Creating Unis

Learn how to create Uni instances.

A Uni represents a stream that can only emit either an item or a failure event. You rarely create instances of Uni yourself, but, instead, use a reactive client exposing a Mutiny API that provides Unis. That being said, it can be handy at times.

The Uni type

A Uni<T> is a specialized stream that emits only an item or a failure. Typically, Uni<T> are great to represent asynchronous actions such as a remote procedure call, an HTTP request, or an operation producing a single result.

Uni<T> provides many operators that create, transform, and orchestrate Uni sequences.

As said, Uni<T> emits either an item or a failure. Note that the item can be null, and the Uni API has specific methods for this case.

Typically, a Uni<Void> always emits null as item event or a failure if the represented operation fails. You can consider the item event as a completion signal indicating the success of the operation.

The offered operators can be used to define a processing pipeline. The event, either the item or failure, flows in this pipeline, and each operator can process or transform the event. Unis are lazy by nature.

To trigger the computation, you must have a final subscriber indicating your interest. The following snippet provides a simple example of pipeline using Uni:

Example of pipeline using Uni
        .onItem().transform(i -> "hello-" + i)

You can see the Uni javadoc here.

Subscribing to a Uni

Remember: if you don’t subscribe, nothing is going to happen. What’s more, the pipeline is materialized for each subscription.

When subscribing to a Uni, you can pass an item callback (invoked when the item is emitted), or two callbacks (one receiving the item and one receiving the failure):

Cancellable cancellable = uni
                item -> System.out.println(item),
                failure -> System.out.println("Failed with " + failure));

Note the returned Cancellable: this object allows canceling the operation if need be.

Creating Unis from item

There are many ways to create Uni instances. Use Uni.createFrom() to see all the possibilities.

You can, for instance, create a Uni from a known value:

Uni<Integer> uni = Uni.createFrom().item(1);

Every subscriber receives the item 1 just after the subscription.

You can also pass a Supplier:

AtomicInteger counter = new AtomicInteger();
Uni<Integer> uni = Uni.createFrom().item(() -> counter.getAndIncrement());

The Supplier is called for every subscriber. So, each of them will get a different value.

Creating failing Unis

Operations represented by Unis can also emit a failure event, indicating that the operation failed.

You can create failed Uni instances with:

// Pass an exception directly:
Uni<Integer> failed1 = Uni.createFrom().failure(new Exception("boom"));

// Pass a supplier called for every subscriber:
Uni<Integer> failed2 = Uni.createFrom().failure(() -> new Exception("boom"));

Creating Uni<Void>

When the represented operation to not produce a result, you still need a way to indicate the operation’s completion. For this, you need to emit a null item:

Uni<Void> uni = Uni.createFrom().nullItem();

Creating Unis using an emitter (advanced)

You can create a Uni using an emitter. This approach is useful when integrating callback-based APIs:

Uni<String> uni = Uni.createFrom().emitter(em -> {
    // When the result is available, emit it

The emitter can also send a failure. It can also get notified of cancellation to, for example, stop the work in progress.

Creating Unis from a CompletionStage (advanced)

You can also Uni objects from CompletionStage / CompletableFuture. This is useful when integrating with APIs that are based on these types:

Uni<String> uni = Uni.createFrom().completionStage(stage);
Subscribing as CompletionStage

You can also create a CompletionStage from a Uni using uni.subscribe().asCompletionStage()