使用 JAXB 将子类实例作为超类传递 [英] Using JAXB to pass subclass instances as superclass

问题描述

我拥有的是一组代表消息类型的 Java 类(接近 25 个).它们都继承自我想抽象的 Message 类.每种消息类型都会向 Message 超类提供的集合添加一些额外的字段.

What I have is a set of Java classes (close to 25) representing message types. They all inherit from a Message class which I'd like to be abstract. Each message type adds a few additional fields to the set provided by the Message superclass.

我正在使用 RESTeasy 实现一些 RESTful Web 服务，并希望有这样的方法:

I'm implementing some RESTful web services using RESTeasy and would like to have methods like this:

public Response persist(Message msg) {
    EntityTransaction tx = em.getTransaction();
    tx.begin();
    try {
        em.persist(msg);
    } catch (Exception e) {
        e.printStackTrace();
    }
    tx.commit();
    em.close();
    return Response.created(URI.create("/message/" + msg.getId())).build();
}

而不是拥有 25 个单独的持久化方法，每个方法都针对特定的消息类型量身定制.

instead of having 25 separate persist methods, each tailored to a particular message type.

目前，我已经像这样注释了我的 Message 类:

Currently, I've annotated my Message class like this:

@MappedSuperclass
@XmlRootElement(name = "message")
public abstract class Message implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    Integer id;
    @Embedded
    Header header;
    @Embedded
    SubHeader subHeader;

我的子类看起来像这样:

My subclass then looks like this:

@Entity
@XmlRootElement(name="regmessage")
@XmlAccessorType(XmlAccessType.FIELD)
public class REGMessage extends Message {

    @XmlElement(required = true)
    int statusUpdateRate;
    @XmlElement(required = true)
    int networkRegistrationFlag;

这会创建一个 看起来 应该可以工作的架构，但是在持久化操作期间在服务器端看到的只是一个 Message 对象(子类型完全丢失，或者至少它没有)t 编组回其正确的子类型).在客户端，要调用我这样做的方法:

This creates a schema which looks like it should work, but all that's seen on the server side during a persist operation is a Message object (the subtype is completely lost, or at least it isn't marshalled back into its proper subtype). On the client side, to invoke the method I do this:

REGMessage msg = new REGMessage();
// populate its fields
Response r = client.createMessage(msg);

我正在尝试的可能吗?我需要使用什么 JAXB 魔法来使翻译以它们应有的方式发生 - 即，将 Java 中的所有内容都视为一条消息，以减少方法数量但仍保留所有特定于子类型的信息?

Is what I'm attempting possible? What JAXB magic do I need to use to make the translations happen the way they should -- ie, to treat everything in Java as if it's a Message to keep the number of methods down yet still preserve all the subtype-specific information?

感谢 Blaise 的博客指点，现在看起来它正在全面运行.这是我所拥有的，它确实有效:

Thanks to Blaise's blog pointers, this now looks like it's on the road to working fully. Here's what I've got, and it does work:

//JAXB annotations
@XmlRootElement(name="message")
@XmlAccessorType(XmlAccessType.FIELD)
@XmlSeeAlso(REGMessage.class)
//JPA annotations
@MappedSuperclass
public class Message {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @XmlAttribute
    private Integer id;

    private JICDHeader header;
    private int subheader;

    @XmlAnyElement
    @Transient
    private Object body;

我今天早上遇到的一个问题是来自 Hibernate 的关于列数不匹配的神秘错误.一旦我意识到body"被映射到表格中，我将其标记为瞬态，瞧！

One of the problems I encountered this morning was a cryptic error from Hibernate about the number of columns being mismatched. Once I realized that "body" was being mapped into the table, I marked it transient and voila!

@XmlRootElement(name="regmessage")
@XmlAccessorType(XmlAccessType.FIELD)
@Entity
public class REGMessage extends Message {

    private int field1;
    private int field2;

现在唯一从此代码生成的表是 regmessage 表.在 RESTeasy 方面:

The only table generated from this code now is the regmessage table. On the RESTeasy side:

@Path("/messages")
public class MessageResource implements IMessageResource {

    private EntityManagerFactory emf;
    private EntityManager em;

    Logger logger = LoggerFactory.getLogger(MessageResource.class);

    public MessageResource() {
        try {
            emf = Persistence.createEntityManagerFactory("shepherd");
            em = emf.createEntityManager();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    @POST
    @Consumes("application/xml")
    public Response saveMessage(Message msg) {

        System.out.println(msg.toString());

        logger.info("starting saveMessage");
        EntityTransaction tx = em.getTransaction();
        tx.begin();

        try {
            em.persist(msg);
        } catch (Exception e) {
            e.printStackTrace();
        }

        tx.commit();
        em.close();
        logger.info("ending saveMessage");

        return Response.created(URI.create("/message/" + msg.getId())).build();
    }
}

这实现了一个接口:

@Path("/messages")
public interface IMessageResource {

    @GET
    @Produces("application/xml")
    @Path("{id}")
    public Message getMessage(@PathParam("id") int id);

    @POST
    @Consumes("application/xml")
    public Response saveMessage(Message msg) throws URISyntaxException;

}

编组&按预期解组工作，持久化到子类的表(根本没有超类表).

Marshalling & unmarshalling work as expected, and persistence is to the subclass's table (and there is no superclass table at all).

我确实看到了 Blaise 关于 JTA 的说明，在我完成 Message &REGMessage 类完全退出.

I did see Blaise's note about JTA, which I may attempt to bring into this mix after I finish fleshing the Message & REGMessage classes back out fully.

推荐答案

您是否尝试将以下内容添加到您的消息类中?@XmlSeeAlso 注解会让 JAXBContext 知道子类.

Have you tried adding the following to your message class? The @XmlSeeAlso annotation will let the JAXBContext know about the subclasses.

import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlSeeAlso;

@XmlRootElement
@XmlSeeAlso(RegMessage.class)
public abstract class Message {

    Integer id;

}

替代策略:

这是我帮助人们使用的策略的链接:

Here is a link to a strategy I have helped people use:

• http://bdoughan.blogspot.com/2010/08/using-xmlanyelement-to-build-generic.html

基本上你有一个消息对象和多个单独的消息有效负载.消息和有效负载之间的关系通过@XmlAnyElement 注解处理.

Essentially you have one message object, and multiple individual message payloads. The relationship between the message and payload is handled through a @XmlAnyElement annotation.

事务处理注意事项

我注意到您正在处理自己的交易.您是否考虑过将 JAX-RS 服务实现为会话 bean 并利用 JTA 进行事务处理?示例见:

I noticed that you are handling your own transactions. Have you considered implementing your JAX-RS service as a session bean and leverage JTA for your transaction handling? For an example see:

• http://bdoughan.blogspot.com/2010/08/creating-restful-web-service-part-45.html

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