用 Jersey 序列化 Java 8 Stream

Question

如何使用 Jersey 序列化 Java 8 java.util.Stream<T>。我试图写一个 MessageBodyWriter，但我需要知道如何为我的 Stream.

编写（装饰）现有的 MessageBodyWriters 和新的 MessageBodyWriter

Stream<String> get(){
  return some stream of strings  
}

public <T> class StreamMessageBodyWriter<Stream<T>> 
           implements MessageBodyWriter<Stream<T>> {

  public void writeTo(.......){
    //How can I get the handle to MessageBodyWriter that will write for type T, 
    //so that I can 'collect' the 'java.util.Stream<T>' and write it to 
    //OutputStream
  }
}

Answer 1

but I need to know how to compose (decorate) existing MessageBodyWriters with a new MessageBodyWriter for my Stream

您可以只注入 Providers and use getMessagBodyWriter(...)，传递所需的详细信息以查找该类型的特定编写器。例如

@Provider
public class StreamBodyWriter implements MessageBodyWriter<Stream> {
    
    @Context
    private Providers providers;

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

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

    @Override
    public void writeTo(Stream stream, Class<?> type, Type genericType, 
            Annotation[] annotations, MediaType mediaType, 
            MultivaluedMap<String, Object> httpHeaders, OutputStream entityStream) 
            throws IOException, WebApplicationException {
     
        Object obj = stream.collect(Collectors.toList());
        Class<?> objType = obj.getClass();
        
        MessageBodyWriter writer = providers.getMessageBodyWriter(objType, 
                null, annotations, mediaType);
        
        writer.writeTo(obj, objType, null, annotations, 
                mediaType, httpHeaders, entityStream);
        
    }
}

如果您查看 writeTo，我首先调用 collect 然后获取返回的类型。然后查找该类型的编写器，然后简单地委托给编写器。

这是一个测试

@Path("stream")
public class StreamResource {
    
    @GET
    @Produces(MediaType.APPLICATION_JSON)
    public Response getStream() {
        List<Person> myList = Arrays.asList(
                              new Person("Stack"), 
                              new Person("Overflow"),
                              new Person("Sam"));
        Stream<Person> stream = myList.stream()
                                      .filter(p -> p.name.startsWith("S"));
        return Response.ok(stream).build();
    }
    
    public static class Person {
        public String name;
        public Person(String name) { this.name = name; }
        public Person() {}
    }
}

C:\>curl -v http://localhost:8080/api/stream
Result:
[{"name":"Stack"},{"name":"Sam"}]

顺便说一句，如果您打算在 writer 中操作 Stream，不妨考虑使用 Interceptor。真的不会有什么不同，但如果你想坚持单一责任原则，这就是 Interceptor 的目的，操纵请求主体。

注：以上为标准JAX-RS

或者...

特别是对于 Jersey，您还可以注入 MessageBodyWorkers，以进行更具体的查找，甚至调用它的 writeTo，如果存在，它将委托给所需的编写器。

Answer 2

符合使用流的目的（不使用stream.collect(Collectors.toList())），有这篇有趣的文章显示how to serialize large data from a database。

是这样的...

@GET
@Produces( "application/json" )
public Response streamGeneratedUuids() {

    return getNoCacheResponseBuilder( Response.Status.OK ).entity( new StreamingOutput() {

        @Override
        public void write( OutputStream os ) throws IOException, WebApplicationException {
            try (PrintWriter writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(os))) ) {
                //iterate the java.util.stream and write to the OutputStream
                writer.print("....");       
            }
        }
    }).build();
}

它不是用 MessageBodyWriter 实现的，但我认为可以进行调整。

用 Jersey 序列化 Java 8 Stream

Serialize Java 8 Stream with Jersey

java

jersey

jax-rs

java-8

jersey-2.0

或者...