StringWrapper.class)
+        public void setList(List<?> l) {
+            _list = l;
+        }
+    }
+
+    static class ArrayContentHolder
+    {
+        Object[] _data;
+
+        @JsonDeserializeAs(content=Long.class)
+        public void setData(Object[] o)
+        { // should have proper type, but no need to coerce here
+            _data = o;
+        }
+    }
+
+    static class MapContentHolder
+    {
+        Map<Object,Object> _map;
+
+        @JsonDeserializeAs(content=Integer.class)
+        public void setMap(Map<Object,Object> m)
+        {
+            _map = m;
+        }
+    }
+
+    /*
+    /**********************************************************
+    /* Test methods for @JsonDeserializeAs#value
+    /**********************************************************
+     */
+
+    private final ObjectMapper MAPPER = newJsonMapper();
+
+    @Test
+    public void testOverrideClassValid() throws Exception
+    {
+        CollectionHolder result = MAPPER.readValue
+            ("{ \"strings\" : [ \"test\" ] }", CollectionHolder.class);
+
+        Collection<String> strs = result._strings;
+        assertEquals(1, strs.size());
+        assertEquals(TreeSet.class, strs.getClass());
+        assertEquals("test", strs.iterator().next());
+    }
+
+    @Test
+    public void testOverrideMapValid() throws Exception
+    {
+        // note: expecting conversion from number to String, as well
+        MapHolder result = MAPPER.readValue
+            ("{ \"strings\" :  { \"a\" : 3 } }", MapHolder.class);
+
+        Map<String,String> strs = result._data;
+        assertEquals(1, strs.size());
+        assertEquals(TreeMap.class, strs.getClass());
+        String value = strs.get("a");
+        assertEquals("3", value);
+    }
+
+    @Test
+    public void testOverrideArrayClass() throws Exception
+    {
+        ArrayHolder result = MAPPER.readValue
+            ("{ \"strings\" : [ \"test\" ] }", ArrayHolder.class);
+
+        String[] strs = result._strings;
+        assertEquals(1, strs.length);
+        assertEquals(String[].class, strs.getClass());
+        assertEquals("test", strs[0]);
+    }
+
+    /*
+    /**********************************************************
+    /* Test methods for @JsonDeserializeAs#value used for root values
+    /**********************************************************
+     */
+
+    @Test
+    public void testRootInterfaceAs() throws Exception
+    {
+        RootInterface value = MAPPER.readValue("{\"a\":\"abc\" }", RootInterface.class);
+        assertTrue(value instanceof RootInterfaceImpl);
+        assertEquals("abc", value.getA());
+    }
+
+    /*
+    /**********************************************************
+    /* Test methods for @JsonDeserializeAs#keys
+    /**********************************************************
+     */
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void testOverrideKeyClassValid() throws Exception
+    {
+        MapKeyHolder result = MAPPER.readValue("{ \"map\" : { \"xxx\" : \"yyy\" } }", MapKeyHolder.class);
+        Map<StringWrapper, String> map = (Map<StringWrapper,String>)(Map<?,?>)result._map;
+        assertEquals(1, map.size());
+        Map.Entry<StringWrapper, String> en = map.entrySet().iterator().next();
+
+        StringWrapper key = en.getKey();
+        assertEquals(StringWrapper.class, key.getClass());
+        assertEquals("xxx", key._string);
+        assertEquals("yyy", en.getValue());
+    }
+
+    /*
+    /**********************************************************
+    /* Test methods for @JsonDeserializeAs#content
+    /**********************************************************
+     */
+
+    @SuppressWarnings("unchecked")
+    @Test
+    public void testOverrideContentClassValid() throws Exception
+    {
+        ListContentHolder result = MAPPER.readValue("{ \"list\" : [ \"abc\" ] }", ListContentHolder.class);
+        List<StringWrapper> list = (List<StringWrapper>)result._list;
+        assertEquals(1, list.size());
+        Object value = list.get(0);
+        assertEquals(StringWrapper.class, value.getClass());
+        assertEquals("abc", ((StringWrapper) value)._string);
+    }
+
+    @Test
+    public void testOverrideArrayContents() throws Exception
+    {
+        ArrayContentHolder result = MAPPER.readValue("{ \"data\" : [ 1, 2, 3 ] }",
+                ArrayContentHolder.class);
+        Object[] data = result._data;
+        assertEquals(3, data.length);
+        assertEquals(Long[].class, data.getClass());
+        assertEquals(1L, data[0]);
+        assertEquals(2L, data[1]);
+        assertEquals(3L, data[2]);
+    }
+
+    @Test
+    public void testOverrideMapContents() throws Exception
+    {
+        MapContentHolder result = MAPPER.readValue("{ \"map\" : { \"a\" : 9 } }",
+                MapContentHolder.class);
+        Map<Object,Object> map = result._map;
+        assertEquals(1, map.size());
+        Object ob = map.values().iterator().next();
+        assertEquals(Integer.class, ob.getClass());
+        assertEquals(Integer.valueOf(9), ob);
+    }
+}
diff --git a/src/test/java/com/fasterxml/jackson/databind/deser/ValueAnnotationsDeserTest.java b/src/test/java/com/fasterxml/jackson/databind/deser/ValueAnnotationsDeserTest.java
