6

I am not trying to solve any problem in particular, but rather I am on a learning path to jersey.

I have an entity class marked like this:

@Entity
@Table(name = "myentity")
@XmlRootElement
public class MyEntity implements serializable {
  // lots of methods...
}

and the corresponding jersey service

@Stateless
@Path("entity")
public class EntityFacade {

    @GET
    @Path("{param}")
    @Produces({"application/xml;charset=UTF-8"})
    public List<MyEntity> find(@PathParam("param") String param) {
       List entities = entityManager.getResultList(); // retrieve list from db
       return entities;
    }

}

Which give a correct XML response. Supposing I want to write a MessageBodyWriter which replicate same behavior, which is producing an XML response, how could I do that ?

@Provider
public class TrasformerMessageBodyWriter implements MessageBodyWriter<Object> {

    @Override
    public long getSize(Object o, Class<?> type, Type genericType,
            Annotation[] annotations, MediaType mediaType) {
        return 0;
    }

    @Override
    public boolean isWriteable(Class<?> type, Type genericType,
            Annotation[] annotations, MediaType mediaType) {
        // return true or false depending if I want to rewrite the response
    }

    @Override
    public void writeTo(Object o, Class<?> type, Type genericType,
            Annotation[] annotations, MediaType mediaType,
            MultivaluedMap<String, Object> httpHeaders,
            OutputStream entityStream) throws IOException,
            WebApplicationException {
        // what do I need to write here...
    }
}

by marking with @Provider annotation I can see the message body writer is correctly invoked.

When writeTo is invoked, Object o is a Vector and Type genericType is a List but at this point I am completely lost on how I could transform the object in XML.

Last, if everything is already provided by jersey and its annotations, how can a MessageBodyWriter be useful ?

Again, I am repeating that this is just an academic exercise.

4

1 回答 1

9

通常，人们会使用 aMessageBodyWriter将对象转换为 Jersey 一无所知的数据格式。TableData这是将域对象转换为 CSV的示例：

@Singleton
@Produces("text/csv")
@Provider
public class FederationCsvWriter implements MessageBodyWriter<TableData> {

  @Override
  public boolean isWriteable(Class<?> type, Type genericType, Annotation[] annotations, MediaType mediaType) {
    return TableData.class.isAssignableFrom(type);
  }

  @Override
  public long getSize(TableData data, Class<?> type, Type genericType, Annotation annotations[], MediaType mediaType) {
    return -1;
  }

  @Override
  public void writeTo(TableData data, Class<?> type, Type genericType, Annotation[] annotations,
      MediaType mediaType, MultivaluedMap<String, Object> headers, OutputStream out) throws IOException {
    Writer osWriter = new OutputStreamWriter(out);
    CSVWriter writer = new CSVWriter(osWriter, ',', '"', "\r\n");
    if (data.getResultCount() > 0) {
      //Write the header
      writer.writeNext(data.getResult().get(0).keySet().toArray(new String[data.getResult().get(0).keySet().size()]));

      //Write the data
      for (ModelData row: data.getResult()) {
        writer.writeNext(row.values().toArray(new String[row.values().size()]));
      }
    }
    writer.flush();
  }

}

在您的示例中，您可以通过自己检查对象并将结果提供给来创建 XML，OutputStream或者您可以使用 JAXB（Jersey 在内部使用）将对象直接编组到OutputStream. 就您的练习而言，JAXB 可能更有趣。

于 2013-05-24T19:40:12.177 回答