Reactive JMS Publisher (wrapper)

Reactive JMS publisher is a simple reactive wrapper for JMS API.

In terms of reactive streams, it is a Cold Publisher. i.e. no data will be lost (via unhandled emitting). Emitting begins right after the client demand's some data.

JMS publisher or rather its Source is unicast by its nature. If multiple clients connect to the same JMS queue, each one will receive unique messages. (The same logic as with common interaction with JMS queue through JMS API).

It's tested with reactive-streams-jvm tck, and verified with amq & wmq brokers.

When to use

If you have some components in your app, that use reactive interfaces, it's a good choice to use one more for easy integration =)

If you want to make some manual/dynamic throughput control, you can check the use cases of Barrier abstraction.

How to use

Creation of JMS Publisher as simple as

final ConnectionFactory factory = ...
final Function<Message, String> messageToString = message -> {
    try {
        return ((TextMessage) message).getText();
    } catch (Exception e) {
        throw new IllegalStateException(e);
    }
};
Publsiher<String> jmsPublisher = new ColdPublisher<>(new UnicastJmsQueueSource<>(factory, messageToString, "MY_COOL_QUEUE"));

Publisher acceps Source<T> instance as data & access provider.

public ColdPublisher(@Nonnull Source<T> source) {
    this.source = Objects.requireNonNull(source, "Source must not be null");
}

You can implement your own source types, it's quite easy:

public interface Source<E> {
    CloseableIterator<E> iterator(int key);
}

Jms source has 2 constructors, that accepts following params:

public UnicastJmsQueueSource(@Nonnull ConnectionFactory factory, @Nonnull Function<Message, T> function,
                             @Nonnull String queue) {
    ...
}

public UnicastJmsQueueSource(@Nonnull ConnectionFactory factory, @Nonnull Function<Message, T> function,
                             @Nonnull String queue, String user, String password, boolean transacted, int acknowledgeMode) {
    ...
}

You can use whatever Subscriber<T> you want with ColdPublisher<T>. There is one build in: BalancingSubscriber<T>.

The main idea is you never ask the given Subscription for an unbounded sequence of elements (usually through Long.MAX_VALUE). Instead, you say how much elements you want to process for a concrete time interval. In a case when the application throughput rises too high, you can obtain additional control through Barrier.

The simplest way to create a subscriber:

Subscriber<T> subscriber = new BalancingSubscriber<T>(System.out::println);

Additionally, BalancingSubscriber has several overloaded constructors:

public BalancingSubscriber(@Nonnull Consumer<T> onNext) {
    ...
}

public BalancingSubscriber(@Nonnull Consumer<T> onNext, @Nonnull Barrier barrier) {
    ...
}

public BalancingSubscriber(@Nonnull Consumer<T> onNext, @Nonnull Barrier barrier, int batchSize, int pollInterval) {
    ...
}

public BalancingSubscriber(@Nonnull Consumer<T> onNext, @Nullable Consumer<Throwable> onError,
                           @Nullable Runnable onComplete, @Nonnull Barrier barrier, int batchSize, int pollInterval) {
    ...
}

Please feel free to send a pr =)