Jackson 的多态反序列化问题

Question

我有以下 classes，我想反序列化 JSON 字符串以使用 Jackson。

PushNotificationMessage.java

public class PushNotificationMessage {
    @JsonProperty("device_info")
    private DeviceInfo deviceInfo;
    private String content;

    //getters & setters
}

DeviceInfo.java

public class DeviceInfo {

    @JsonProperty(value = "device_type")
    private String deviceType;

    //getters & setters
}

IOSDeviceInfo.java

public class IOSDeviceInfo extends DeviceInfo {

    @JsonProperty(value = "device_id")
    private String deviceId;
    private String arn;
    @JsonProperty(value = "user_data")
    private String userData;

    //getters & setters
}

网络DeviceInfo.java

public class WebDeviceInfo extends DeviceInfo {
    private String endpoint;
    private String key;
    private String auth;

    //getters & setters
}

我有以下 JSON 内容要反序列化：

{
    "device_info": {
        "endpoint": "https://android.googleapis.com/gcm/send/blah",
        "key": "blahkey",
        "auth": "blahauth",
        "device_type": "web"
    },
    "content": "Notification content"
}

我只是使用 ObjectMapper 来尝试执行反序列化。

final ObjectMapper objectMapper = new ObjectMapper();
final PushNotificationMessage message = objectMapper.readValue(jsonString, PushNotifictionMessage.class);

当我这样做时，我得到：

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "endpoint" (class com.blah.DeviceInfo), not marked as ignorable (one known property: "device_type"])

我怎样才能让 Jackson 认识到它需要被映射到一个 WebDeviceInfo 实例，而不是试图将它映射到 DeviceInfo superclass，它不有 endpoint 字段？

我试过在不同的 class 中使用 @JsonTypeInfo 和 @JsonSubTypes 注释，但我找不到如何使用它们的好例子。

编辑： 我将 @JsonDeserialize(using = DeviceInfoDeserializer.class) 注释添加到我的 DeviceInfo class，并创建了以下 DeviceInfoDeserializer。

public class DeviceInfoDeserializer extends JsonDeserializer<DeviceInfo> {

    private static final String DEVICE_TYPE = "device_type";
    private static final String WEB = "web";
    private static final String IOS = "ios";

    @Override
    public DeviceInfo deserialize(final JsonParser jsonParser, final DeserializationContext deserializationContext) throws IOException {
        final ObjectMapper objectMapper = (ObjectMapper) jsonParser.getCodec();
        final ObjectNode root = objectMapper.readTree(jsonParser);
        if (root.has(DEVICE_TYPE)) {
            final JsonNode jsonNode = root.get(DEVICE_TYPE);
            if (jsonNode.asText().equalsIgnoreCase(WEB)) {
                return objectMapper.readValue(root.toString(), WebDeviceInfo.class);
            } else if (jsonNode.asText().equalsIgnoreCase(IOS)) {
                return objectMapper.readValue(root.toString(), IOSDeviceInfo.class);
            }
        }
        throw deserializationContext.mappingException("Failed to de-serialize device info, as device_type was not \"web\" or \"ios\"");
    }
}

现在，我在尝试反序列化我的 PushNotificationMessage JSON:

时遇到了不同的错误

java.lang.WhosebugError: null
    at com.fasterxml.jackson.databind.deser.std.BaseNodeDeserializer.deserializeObject(JsonNodeDeserializer.java:210)
    at com.fasterxml.jackson.databind.deser.std.JsonNodeDeserializer.deserialize(JsonNodeDeserializer.java:69)
    at com.fasterxml.jackson.databind.deser.std.JsonNodeDeserializer.deserialize(JsonNodeDeserializer.java:15)
    at com.fasterxml.jackson.databind.ObjectMapper._readValue(ObjectMapper.java:3770)
    at com.fasterxml.jackson.databind.ObjectMapper.readTree(ObjectMapper.java:2207)
    at com.blah.serialization.DeviceInfoDeserializer.deserialize(DeviceInfoDeserializer.java:25)
    at com.blah.serialization.DeviceInfoDeserializer.deserialize(DeviceInfoDeserializer.java:16)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3798)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2842)
    ... (above trace repeated many times)

编辑： 只需将 @JsonDeserialize(as = WebDeviceInfo.class) 和 @JsonDeserialize(as = IOSDeviceInfo.class) 添加到我的子classes，现在它按预期工作了。非常感谢。

Answer 1

Jackson 不了解多态性，它只是尝试创建具体 DeviceInfo 的实例 class。

但是，您可以实现自定义解串器，以编程方式解析设备信息 JSON 并知道何时实例化其中一个子 class，因为某些字段的唯一性，例如 endpoint.

https://fasterxml.github.io/jackson-databind/javadoc/2.2.0/com/fasterxml/jackson/databind/annotation/JsonDeserialize.html

@JsonDeserialize(using = DeviceInfoDeserializer.class)
public class DeviceInfo {
}

可在此处找到示例：http://sunilkumarpblog.blogspot.nl/2015/12/javajson-polymorphic-serialization-de.html

Jackson 的多态反序列化问题

Polymorphic Deserialization Issue with Jackson

java

json

jackson

deserialization

json-deserialization