杰克逊反序列化 - 包含ArrayList <T>

Question

美好的一天,

我目前正在集成尝试使用Jackson(使用Jersey)使用生成JSON(用.NET编写)的REST服务.JSON由可能的错误消息和对象数组组成.下面是Jersey的日志记录过滤器返回的JSON示例:

{
    "error":null,
    "object":"[{\"Id\":16,\"Class\":\"ReportType\",\"ClassID\":\"4\",\"ListItemParent_ID\":4,\"Item\":\"Pothole\",\"Description\":\"Pothole\",\"Sequence\":1,\"LastEditDate\":null,\"LastEditor\":null,\"ItemStatus\":\"Active\",\"ItemColor\":\"#00AF64\"}]"
}

我有两个类来表示类型(外部ListResponse):

public class ListResponse { 

    public String error;    
    public ArrayList<ListItem> object;  

    public ListResponse() { 
    }
}

和(内部ListItem):

public class ListItem {
    @JsonProperty("Id")
    public int id;      
    @JsonProperty("Class")
    public String classType;
    @JsonProperty("ClassID")
    public String classId;  
    @JsonProperty("ListItemParent_ID")
    public int parentId;    
    @JsonProperty("Item")
    public String item; 
    @JsonProperty("Description")
    public String description;

    @JsonAnySetter 
    public void handleUnknown(String key, Object value) {}

    public ListItem() {
    }
}

调用并返回JSON的类如下所示:

public class CitizenPlusService {
    private Client client = null;   
    private WebResource service = null;     

    public CitizenPlusService() {
        initializeService("http://localhost:59105/PlusService/"); 
    }

    private void initializeService(String baseURI) {    
        // Use the default client configuration. 
        ClientConfig clientConfig = new DefaultClientConfig();      
        clientConfig.getClasses().add(JacksonJsonProvider.class);                       

        client = Client.create(clientConfig);

        // Add a logging filter to track communication between server and client. 
        client.addFilter(new LoggingFilter()); 
        // Add the base URI
        service = client.resource(UriBuilder.fromUri(baseURI).build()); 
    }

    public ListResponse getListItems(String id) throws Exception
    {           
        ListResponse response = service.path("GetListItems").path(id).accept(MediaType.APPLICATION_JSON_TYPE, MediaType.APPLICATION_XML_TYPE).get(ListResponse.class);                                  
        return response;            
    }
}

这里重要的调用是getListItems方法.在测试工具中运行代码,生成以下内容:

org.codehaus.jackson.map.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of VALUE_STRING token
at [Source: java.io.StringReader@49497eb8; line: 1, column: 14] (through reference chain: citizenplus.types.ListResponse["object"])

请协助.

此致,Carl-Peter Meyer

Answer 1

您可能缺少@JsonDeserialize属性,因为类型信息在运行时在泛型中丢失.如果可以的话,你应该避免使用具体的集合类.

public class ListResponse { 

    public String error;

    @JsonDeserialize(as=ArrayList.class, contentAs=ListItem.class)
    public List<ListItem> object;  

}

Answer 2

您的问题是“对象”属性值是字符串而不是数组！该字符串包含一个 JSON 数组，但 Jackson 需要一个本机数组（不带引号）。

我遇到了同样的问题，我创建了一个自定义反序列化器，它将字符串值反序列化为所需类型的通用集合：

public class JsonCollectionDeserializer extends StdDeserializer<Object> implements ContextualDeserializer {

  private final BeanProperty    property;

  /**
   * Default constructor needed by Jackson to be able to call 'createContextual'.
   * Beware, that the object created here will cause a NPE when used for deserializing!
   */
  public JsonCollectionDeserializer() {
    super(Collection.class);
    this.property = null;
  }

  /**
   * Constructor for the actual object to be used for deserializing.
   *
   * @param property this is the property/field which is to be serialized
   */
  private JsonCollectionDeserializer(BeanProperty property) {
    super(property.getType());
    this.property = property;
  }

  @Override
  public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property) throws JsonMappingException {
    return new JsonCollectionDeserializer(property);
  }


  @Override
  public Object deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
    switch (jp.getCurrentToken()) {
      case VALUE_STRING:
        // value is a string but we want it to be something else: unescape the string and convert it
        return JacksonUtil.MAPPER.readValue(StringUtil.unescapeXml(jp.getText()), property.getType());
      default:
        // continue as normal: find the correct deserializer for the type and call it
        return ctxt.findContextualValueDeserializer(property.getType(), property).deserialize(jp, ctxt);
    }
  }
}

请注意，如果值实际上是数组而不是字符串，则该反序列化器也将起作用，因为它相应地委托了实际的反序列化。

在您的示例中，您现在必须像这样注释您的集合字段：

public class ListResponse { 

    public String error;    
    @JsonDeserialize(using = JsonCollectionDeserializer.class)
    public ArrayList<ListItem> object;  

    public ListResponse() {}    
}

应该就是这样。

注意：JacksonUtil 和 StringUtil 是自定义类，但您可以轻松替换它们。例如通过使用new ObjectMapper()和org.apache.commons.lang3.StringEscapeUtils。