io.reactivex.rxjava3.subjects

Class BehaviorSubject<T>

java.lang.Object

io.reactivex.rxjava3.core.Observable<T>

io.reactivex.rxjava3.subjects.Subject<T>

io.reactivex.rxjava3.subjects.BehaviorSubject<T>

Type Parameters:

T - the type of item expected to be observed by the Subject

All Implemented Interfaces:

ObservableSource<T>, Observer<T>

public final class BehaviorSubject<T>
extends Subject<T>

Subject that emits the most recent item it has observed and all subsequent observed items to each subscribed Observer.

This subject does not have a public constructor by design; a new empty instance of this BehaviorSubject 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 Observable.create that creates an Observable, not a BehaviorSubject).

Since a Subject is conceptionally derived from the Processor type in the Reactive Streams specification, nulls are not allowed (Rule 2.13) as default initial values in createDefault(Object) or as parameters to onNext(Object) and onError(Throwable). Such calls will result in a NullPointerException being thrown and the subject's state is not changed.

Since a BehaviorSubject is an Observable, it does not support backpressure.

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

The BehaviorSubject 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 Observers subscribe through a filter whose predicate filters out this special item:

 BehaviorSubject<Integer> subject = BehaviorSubject.create();

 final Integer EMPTY = Integer.MIN_VALUE;

 Observable<Integer> observable = subject.filter(v -> v != EMPTY);

 TestObserver<Integer> to1 = observable.test();

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

 TestObserver<Integer> to2 = observable.test();

 subject.onNext(2);
 subject.onComplete();

 // to1 received both non-empty items
 to1.assertResult(1, 2);

 // to2 received only 2 even though the current item was EMPTY
 // when it got subscribed
 to2.assertResult(2);

 // Observers coming after the subject was terminated receive
 // no items and only the onComplete event in this case.
 observable.test().assertResult();
 

Even though BehaviorSubject implements the Observer interface, calling onSubscribe is not required (Rule 2.12) if the subject is used as a standalone source. However, calling onSubscribe after the BehaviorSubject reached its terminal state will result in the given Disposable being disposed immediately.

Calling onNext(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 Subject.toSerialized() method available to all Subjects provides such serialization and also protects against reentrance (i.e., when a downstream Observer consuming this subject also wants to call onNext(Object) on this subject recursively).

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

Scheduler:

BehaviorSubject does not operate by default on a particular Scheduler and the Observers get notified on the thread the respective onXXX methods were invoked.

Error handling:

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

Example usage:

 

  // observer will receive all 4 events (including "default").
  BehaviorSubject<Object> subject = BehaviorSubject.createDefault("default");
  subject.subscribe(observer);
  subject.onNext("one");
  subject.onNext("two");
  subject.onNext("three");

  // observer will receive the "one", "two" and "three" events, but not "zero"
  BehaviorSubject<Object> subject = BehaviorSubject.create();
  subject.onNext("zero");
  subject.onNext("one");
  subject.subscribe(observer);
  subject.onNext("two");
  subject.onNext("three");

  // observer will receive only onComplete
  BehaviorSubject<Object> subject = BehaviorSubject.create();
  subject.onNext("zero");
  subject.onNext("one");
  subject.onComplete();
  subject.subscribe(observer);

  // observer will receive only onError
  BehaviorSubject<Object> subject = BehaviorSubject.create();
  subject.onNext("zero");
  subject.onNext("one");
  subject.onError(new RuntimeException("error"));
  subject.subscribe(observer);
   

Method Summary

Modifier and Type	Method and Description
static <T> BehaviorSubject<T>	create() Creates a BehaviorSubject without a default item.
static <T> BehaviorSubject<T>	createDefault(T defaultValue) Creates a BehaviorSubject that emits the last item it observed and all subsequent items to each Observer that subscribes to it.
Throwable	getThrowable() Returns the error that caused the Subject to terminate or null if the Subject hasn't terminated yet.
T	getValue() Returns a single value the Subject currently has or null if no such value exists.
boolean	hasComplete() Returns true if the subject has reached a terminal state through a complete event.
boolean	hasObservers() Returns true if the subject has any Observers.
boolean	hasThrowable() Returns true if the subject has reached a terminal state through an error event.
boolean	hasValue() Returns true if the subject has any value.
void	onComplete() Notifies the Observer that the Observable has finished sending push-based notifications.
void	onError(Throwable t) Notifies the Observer that the Observable has experienced an error condition.
void	onNext(T t) Provides the Observer with a new item to observe.
void	onSubscribe(Disposable d) Provides the Observer with the means of cancelling (disposing) the connection (channel) with the Observable in both synchronous (from within Observer.onNext(Object)) and asynchronous manner.
protected void	subscribeActual(Observer<? super T> observer) Operator implementations (both source and intermediate) should implement this method that performs the necessary business logic and handles the incoming Observers.

