使用 Jackson 反序列化自定义 Map

Question

我目前正在尝试使用 Jackson 2.8.4 将自定义 Map 类型序列化和反序列化为 JSON。我已经设法使基于this answer 的序列化工作，但我在反序列化方面苦苦挣扎。这是一个例子：

public class TestMapSerialize {

    public interface TestMapGetters {
        Map<String, String> getFooMap();
        Map<String, String> getBarMap();
    }

    @JsonSerialize(as = TestMapGetters.class)
    public static class TestMap extends ForwardingMap<String, String>
            implements TestMapGetters {

        private Map<String, String> fooMap;
        private Map<String, String> barMap;

        @Override
        protected Map<String, String> delegate() {
            return ImmutableMap.<String, String>builder()
                    .putAll(fooMap)
                    .putAll(barMap)
                    .build();
        }

        // Getters and setters...

    }

    public static void main(String[] args) throws IOException {
        final ObjectMapper mapper = new ObjectMapper();
        final TestMap map = new TestMap();

        map.setFooMap(ImmutableMap.of("a", "b", "c", "d"));
        map.setBarMap(ImmutableMap.of("e", "f", "g", "h"));

        final String json = mapper.writeValueAsString(map);

        // Prints {"fooMap":{"a":"b","c":"d"},"barMap":{"e":"f","g":"h"}} as expected.
        System.out.println(json);

        // Throws JsonMappingException, see below.
        final TestMap copy = mapper.readValue(json, TestMap.class);
    }

}

当我尝试像上面那样反序列化时，我得到一个异常：

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of START_OBJECT token
 at [Source: {"fooMap":{"a":"b","c":"d"},"barMap":{"e":"f","g":"h"}}; line: 1, column: 11] (through reference chain: test.TestMapSerialize$TestMap["fooMap"])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:270)
    at com.fasterxml.jackson.databind.DeserializationContext.reportMappingException(DeserializationContext.java:1234)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1122)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1075)
    at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:60)
    at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:11)
    at com.fasterxml.jackson.databind.deser.std.MapDeserializer._readAndBindStringKeyMap(MapDeserializer.java:517)
    at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:362)
    at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:27)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3798)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2842)
    at test.TestMapSerialize.main(TestMapSerialize.java:66)

所以 Jackson 仍在尝试将我的课程反序列化，就好像它是一个通用的 Map，因为它是一个 bean，所以它不起作用。我也尝试过添加@JsonDeserialize(as = TestMapGetters.class)，但后来我得到：

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Failed to narrow type [map type; class test.TestMapSerialize$TestMap, [simple type, class java.lang.String] -> [simple type, class java.lang.String]] with annotation (value test.TestMapSerialize$TestMapGetters), from 'test.TestMapSerialize$TestMap': Class test.TestMapSerialize$TestMapGetters not subtype of [map type; class test.TestMapSerialize$TestMap, [simple type, class java.lang.String] -> [simple type, class java.lang.String]]
    at com.fasterxml.jackson.databind.AnnotationIntrospector.refineDeserializationType(AnnotationIntrospector.java:1194)
    at com.fasterxml.jackson.databind.deser.DeserializerCache.modifyTypeByAnnotation(DeserializerCache.java:519)
    at com.fasterxml.jackson.databind.deser.DeserializerCache._createDeserializer(DeserializerCache.java:333)
    at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCache2(DeserializerCache.java:264)
    at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCacheValueDeserializer(DeserializerCache.java:244)
    at com.fasterxml.jackson.databind.deser.DeserializerCache.findValueDeserializer(DeserializerCache.java:142)
    at com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer(DeserializationContext.java:476)
    at com.fasterxml.jackson.databind.ObjectMapper._findRootDeserializer(ObjectMapper.java:3899)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3794)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2842)
    at test.TestMapSerialize.main(TestMapSerialize.java:68)
Caused by: java.lang.IllegalArgumentException: Class test.TestMapSerialize$TestMapGetters not subtype of [map type; class test.TestMapSerialize$TestMap, [simple type, class java.lang.String] -> [simple type, class java.lang.String]]
    at com.fasterxml.jackson.databind.type.TypeFactory.constructSpecializedType(TypeFactory.java:359)
    at com.fasterxml.jackson.databind.AnnotationIntrospector.refineDeserializationType(AnnotationIntrospector.java:1192)
    ... 10 more

这是有道理的，因为它不是子类型。我宁愿不必编写自定义反序列化器，因为如果我的对象不是Map 的子类型，这将很好地反序列化。有什么方法可以告诉 Jackson 忽略我的课程是 Map 并且只是反序列化为 bean 的事实？

Answer 1

我发现我可以在 TestMap 类中添加一个静态创建者方法：

@JsonCreator
static TestMap fromJson(
        @JsonProperty("fooMap") ImmutableMap<String, String> fooMap,
        @JsonProperty("barMap") ImmutableMap<String, String> barMap) {
    final TestMap map = new TestMap();
    map.setFooMap(fooMap);
    map.setBarMap(barMap);
    return map;
}

如果有办法说“只是作为 POJO 进行序列化和反序列化”以避免整个 @JsonCreator 和 @JsonSerialize(as) rigamarole，那就太好了。