Map<String, Object> 中的 Jackson java.util.Date 值（反）序列化

Question

考虑这个属性

@JsonProperty
private Map<String, Object> myMap;

当包含的java.util.Date 值被序列化时，它不会再次反序列化为Date，因为Map<String, Object> 中不存在类型信息。我怎样才能绕过这个问题？我阅读了有关this question 的答案，这将是一种解决方法，但无法区分包含日期的字符串与在地图中序列化为字符串的日期。 我可以告诉 Jackson 包含每个映射值的类型信息，以便 Jackson 可以正确反序列化它们吗？

Answer 1

最后，我想出了这个解决方案。反序列化器：

private TypeReference<Map<String, Object>> typeRef = new TypeReference<Map<String, Object>>() {
};

@Override
public Map<String, Object> deserialize(JsonParser p, DeserializationContext ctxt, Map<String, Object> target) throws IOException {
    Map<String, Object> map = new ObjectMapper().readValue(p, typeRef);

    for (Map.Entry<String, Object> e : map.entrySet()) {
        if (e.getKey().endsWith("[date]")) {
            target.put(e.getKey().substring(0, e.getKey().length() - 6), new Date((Long) e.getValue()));
        }
        else {
            target.put(e.getKey(), e.getValue());
        }
    }

    return target;
}

序列化器：

@Override
public void serialize(Map<String, Object> value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
    Map<String, Object> adaptedValue = new HashMap<>(value);

    for (Map.Entry<String, Object> e : value.entrySet()) {
        if (e.getValue() instanceof Date) {
            adaptedValue.put(e.getKey() + "[date]", ((Date) e.getValue()).getTime());
            adaptedValue.remove(e.getKey());
        }
    }

    new ObjectMapper().writeValue(gen, adaptedValue);
}

映射键根据数据类型进行调整。这很容易扩展。

Answer 2

实现自定义反序列化器并将注释@JsonDeserialize(using = DateDeserializer.class) 添加到您的字段。

看看这个例子：

你的 Json-Bean：

public class Foo {

    private String            name;

    @JsonProperty
    @JsonDeserialize(using = DateDeserializer.class)
    private Map<String, Object> dates;

    [...] // getter, setter, equals, hashcode
}

反序列化器：

public class DateDeserializer extends JsonDeserializer<Map<String, Object>> {

    private TypeReference<HashMap<String, Object>> typeRef = new TypeReference<HashMap<String, Object>>() {};

    @Override
    public Map<String, Object> deserialize(JsonParser p, DeserializationContext ctxt, Map<String, Object> target) throws IOException, JsonProcessingException {

        Map<String, Long> map = new ObjectMapper().readValue(p, typeRef);

        for(Entry<String, Long> e : map.entrySet()){

            Long value = e.getValue();
            String key = e.getKey();

            if(value instanceof Long){ // or if("date".equals(key)) ...
                target.put(key, new Date(value));
            } else {
                target.put(key, value); // leave as is
            }

        }

        return target;
    }

    @Override
    public Map<String, Object> deserialize(JsonParser paramJsonParser, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        return this.deserialize(paramJsonParser, ctxt, new HashMap<>());
    }

}

简单测试：

public static void main(String[] args) throws Exception {

    Foo foo1 = new Foo();
    foo1.setName("foo");
    foo1.setData(new HashMap<String, Object>(){{
        put("date",   new Date());
        put("bool",   true);
        put("string", "yeah");
    }});
    ObjectMapper mapper = new ObjectMapper();
    String jsonStr = mapper.writeValueAsString(foo1);
    System.out.println(jsonStr);
    Foo foo2 = mapper.readValue(jsonStr, Foo.class);

    System.out.println(foo2.equals(foo1));

}