Class BehaviorSubject<T>

java.lang.Object
io.reactivex.rxjava4.core.Observable<T>
io.reactivex.rxjava4.subjects.Subject<T>
io.reactivex.rxjava4.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();

subject.onNext(1);
// this will "clear" the cache
subject.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 Details

    • create

      @CheckReturnValue @NonNull public static <T> @NonNull 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 <@NonNull T> @NonNull BehaviorSubject<T> createDefault(@NonNull 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
      Throws:
      NullPointerException - if defaultValue is null

    • 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(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(Throwable).

    • hasObservers

      @CheckReturnValue 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 @CheckReturnValue public @Nullable 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 @CheckReturnValue 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

      @CheckReturnValue 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:

    • hasThrowable

      @CheckReturnValue 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:

    • hasValue

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

      The method is thread-safe.

      Returns:
      true if the subject has any value