Chapter 10. Asynchronous Services and Clients

As operations in asynchronous cases might take long time and they are not always finished within a single resource method invocation, JAX-RS offers facility to register callbacks to be invoked based on suspended async response state changes. In Jersey you can register two JAX-RS callbacks:

@Path("/resource")
public class AsyncResource {
    private static int numberOfSuccessResponses = 0;
    private static int numberOfFailures = 0;
    private static Throwable lastException = null;

    @GET
    public void asyncGetWithTimeout(@Suspended final AsyncResponse asyncResponse) {
        asyncResponse.register(new CompletionCallback() {
            @Override
            public void onComplete(Throwable throwable) {
                if (throwable == null) {
                    // no throwable - the processing ended successfully
                    // (response already written to the client)
                    numberOfSuccessResponses++;
                } else {
                    numberOfFailures++;
                    lastException = throwable;
                }
            }
        });

        new Thread(new Runnable() {
            @Override
            public void run() {
                String result = veryExpensiveOperation();
                asyncResponse.resume(result);
            }

            private String veryExpensiveOperation() {
                // ... very expensive operation
            }
        }).start();
    }
}

A completion callback is registered using register(...) method on the AsyncResponse instance. A registered completion callback is bound only to the response(s) to which it has been registered. In the example the CompletionCallback is used to calculate successfully processed responses, failures and to store last exception. This is only a simple case demonstrating the usage of the callback. You can use completion callback to release the resources, change state of internal resources or representations or handle failures. The method has an argument Throwable which is set only in case of an error. Otherwise the parameter will be null, which means that the response was successfully written. The callback is executed only after the response is written to the client (not immediately after the response is resumed).

The AsyncResponse register(...) method is overloaded and offers options to register a single callback as an Object (in the example), as a Class or multiple callbacks using varags.

As some async requests may take long time to process the client may decide to terminate it's connection to the server before the response has been resumed or before it has been fully written to the client. To deal with these use cases a ConnectionCallback can be used. This callback will be executed only if the connection was prematurely terminated or lost while the response is being written to the back client. Note that this callback will not be invoked when a response is written successfully and the client connection is closed as expected. See javadoc of ConnectionCallback for more information.

Jersey offers a facility for sending response to the client in multiple more-or-less independent chunks using a chunked output. Each response chunk usually takes some (longer) time to prepare before sending it to the client. The most important fact about response chunks is that you want to send them to the client immediately as they become available without waiting for the remaining chunks to become available too. The first bytes of each chunked response consists of the HTTP headers that are sent to the client. The size -1 is set in the response Content-Length header to indicate that the response is chunked. As noted above, the entity of the response is then sent in chunks as they become available. Client knows that the response is going to be chunked, so it reads each chunk of the response separately, processes it, and waits for more chunks to arrive on the same connection. After some time, the server generates another response chunk and send it again to the client. Server keeps on sending response chunks until it closes the connection after sending the last chunk when the response processing is finished.

In Jersey you can use ChunkedOutput to send response to a client in chunks. Chunks are strictly defined pieces of a response body can be marshalled as a separate entities using Jersey/JAX-RS MessageBodyWriter<T> providers. A chunk can be String, Long or JAXB bean serialized to XML or JSON or any other dacustom type for which a MessageBodyWriter<T> is available.

The resource method that returns ChunkedOutput informs the Jersey runtime that the response will be chunked and that the processing works asynchronously as such. You do not need to inject AsyncResponse to start the asynchronous processing mode in this case. Returning a ChunkedOutput instance from the method is enough to indicate the asynchronous processing. Response headers will be sent to a client when the resource method returns and the client will wait for the stream of chunked data which you will be able to write from different thread using the same ChunkedOutput instance returned from the resource method earlier. The following example demonstrates this use case:

@Path("/resource")
public class AsyncResource {
    @GET
    public ChunkedOutput<String> getChunkedResponse() {
        final ChunkedOutput<String> output = new ChunkedOutput<String>(String.class);

        new Thread() {
            public void run() {
                try {
                    String chunk;

                    while ((chunk = getNextString()) != null) {
                        output.write(chunk);
                    }
                } catch (IOException e) {
                    // IOException thrown when writing the
                    // chunks of response: should be handled
                } finally {
                    output.close();
                        // simplified: IOException thrown from
                        // this close() should be handled here...
                }
            }
        }.start();

        // the output will be probably returned even before
        // a first chunk is written by the new thread
        return output;
    }

    private String getNextString() {
        // ... long running operation that returns
        //     next string or null if no other string is accessible
    }
}

The example above defines a GET method that returns a ChunkedOutput instance. The generic type of ChunkedOutput defines the chunk types (in this case chunks are Strings). Before the instance is returned a new thread is started that writes individual chunks into the chunked output instance named output. Once the original thread returns from the resource method, Jersey runtime writes headers to the container response but does not close the client connection yet and waits for the response data to be written to the chunked output. New thread in a loop calls the method getNextString() which returns a next String or null if no other String exists (the method could for example load latest data from the database). Returned Strings are written to the chunked output. Such a written chunks are internally written to the container response and client can read them. At the end the chunked output is closed which determines the end of the chunked response. Please note that you must close the output explicitly in order to close the client connection as Jersey does not implicitly know when you are finished with writing the chunks.

