JsonMappingException:如何通过Restlet传输对象化实体(带有键) [英] JsonMappingException: How to transfert a Objectify Entity (with Key) through Restlet
问题描述
我正在开发一个与GAE服务器+ Objectify DB通讯的Android应用程序.
I am developping an Android Application, comunicating with a GAE server + Objectify DB.
我选择Restlet作为Rest框架.
I choose Restlet for rest framework.
当我尝试检索具有Key属性的实体时遇到问题.服务器抛出错误:
I have a problem when I try to retrieve an Entity with a Key attribute. The server throws an error:
org.codehaus.jackson.map.JsonMappingException: Direct self-reference leading to cycle
(through reference chain: java.util.ArrayList[0]->com.my.model.MyMessage["senderKey"]->com.googlecode.objectify.Key["root"])
这是我的模型(非常简单):
Here is my model (very simple):
public class MyMessage implements Serializable {
private static final long serialVersionUID = -1075184303389185795L;
@Id
private Long id;
@Unindexed
private String sendMessage;
@Parent
Key<MyUser> senderKey;
private MyMessage() {
}
public MyMessage(MyUser user, String message) {
super();
this.sendMessage = message;
this.senderKey = new Key<MyUser>(MyUser.class, user.getId());
}
[... getters and setters ...]
}
.
public class MyUser implements Serializable {
private static final long serialVersionUID = 7390103290165670089L;
@Id private String id;
private MyUser() {
this.setId("default");
}
public MyUser(String mail) {
this.setId(mail);
}
[... getters and setters ...]
}
我该怎么办才能解决这个问题?
What can I do to solve this problem??
推荐答案
Objectify的键具有便捷的方法getRoot()，该方法返回祖先链中的根元素.当键是的根时，getRoot()返回 this .杰克逊将其检测为周期，因此您会得到错误.
Objectify's Key has a convenience method getRoot() that returns the root element in the ancestor chain. When a key is the root, getRoot() returns this. Jackson detects this as a cycle and thus you get the error.
处理此问题的最佳方法是不要尝试将Keys序列化为json对象.密钥在JSON中更好地表示为字符串化版本.
The best way to deal with this is not to try to serialize Keys as json objects. Keys are much better represented in JSON as the stringified version.
我不知道您如何在Restlet中执行此操作，但是您需要修改ObjectMapper实例以提供执行此转换的KeySerializer.查看Objectify4中的ObjectifyJacksonModule以获取指导:
I don't know how you do this in Restlet, but you need to modify the ObjectMapper instance to provide a KeySerializer that does this translation. Look at the ObjectifyJacksonModule in Objectify4 for guidance:
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