Methods inherited from class io.reactivex.rxjava3.subjects.Subject

toSerialized

Methods inherited from class io.reactivex.rxjava3.core.Observable

all, amb, ambArray, ambWith, any, blockingFirst, blockingFirst, blockingForEach, blockingIterable, blockingIterable, blockingLast, blockingLast, blockingLatest, blockingMostRecent, blockingNext, blockingSingle, blockingSingle, blockingSubscribe, blockingSubscribe, blockingSubscribe, blockingSubscribe, blockingSubscribe, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, buffer, bufferSize, cache, cacheWithInitialCapacity, cast, collect, collectInto, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatest, combineLatestArray, combineLatestArray, combineLatestDelayError, combineLatestDelayError, combineLatestDelayError, combineLatestDelayError, compose, concat, concat, concat, concat, concat, concat, concatArray, concatArrayDelayError, concatArrayEager, concatArrayEager, concatArrayEagerDelayError, concatArrayEagerDelayError, concatDelayError, concatDelayError, concatDelayError, concatEager, concatEager, concatEager, concatEager, concatMap, concatMap, concatMap, concatMapCompletable, concatMapCompletable, concatMapCompletableDelayError, concatMapCompletableDelayError, concatMapCompletableDelayError, concatMapDelayError, concatMapDelayError, concatMapDelayError, concatMapEager, concatMapEager, concatMapEagerDelayError, concatMapEagerDelayError, concatMapIterable, concatMapIterable, concatMapMaybe, concatMapMaybe, concatMapMaybeDelayError, concatMapMaybeDelayError, concatMapMaybeDelayError, concatMapSingle, concatMapSingle, concatMapSingleDelayError, concatMapSingleDelayError, concatMapSingleDelayError, concatWith, concatWith, concatWith, concatWith, contains, count, create, debounce, debounce, debounce, defaultIfEmpty, defer, delay, delay, delay, delay, delay, delay, delaySubscription, delaySubscription, delaySubscription, dematerialize, distinct, distinct, distinct, distinctUntilChanged, distinctUntilChanged, distinctUntilChanged, doAfterNext, doAfterTerminate, doFinally, doOnComplete, doOnDispose, doOnEach, doOnEach, doOnError, doOnLifecycle, doOnNext, doOnSubscribe, doOnTerminate, elementAt, elementAt, elementAtOrError, empty, error, error, filter, first, firstElement, firstOrError, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMap, flatMapCompletable, flatMapCompletable, flatMapIterable, flatMapIterable, flatMapMaybe, flatMapMaybe, flatMapSingle, flatMapSingle, forEach, forEachWhile, forEachWhile, forEachWhile, fromArray, fromCallable, fromFuture, fromFuture, fromFuture, fromFuture, fromIterable, fromPublisher, fromSupplier, generate, generate, generate, generate, generate, groupBy, groupBy, groupBy, groupBy, groupBy, groupJoin, hide, ignoreElements, interval, interval, interval, interval, intervalRange, intervalRange, isEmpty, join, just, just, just, just, just, just, just, just, just, just, last, lastElement, lastOrError, lift, map, materialize, merge, merge, merge, merge, merge, merge, merge, merge, mergeArray, mergeArray, mergeArrayDelayError, mergeArrayDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeDelayError, mergeWith, mergeWith, mergeWith, mergeWith, never, observeOn, observeOn, observeOn, ofType, onErrorResumeNext, onErrorResumeWith, onErrorReturn, onErrorReturnItem, onTerminateDetach, publish, publish, range, rangeLong, reduce, reduce, reduceWith, repeat, repeat, repeatUntil, repeatWhen, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, replay, retry, retry, retry, retry, retry, retryUntil, retryWhen, safeSubscribe, sample, sample, sample, sample, sample, sample, scan, scan, scanWith, sequenceEqual, sequenceEqual, sequenceEqual, sequenceEqual, serialize, share, single, singleElement, singleOrError, skip, skip, skip, skipLast, skipLast, skipLast, skipLast, skipLast, skipLast, skipUntil, skipWhile, sorted, sorted, startWith, startWithArray, startWithItem, startWithIterable, subscribe, subscribe, subscribe, subscribe, subscribe, subscribeOn, subscribeWith, switchIfEmpty, switchMap, switchMap, switchMapCompletable, switchMapCompletableDelayError, switchMapDelayError, switchMapDelayError, switchMapMaybe, switchMapMaybeDelayError, switchMapSingle, switchMapSingleDelayError, switchOnNext, switchOnNext, switchOnNextDelayError, switchOnNextDelayError, take, take, take, takeLast, takeLast, takeLast, takeLast, takeLast, takeLast, takeLast, takeLast, takeLast, takeUntil, takeUntil, takeWhile, test, test, throttleFirst, throttleFirst, throttleLast, throttleLast, throttleLatest, throttleLatest, throttleLatest, throttleLatest, throttleWithTimeout, throttleWithTimeout, timeInterval, timeInterval, timeInterval, timeInterval, timeout, timeout, timeout, timeout, timeout, timeout, timeout, timeout, timer, timer, timestamp, timestamp, timestamp, timestamp, to, toFlowable, toFuture, toList, toList, toList, toMap, toMap, toMap, toMultimap, toMultimap, toMultimap, toMultimap, toSortedList, toSortedList, toSortedList, toSortedList, unsafeCreate, unsubscribeOn, using, using, window, window, window, window, window, window, window, window, window, window, window, window, window, window, window, window, window, withLatestFrom, withLatestFrom, withLatestFrom, withLatestFrom, withLatestFrom, withLatestFrom, wrap, zip, zip, zip, zip, zip, zip, zip, zip, zip, zip, zip, zip, zipArray, zipWith, zipWith, zipWith, zipWith

