即使我添加了定制的反序列化器,杰克逊也无法将枚举反序列化为对象

Question

我想使用Jackson JSON序列化/反序列化包含枚举对象的类.我的班级是:

class Bar {

    @JsonProperty("rateType")
    @JsonDeserialize(using = ReturnedRateTypeDeserializer.class)
    private ReturnedRateType rateType;

    public ReturnedRateType getRateType() {
        return rateType;
    }

    public void setRateType(ReturnedRateType rateType) {
        this.rateType = rateType;
    }
}

枚举类ReturnedRateType定义为:

@JsonFormat(shape = JsonFormat.Shape.OBJECT)
public enum ReturnedRateType {
    AA("AA"),
    BB("BB"),
    CC("CC");

    @JsonProperty("value")
    private String value;

    ReturnedRateType(String value) {
        this.value = value;
    }

    @JsonCreator
    public static ReturnedRateType fromValue(final String value) {
        if (value != null) {
            for (ReturnedRateType type : ReturnedRateType.values()) {
                if (value.equalsIgnoreCase(type.value)) {
                    return type;
                }
            }
        }
        return null;
    }
}

如您所见,我添加了@JsonFormat注释,告诉Jackson将此枚举序列化为POJO,并添加@JsonCreator注释以获取从给定字符串到枚举对象的静态工厂方法.由于Jackson只能序列化但无法从对象表示反序列化为枚举,因此我为枚举添加了以下自定义反序列化器ReturnedRateType:

public class ReturnedRateTypeDeserializer extends JsonDeserializer<ReturnedRateType> {

    @Override
    public ReturnedRateType deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        ReturnedRateType type = ReturnedRateType.fromValue(jp.getValueAsString());
        if(type != null)
            return type;
        throw new JsonMappingException("invalid value for ReturnedRateType");
    }
} 

但是,当我测试从JSON字符串到枚举的反序列化时,我得到了错误.JSON字符串是:

{"rateType": {"value": "AA"}}

我的测试代码是:

@Test
public void RateTypeToEnum() {
    String json = "{\"rateType\": {\"value\": \"AA\"}}";
    System.out.println(json);
    ObjectMapper mapper = new ObjectMapper();
    Bar bar = null;
    try {
        bar = mapper.readValue(json, Bar.class);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    System.out.println(bar.getRateType());
}

我希望看到输出应该是AA.但jp.getValueAsString()在我的自定义反序列化器ReturnedRateTypeDeserializer执行期间为null:

ReturnedRateType type = ReturnedRateType.fromValue(jp.getValueAsString());  //jp.getValueAsString() is null here!

因此它返回错误.那么这里有什么问题？

Answer 1

根据杰克逊2.5.X关于JsonFormat注释的文档,这Shape.Object不适用于枚举反序列化:

• 枚举:形状JsonFormat.Shape.STRING和JsonFormat.Shape.NUMBER可用于在数字(索引)和文本(name或toString())之间进行更改; 但也可以使用JsonFormat.Shape.OBJECT序列化(但不反序列化).

我让JsonCreator静态方法接受JsonNode并从中读取字符串值.

请注意,这将起作用2.5.X.在早期版本中,您需要编写自定义反序列化器.这是一个例子:

public class JacksonEnumObjectShape {
    @JsonFormat(shape = JsonFormat.Shape.OBJECT)
    @JsonDeserialize(using = ReturnedRateTypeDeserializer.class)
    public enum ReturnedRateType {
        AA("AA"),
        BB("BB"),
        CC("CC");

        @JsonProperty("value")
        private String value;

        ReturnedRateType(String value) {
            this.value = value;
        }

        @JsonCreator
        public static ReturnedRateType fromValue(final JsonNode jsonNode) {

            for (ReturnedRateType type : ReturnedRateType.values()) {
                if (type.value.equals(jsonNode.get("value").asText())) {
                    return type;
                }
            }
            return null;
        }
    }
    // can be avoided since 2.5
    public static class ReturnedRateTypeDeserializer extends JsonDeserializer<ReturnedRateType> {

        @Override
        public ReturnedRateType deserialize(
                final JsonParser jp,
                final DeserializationContext ctxt) throws IOException {
            final JsonNode jsonNode = jp.readValueAsTree();
            return ReturnedRateType.fromValue(jsonNode);
        }
    }

    public static void main(String[] args) throws IOException {
        final ObjectMapper mapper = new ObjectMapper();
        final String json = mapper.writeValueAsString(ReturnedRateType.AA);
        System.out.println(json);
        System.out.println(mapper.readValue(json, ReturnedRateType.class));
    }
}

输出:

{"value":"AA"}
AA