A chunked output can be processed also from threads created from another request as it is explained in the sections above. This means that one resource method may e.g. only return a ChunkedOutput instance and other resource method(s) invoked from another request thread(s) can write data into the chunked output and/or close the chunked response.

Similarly to the server side, in the client API you can register asynchronous callbacks too. You can use these callbacks to be notified when a response arrives instead of waiting for the response on Future.get() or checking the status by Future.isDone() in a loop. A client-side asynchronous invocation callback can be registered as shown in the following example:

final Future<Response> responseFuture = target().path("http://example.com/resource/")
        .request().async().get(new InvocationCallback<Response>() {
            @Override
            public void completed(Response response) {
                System.out.println("Response status code "
                        + response.getStatus() + " received.");
            }

            @Override
            public void failed(Throwable throwable) {
                System.out.println("Invocation failed.");
                throwable.printStackTrace();
            }
        });

The registered callback is expected to implement the InvocationCallback interface that defines two methods. First method completed(Response) gets invoked when an invocation successfully finishes. The result response is passed as a parameter to the callback method. The second method failed(Throwable) is invoked in case the invocation fails and the exception describing the failure is passed to the method as a parameter. In this case since the callback generic type is Response, the failed(Throwable) method would only invoked in case the invocation fails because of an internal client-side processing error. It would not be invoked in case a server responds with an HTTP error code, for example if the requested resource is not found on the server and HTTP 404 response code is returned. In such case completed(Response) callback method would be invoked and the response passed to the method would contain the returned error response with HTTP 404 error code. This is a special behavior in case the generic callback return type is Response. In the next example an exception is thrown (or failed(Throwable) method on the invocation callback is invoked) even in case a non-2xx HTTP error code is returned.

As with the synchronous client API, you can retrieve the response entity as a Java type directly without requesting a Response first. In case of an InvocationCallback, you need to set its generic type to the expected response entity type instead of using the Response type as demonstrated in the example below:

final Future<String> entityFuture = target().path("http://example.com/resource/")
        .request().async().get(new InvocationCallback<String>() {
            @Override
            public void completed(String response) {
                System.out.println("Response entity '" + response + "' received.");
            }

            @Override
            public void failed(Throwable throwable) {
                System.out.println("Invocation failed.");
                throwable.printStackTrace();
            }
        });
System.out.println(entityFuture.get());

Here, the generic type of the invocation callback information is used to unmarshall the HTTP response content into a desired Java type.

In an earlier section the ChunkedOutput was described. It was shown how to use a chunked output on the server. In order to read chunks on the client the ChunkedInput can be used to complete the story.

You can, of course, process input on the client as a standard input stream but if you would like to leverage Jersey infrastructure to provide support of translating message chunk data into Java types using a ChunkedInput is much more straightforward. See the usage of the ChunkedInput in the following example:

final Response response = target().path("http://example.com/resource/")
        .request().get();
final ChunkedInput<String> chunkedInput =
        response.readEntity(new GenericType<ChunkedInput<String>>() {});
String chunk;
while ((chunk = chunkedInput.read()) != null) {
    System.out.println("Next chunk received: " + chunk);
}

The response is retrieved in a standard way from the server. The entity is read as a ChunkedInput entity. In order to do that the GenericEntity<T> is used to preserve a generic information at run time. If you would not use GenericEntity<T>, Java language generic type erasure would cause that the generic information would get lost at compile time and an exception would be thrown at run time complaining about the missing chunk type definition.

In the next lines in the example, individual chunks are being read from the response. Chunks can come with some delay, so they will be written to the console as they come from the server. After receiving last chunk the null will be returned from the read() method. This will mean that the server has sent the last chunk and closed the connection. Note that the read() is a blocking operation and the invoking thread is blocked until a new chunk comes.

Writing chunks with ChunkedOutput is simple, you only call method write() which writes exactly one chunk to the output. With the input reading it is slightly more complicated. The ChunkedInput does not know how to distinguish chunks in the byte stream unless being told by the developer. In order to define custom chunks boundaries, the ChunkedInput offers possibility to register a ChunkParser which reads chunks from the input stream and separates them. Jersey provides several chunk parser implementation and you can implement your own parser to separate your chunks if you need. In our example above the default parser provided by Jersey is used that separates chunks based on presence of a \r\n delimiting character sequence.

Each incoming input stream is firstly parsed by the ChunkParser, then each chunk is processed by the proper MessageBodyReader<T>. You can define the media type of chunks to aid the selection of a proper MessageBodyReader<T> in order to read chunks correctly into the requested entity types (in our case into Strings).