在Jackson中反序列化同名但不同类型的属性？

Question

我有一个REST API，它返回JSON响应为：

{
    "channel" : "JHBHS"
}

有时它会返回：

{
    "channel": {
                    "id": 12321,
                    "name": "Some channel"
               }
}

我有一个这样的POJO：

public class Event {
    private String channel;
    @JsonProperty("channel")
    private Channel channelObj;
}

public class Channel {
    private int id;
    private String name;
}

那么，有没有办法（除了编写自己的自定义反序列化程序）在 Jackson2 中帮助我将 JSON 中的通道映射到字符串类型（当它是字符串时）和通道类型（当它是 JSON 对象时）？

或者换句话说，Jackson中是否有一种方法可以通过变量的type映射，而不仅仅是通过name？

Answer 1

我可以建议你这样使用JsonNode:

class Event {

    @JsonProperty("channel")
    private JsonNode channelInternal;

    private Channel channel;

    private String channelStr;

    /**
     * Custom getter with channel parsing
     * @return channel
     */
    public Channel getChannel() {
        if (channel == null && channelInternal != null) {
            if (channelInternal.isObject()) {
                int id = channelInternal.get("id").intValue();
                String name = channelInternal.get("name").asText();
                channel = new Channel(id, name);
            }
        }
        return channel;
    }

    /**
     * Custom getter with channel string parsing
     * @return channel string
     */
    public String getChannelStr() {
        if (channelStr == null && channelInternal != null) {
            if (channelInternal.isTextual()) {
                channelStr = channelInternal.asText();
            }
        }
        return channelStr;
    }
}

或者像这样:

class Event {

    private Channel channel;

    private String channelStr;

    @JsonSetter("channel")
    public void setChannelInternal(JsonNode channelInternal) {
        if (channelInternal != null) {
            if (channelInternal.isTextual()) {
                channelStr = channelInternal.asText();
            } else if (channelInternal.isObject()) {
                int id = channelInternal.get("id").intValue();
                String name = channelInternal.get("name").asText();
                channel = new Channel(id, name);
            }
        }
    }

    public Channel getChannel() {
        return channel;
    }

    public String getChannelStr() {
        return channelStr;
    }
}

但我认为使用自定义反序列化器更常见。

这是测试代码

public static void main(String[] args) throws IOException {
    ObjectMapper objectMapper = new ObjectMapper();
    String source1 = "{\n" +
            "    \"channel\" : \"JHBHS\"\n" +
            "}";
    String source2 = "{\n" +
            "    \"channel\": {\n" +
            "                    \"id\": 12321,\n" +
            "                    \"name\": \"Some channel\"\n" +
            "               }\n" +
            "}";

    //Test object parsing
    Event result = objectMapper.readValue(source2, Event.class);
    System.out.println(String.format("%s : %s", result.getChannel().getId(), result.getChannel().getName()));

    //Test string parsing
    result = objectMapper.readValue(source1, Event.class);
    System.out.println(result.getChannelStr());
}

和输出：

12321 : Some channel
JHBHS