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

create

@CheckReturnValue
 @NonNull
public static <T> BehaviorSubject<T> create()

Creates a BehaviorSubject without a default item.

Type Parameters:

T - the type of item the Subject will emit

Returns:

the constructed BehaviorSubject

createDefault

@CheckReturnValue
 @NonNull
public static <T> BehaviorSubject<T> createDefault(T defaultValue)

Creates a BehaviorSubject that emits the last item it observed and all subsequent items to each Observer that subscribes to it.

Type Parameters:

T - the type of item the Subject will emit

Parameters:

defaultValue - the item that will be emitted first to any Observer as long as the BehaviorSubject has not yet observed any items from its source Observable

Returns:

the constructed BehaviorSubject

subscribeActual

protected void subscribeActual(Observer<? super T> observer)

Description copied from class: Observable

Operator implementations (both source and intermediate) should implement this method that performs the necessary business logic and handles the incoming Observers.

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

Specified by:

subscribeActual in class Observable<T>

Parameters:

observer - the incoming Observer, never null

onSubscribe

public void onSubscribe(Disposable d)

Description copied from interface: Observer

Provides the Observer with the means of cancelling (disposing) the connection (channel) with the Observable in both synchronous (from within Observer.onNext(Object)) and asynchronous manner.

Parameters:

d - the Disposable instance whose Disposable.dispose() can be called anytime to cancel the connection

onNext

public void onNext(T t)

Description copied from interface: Observer

Provides the Observer with a new item to observe.

The Observable may call this method 0 or more times.

The Observable will not call this method again after it calls either Observer.onComplete() or Observer.onError(java.lang.Throwable).

Parameters:

t - the item emitted by the Observable

onError

public void onError(Throwable t)

Description copied from interface: Observer

Notifies the Observer that the Observable has experienced an error condition.

If the Observable calls this method, it will not thereafter call Observer.onNext(T) or Observer.onComplete().

Parameters:

t - the exception encountered by the Observable

onComplete

public void onComplete()

Description copied from interface: Observer

Notifies the Observer that the Observable has finished sending push-based notifications.

The Observable will not call this method if it calls Observer.onError(java.lang.Throwable).

hasObservers

public boolean hasObservers()

Description copied from class: Subject

Returns true if the subject has any Observers.

The method is thread-safe.

Specified by:

hasObservers in class Subject<T>

Returns:

true if the subject has any Observers

getThrowable

@Nullable
public Throwable getThrowable()

Description copied from class: Subject

Returns the error that caused the Subject to terminate or null if the Subject hasn't terminated yet.

The method is thread-safe.

Specified by:

getThrowable in class Subject<T>

Returns:

the error that caused the Subject to terminate or null if the Subject hasn't terminated yet

getValue

@Nullable
public T getValue()

Returns a single value the Subject currently has or null if no such value exists.

The method is thread-safe.

Returns:

a single value the Subject currently has or null if no such value exists

hasComplete

public boolean hasComplete()

Description copied from class: Subject

Returns true if the subject has reached a terminal state through a complete event.

The method is thread-safe.

Specified by:

hasComplete in class Subject<T>

Returns:

true if the subject has reached a terminal state through a complete event

See Also:

Subject.hasThrowable()

hasThrowable

public boolean hasThrowable()

Description copied from class: Subject

Returns true if the subject has reached a terminal state through an error event.

The method is thread-safe.

Specified by:

hasThrowable in class Subject<T>

Returns:

true if the subject has reached a terminal state through an error event

See Also:

Subject.getThrowable(), Subject.hasComplete()

hasValue

public boolean hasValue()

Returns true if the subject has any value.

The method is thread-safe.

Returns:

true if the subject has any value