使用Jackson反序列化值为List的Map<Long, List<Long>>时编译报错求助

尝试用Jackson的com.fasterxml.jackson.databind.ObjectMapper反序列化Map<Long, List<Long>>时出现编译错误，以下是相关代码及错误详情：

ObjectMapper初始化代码 #

// ObjectMapper initialization
public static final ObjectMapper objectMapper = JsonMapper.builder()
  .addModule(new JavaTimeModule())
  .addModule(new SimpleModule()
      .addAbstractTypeMapping(Claims.class, DefaultClaims.class)
      .addAbstractTypeMapping(GrantedAuthority.class, SimpleGrantedAuthority.class)
  )
  .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
  .addMixIn(PersonTraits.class, PersonTraitsMixIn.class)
  .addMixIn(SimpleGrantedAuthority.class, SimpleGrantedAuthorityMixin.class)
  .build();

反序列化方法 #

public static <K, V, T extends Map<?, ?>> Map<K, List<V>> deserializeMap(String genericObject, Class<T> mapClass,
          Class<K> keyClass, Class<List<V>> valueClass) throws Exception {
        if (genericObject == null) {
          return null;
        }
        try {
          return objectMapper.readValue(genericObject, objectMapper.getTypeFactory().constructMapType(mapClass, keyClass, valueClass));
        } catch (JsonProcessingException e) {
          // Do something
          throw e;
        }
      }

JUnit测试用例 #

@Test
public void testDeserializeMapContainingListAsValue() throws Exception {
  // given
  Map<Long, List<Long>> mapToSerialize = new HashMap<>();
  List<Long> valueList = new ArrayList<>();
  valueList.add(2L);
  mapToSerialize.put(1L, valueList);
  String serializedMap = SerializationUtil.serializeObject(mapToSerialize);

  // when
  final Map<Long, List<Long>> result = SerializationUtil.deserializeMap(serializedMap, HashMap.class, Long.class,
      List.class);

  // then
  assertNotNull(result);
}

编译错误信息 #

调用语句：

final Map<Long, List<Long>> result = SerializationUtil.deserializeMap(serializedMap1, HashMap.class, Long.class,
    List.class);

错误详情：

Required type.      Provided
mapClass:    Class<T>.           Class<HashMap>
keyClass:    Class<K>            Class<Long>
valueClass:  Class<List<V>>.     Class<List>
reason: Incompatible equality constraint: List<V> and List

问题原因 #

Java泛型存在类型擦除机制，运行时List<Long>和List的Class对象是同一个，无法直接传递Class<List<V>>类型的参数。你传入的List.class实际是Class<List>，和方法定义的Class<List<V>>不匹配，导致编译错误。

解决方案 #

修改反序列化方法，将参数从Class<List<V>> valueClass改为Class<V> valueElementType，然后在方法内部构造List<V>的类型：

public static <K, V, T extends Map<?, ?>> Map<K, List<V>> deserializeMap(String genericObject, Class<T> mapClass,
          Class<K> keyClass, Class<V> valueElementType) throws Exception {
        if (genericObject == null) {
          return null;
        }
        try {
          // 构造List<V>的类型
          Type listType = objectMapper.getTypeFactory().constructCollectionType(List.class, valueElementType);
          // 构造Map<K, List<V>>的类型
          JavaType mapType = objectMapper.getTypeFactory().constructMapType(mapClass, keyClass, listType);
          return objectMapper.readValue(genericObject, mapType);
        } catch (JsonProcessingException e) {
          // Do something
          throw e;
        }
      }

然后修改测试用例的调用，传递Long.class作为valueElementType：

final Map<Long, List<Long>> result = SerializationUtil.deserializeMap(serializedMap, HashMap.class, Long.class,
    Long.class);

这样就能正确编译并完成反序列化了。

内容的提问来源于stack exchange，提问作者Sandeep Barnabas