Jackson ObjectMapper用方括号包装每个元素（甚至是单个值）

Question

我有两个Map对象，将它们合并到Guava的Multimap中。合并后，我试图通过杰克逊序列化映射最终ObjectMapper用GuavaModule（否则它不序列正确）。但是，问题是，即使只有一个值，键的每个值也会作为列表的元素打印出来（objectmapper用方括号将每个值包装起来）。我得到的基本输出如下

{
    "address": {
        "city": ["san francisco"]
    },
    "company": {
        "employees": [
            [{
                "name": "David",
                "lastname": "Foobar",
                "age": 22
            }, {
                "name": "Michael",
                "lastname": "FizBuzz",
                "age": 35
            }]
        ]
    },
    "config": {
        "key1": ["value1", "value3"],
        "key2": ["value2"],
        "key4": ["value4"]
    }
}

如您所见，“地址”->“城市”->“旧金山”用方括号company -> employees包裹，也用两次包裹。此外，我已经尝试过SerializationFeature.WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED, trueFuture，但没有任何改变。

    Person person = new Person();
    person.setName("David");
    person.setLastname("Foobar");
    person.setAge(22);

    Person person2 = new Person();
    person2.setName("Michael");
    person2.setLastname("FizBuzz");
    person2.setAge(35);

    Map<String, Map<String, Object>> map1 = new ImmutableMap.Builder<String, Map<String, Object>>()
            .put("config", ImmutableMap.of(
                    "key1", "value1",
                    "key2", "value2"))
            .put("address", ImmutableMap.of("city", "san francisco"))
            .put("company", ImmutableMap.of("employees", Arrays.asList(person, person2)))
            .build();

    Map<String, Map<String, Object>> map2 = new ImmutableMap.Builder<String, Map<String, Object>>()
            .put("config", ImmutableMap.of(
                    "key1", "value3",
                    "key4", "value4"))
            .build();

    Map<String, Multimap<String, Object>> merged = new HashMap<>();
    BiFunction<Multimap<String, Object>,
                Multimap<String, Object>,
                Multimap<String, Object>> remappingFunction = (value1, value2) -> {
        Multimap<String, Object> multimap = HashMultimap.<String, Object>create();
        multimap.putAll(value1);
        multimap.putAll(value2);
        return multimap;
    };


    map1.forEach((key, value) -> merged.merge(key, Multimaps.forMap(value), remappingFunction));
    map2.forEach((key, value) -> merged.merge(key, Multimaps.forMap(value), remappingFunction));

    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.registerModule(new GuavaModule());
    objectMapper.configure(SerializationFeature.WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED, true);
    String str = objectMapper.writeValueAsString(merged);
    System.out.print(str);
}

感谢您的帮助。谢谢

Answer 1

根据Jackson的文档，MultimapSerializer“将所有值集都封装在JSON数组中，而与值的数量无关”。我相信这意味着它实际上是在忽略该WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED功能。

您可以通过在序列化之前将所有内部Multimap<String, Object>实例转换为标准Map<String, Collection<Object>>实例来避免此问题。您可以使用Multimap.asMap()以下方法执行此操作：

Map<String, Map<String, Collection<Object>>> newMerged = new HashMap<>();
merged.forEach((key, value) -> newMerged.put(key, value.asMap()));
String newStr = objectMapper.writeValueAsString(newMerged);
System.out.println(newStr);

这将输出您期望的输出：

{"address":{"city":"san francisco"},"company":{"employees":[{"name":"David","lastname":"Foobar","age":22},{"name":"Michael","lastname":"FizBuzz","age":35}]},"config":{"key1":["value1","value3"],"key2":"value2","key4":"value4"}}