杰克逊

Question

我有以下类要将JSON字符串反序列化为使用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
}

WebDeviceInfo.java

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

    //getters & setters
}

我有以下要反序列化的JSON内容：

{
    "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：无法识别的字段＆＃34;端点＆＃34; （com.blah.DeviceInfo类），未标记为可忽略（一个已知属性：＆＃34; device_type＆＃34;]）

如何让杰克逊认识到需要将其映射到WebDeviceInfo实例，而不是尝试将其映射到DeviceInfo超类，而这些超类没有endpoint场？

我尝试在不同的课程中使用@JsonTypeInfo和@JsonSubTypes注释，但我找不到如何使用它们的好例子。

编辑：我在@JsonDeserialize(using = DeviceInfoDeserializer.class)课程中添加了DeviceInfo注释，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，在尝试反序列化PushNotificationMessage JSON时出现了不同的错误：

java.lang.StackOverflowError: 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)添加到我的子类，现在它可以按预期工作。非常感谢@Luciano van der Veekens。

Answer 1

Jackson并不知道多态，它只是尝试创建具体的DeviceInfo类的实例。

但是，您可以实现一个自定义反序列化器，它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类，因为endpoint等某些字段的唯一性。

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

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

可在此处找到一个示例：http://sunilkumarpblog.blogspot.nl/2015/12/javajson-polymorphic-serialization-de.html