Package io.reactivex.rxjava3.processors

Class BehaviorProcessor<T>

  • Type Parameters:
    T - the type of item expected to be observed and emitted by the Processor
    All Implemented Interfaces:
    FlowableSubscriber<T>, org.reactivestreams.Processor<T,​T>, org.reactivestreams.Publisher<T>, org.reactivestreams.Subscriber<T>
    public final class BehaviorProcessor<@NonNull T>
extends FlowableProcessor<T>
    Processor that emits the most recent item it has observed and all subsequent observed items to each subscribed Subscriber.

    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 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 FlowableProcessor.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 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 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);

    • Field Detail

      • EMPTY_ARRAY

        static final java.lang.Object[] EMPTY_ARRAY

      • lock

        final java.util.concurrent.locks.ReadWriteLock lock

      • readLock

        final java.util.concurrent.locks.Lock readLock

      • writeLock

        final java.util.concurrent.locks.Lock writeLock

      • value

        final java.util.concurrent.atomic.AtomicReference<java.lang.Object> value

      • terminalEvent

        final java.util.concurrent.atomic.AtomicReference<java.lang.Throwable> terminalEvent

      • index

        long index

    • Constructor Detail

      • BehaviorProcessor

        BehaviorProcessor()
        Constructs an empty BehaviorProcessor.
        Since:
        2.0

      • BehaviorProcessor

        BehaviorProcessor​(@NonNull T defaultValue)
        Constructs a BehaviorProcessor with the given initial value.
        Parameters:
        defaultValue - the initial value, not null (verified)
        Throws:
        java.lang.NullPointerException - if defaultValue is null
        Since:
        2.0

    • Method Detail

      • createDefault

        @CheckReturnValue
@NonNull
public static <@NonNull T> @NonNull BehaviorProcessor<T> createDefault​(@NonNull T defaultValue)
        Creates a BehaviorProcessor that emits the last item it observed and all subsequent items to each Subscriber that subscribes to it.
        Type Parameters:
        T - the type of item the BehaviorProcessor will emit
        Parameters:
        defaultValue - the item that will be emitted first to any Subscriber as long as the BehaviorProcessor has not yet observed any items from its source Observable
        Returns:
        the constructed BehaviorProcessor
        Throws:
        java.lang.NullPointerException - if defaultValue is null

      • subscribeActual

        protected void subscribeActual​(@NonNull
                               @NonNull org.reactivestreams.Subscriber<? super @NonNull T> s)
        Description copied from class: Flowable
        Operator implementations (both source and intermediate) should implement this method that performs the necessary business logic and handles the incoming Subscribers.

        There is no need to call any of the plugin hooks on the current Flowable instance or the Subscriber; all hooks and basic safeguards have been applied by Flowable.subscribe(Subscriber) before this method gets called.

        Specified by:
        subscribeActual in class Flowable<T>
        Parameters:
        s - the incoming Subscriber, never null

      • onSubscribe

        public void onSubscribe​(@NonNull
                        @NonNull org.reactivestreams.Subscription s)
        Description copied from interface: FlowableSubscriber
        Implementors of this method should make sure everything that needs to be visible in Subscriber.onNext(Object) is established before calling Subscription.request(long). In practice this means no initialization should happen after the request() call and additional behavior is thread safe in respect to onNext.

      • onComplete

        public void onComplete()

      • offer

        @CheckReturnValue
public boolean offer​(@NonNull
                     @NonNull T t)
        Tries to emit the item to all currently subscribed Subscribers if all of them has requested some value, returns false otherwise.

        This method should be called in a sequential manner just like the onXXX methods of this BehaviorProcessor.

        History: 2.0.8 - experimental

        Parameters:
        t - the item to emit, not null
        Returns:
        true if the item was emitted to all Subscribers
        Throws:
        java.lang.NullPointerException - if t is null
        Since:
        2.2

      • getThrowable

        @Nullable
@CheckReturnValue
public @Nullable java.lang.Throwable getThrowable()
        Description copied from class: FlowableProcessor
        Returns the error that caused the FlowableProcessor to terminate or null if the FlowableProcessor hasn't terminated yet.

        The method is thread-safe.

        Specified by:
        getThrowable in class FlowableProcessor<T>
        Returns:
        the error that caused the FlowableProcessor to terminate or null if the FlowableProcessor hasn't terminated yet

      • getValue

        @Nullable
@CheckReturnValue
public T getValue()
        Returns a single value the BehaviorProcessor currently has or null if no such value exists.

        The method is thread-safe.

        Returns:
        a single value the BehaviorProcessor currently has or null if no such value exists

      • hasValue

        @CheckReturnValue
public boolean hasValue()
        Returns true if the BehaviorProcessor has any value.

        The method is thread-safe.

        Returns:
        true if the BehaviorProcessor has any value

      • setCurrent

        void setCurrent​(java.lang.Object o)