This processor does not have a public constructor by design; a new empty instance of this BehaviorProcessor can be created via the create() method and a new non-empty instance can be created via createDefault(Object) (named as such to avoid overload resolution conflict with Flowable.create that creates a Flowable, not a BehaviorProcessor).

In accordance with the Reactive Streams specification (Rule 2.13) nulls are not allowed as default initial values in createDefault(Object) or as parameters to onNext(Object) and onError(Throwable).

When this BehaviorProcessor is terminated via onError(Throwable) or onComplete(), the last observed item (if any) is cleared and late org.reactivestreams.Subscribers only receive the respective terminal event.

The BehaviorProcessor does not support clearing its cached value (to appear empty again), however, the effect can be achieved by using a special item and making sure Subscribers subscribe through a filter whose predicate filters out this special item:

BehaviorProcessor<Integer> processor = BehaviorProcessor.create();

final Integer EMPTY = Integer.MIN_VALUE;

Flowable<Integer> flowable = processor.filter(v -> v != EMPTY);

TestSubscriber<Integer> ts1 = flowable.test();

processor.onNext(1);
// this will "clear" the cache
processor.onNext(EMPTY);

TestSubscriber<Integer> ts2 = flowable.test();

processor.onNext(2);
processor.onComplete();

// ts1 received both non-empty items
ts1.assertResult(1, 2);

// ts2 received only 2 even though the current item was EMPTY
// when it got subscribed
ts2.assertResult(2);

// Subscribers coming after the processor was terminated receive
// no items and only the onComplete event in this case.
flowable.test().assertResult();

Even though BehaviorProcessor implements the Subscriber interface, calling onSubscribe is not required (Rule 2.12) if the processor is used as a standalone source. However, calling onSubscribe after the BehaviorProcessor reached its terminal state will result in the given Subscription being cancelled immediately.

Calling onNext(Object), offer(Object), onError(Throwable) and onComplete() is required to be serialized (called from the same thread or called non-overlappingly from different threads through external means of serialization). The toSerialized() method available to all FlowableProcessors provides such serialization and also protects against reentrance (i.e., when a downstream Subscriber consuming this processor also wants to call onNext(Object) on this processor recursively). Note that serializing over offer(Object) is not supported through toSerialized() because it is a method available on the PublishProcessor and BehaviorProcessor classes only.

This BehaviorProcessor supports the standard state-peeking methods hasComplete(), hasThrowable(), getThrowable() and hasSubscribers() as well as means to read the latest observed value in a non-blocking and thread-safe manner via hasValue() or getValue().

Note that this processor signals MissingBackpressureException if a particular Subscriber is not ready to receive onNext events. To avoid this exception being signaled, use offer(Object) to only try to emit an item when all Subscribers have requested item(s).

Backpressure:

The BehaviorProcessor does not coordinate requests of its downstream Subscribers and expects each individual Subscriber is ready to receive onNext items when onNext(Object) is called. If a Subscriber is not ready, a MissingBackpressureException is signalled to it. To avoid overflowing the current Subscribers, the conditional offer(Object) method is available that returns true if any of the Subscribers is not ready to receive onNext events. If there are no Subscribers to the processor, offer() always succeeds. If the BehaviorProcessor is (optionally) subscribed to another Publisher, this upstream Publisher is consumed in an unbounded fashion (requesting Long#MAX_VALUE).

Scheduler:

BehaviorProcessor does not operate by default on a particular io.reactivex.rxjava3.core.Scheduler and the Subscribers get notified on the thread the respective onXXX methods were invoked.

Error handling:

When the onError(Throwable) is called, the BehaviorProcessor enters into a terminal state and emits the same Throwable instance to the last set of Subscribers. During this emission, if one or more Subscribers cancel their respective Subscriptions, the Throwable is delivered to the global error handler via io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable) (multiple times if multiple Subscribers cancel at once). If there were no Subscribers subscribed to this BehaviorProcessor when the onError() was called, the global error handler is not invoked.

Example usage:

 // subscriber will receive all events.
 BehaviorProcessor<Object> processor = BehaviorProcessor.create("default");
 processor.subscribe(subscriber);
 processor.onNext("one");
 processor.onNext("two");
 processor.onNext("three");

 // subscriber will receive the "one", "two" and "three" events, but not "zero"
 BehaviorProcessor<Object> processor = BehaviorProcessor.create("default");
 processor.onNext("zero");
 processor.onNext("one");
 processor.subscribe(subscriber);
 processor.onNext("two");
 processor.onNext("three");

 // subscriber will receive only onComplete
 BehaviorProcessor<Object> processor = BehaviorProcessor.create("default");
 processor.onNext("zero");
 processor.onNext("one");
 processor.onComplete();
 processor.subscribe(subscriber);

 // subscriber will receive only onError
 BehaviorProcessor<Object> processor = BehaviorProcessor.create("default");
 processor.onNext("zero");
 processor.onNext("one");
 processor.onError(new RuntimeException("error"));
 processor.subscribe(subscriber);