java 如何使用 jersey 编写 XML MessageBodyWriter 提供程序
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How to write an XML MessageBodyWriter provider with jersey
提问by Leonardo
I am not trying to solve any problem in particular, but rather I am on a learning path to jersey.
我并不是特别想解决任何问题，而是我正在学习球衣。
I have an entity class marked like this:
我有一个这样标记的实体类：
@Entity
@Table(name = "myentity")
@XmlRootElement
public class MyEntity implements serializable {
// lots of methods...
}
and the corresponding jersey service
以及相应的球衣服务
@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 ?
这给出了正确的 XML 响应。假设我想编写一个 MessageBodyWriter 来复制相同的行为，即产生 XML 响应，我该怎么做？
@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.
通过使用@Provider 批注进行标记，我可以看到正确调用了消息正文编写器。
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.
当 writeTo 被调用时，Object o 是一个 Vector 而 Type genericType 是一个 List 但此时我完全不知道如何在 XML 中转换对象。
Last, if everything is already provided by jersey and its annotations, how can a MessageBodyWriter be useful ?
最后，如果 jersey 及其注释已经提供了一切，那么 MessageBodyWriter 如何有用？
Again, I am repeating that this is just an academic exercise.
我再次重申，这只是一个学术练习。
回答by condit
Typically one would use a MessageBodyWriterto convert objects to data formats that Jersey knows nothing about. Here's an example of translating a TableDatadomain object to CSV:
通常，人们会使用 aMessageBodyWriter将对象转换为 Jersey 一无所知的数据格式。以下是将TableData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();
}
}
In your example you could either create XML by inspecting the object yourself and feeding the result to the OutputStreamor your could use JAXB (which Jersey uses internally) to marshall the objects directly to the OutputStream. For the purposes of your exercise JAXB is probably more interesting.
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. 就练习而言，JAXB 可能更有趣。
