Plugins

Plugins allow you to modify the default behavior of RxJava in several respects:

• by changing the set of default computation, i/o, and new thread Schedulers
• by registering a handler for extraordinary errors that RxJava may encounter
• by registering functions that can take note of the occurrence of several regular RxJava activities

As of 1.1.6 the regular RxJavaPlugins and the other hook classes have been deprecated in favor of RxJavaHooks.

RxJavaHooks

The new RxJavaHooks allows you to hook into the lifecycle of the Observable, Single and Completable types, the Schedulers returned by Schedulers and offers a catch-all for undeliverable errors.

You can now change these hooks at runtime and there is no need to prepare hooks via system parameters anymore. Since users may still rely on the old hooking system, RxJavaHooks delegates to those old hooks by default.

The RxJavaHooks has setters and getters of the various hook types:

Hook	Type	Description
onError	Action1<Throwable>	Sets the catch-all callback
onObservableCreate	Func1<Observable.OnSubscribe, Observable.OnSubscribe>	Called when operators and sources are instantiated on Observable
onObservableStart	Func2<Observable, Observable.OnSubscribe, Observable.OnSubscribe>
onObservableSubscribeError	Func1<Throwable, Throwable>
onObservableReturn	Func1<Subscription, Subscription>
onObservableLift	Func1<Observable.Operator, Observable.Operator>
onSingleCreate	Func1<Single.OnSubscribe, Single.OnSubscribe>	Called when operators and sources are instantiated on Single
onSingleStart	Func2<Single, Observable.OnSubscribe, Observable.OnSubscribe>
onSingleSubscribeError	Func1<Throwable, Throwable>
onSingleReturn	Func1<Subscription, Subscription>
onSingleLift	Func1<Observable.Operator, Observable.Operator>	called when the operator lift is used (note: Observable.Operator is deliberate here)
onCompletableCreate	Func1<Completable.OnSubscribe, Completable.OnSubscribe>	Called when operators and sources are instantiated on Completable
onCompletableStart	Func2<Completable, Completable.OnSubscribe, Completable.OnSubscribe>
onCompletableSubscribeError	Func1<Throwable, Throwable>
onCompletableLift	Func1<Completable.Operator, Completable.Operator>
onComputationScheduler	Func1<Scheduler, Scheduler>
onIOScheduler	Func1<Scheduler, Scheduler>
onNewThreadScheduler	Func1<Scheduler, Scheduler>
onScheduleAction	Func1<Action0, Action0>	Called when a task gets scheduled in any of the Schedulers
onGenericScheduledExecutorService	Func0<ScheduledExecutorService>	that should return single-threaded executors to support background timed tasks of RxJava itself

Reading and changing these hooks is thread-safe.

You can also clear all hooks via clear() or reset to the default behavior (of delegating to the old RxJavaPlugins system) via reset().

Example:

RxJavaHooks.setOnObservableCreate(o -> { System.out.println("Creating " + o.getClass()); return o; });
try {
    Observable.range(1, 10)
    .map(v -> v * 2)
    .filter(v -> v % 4 == 0)
    .subscribe(System.out::println);
} finally {
    RxJavaHooks.reset();
}

In addition, the RxJavaHooks offers the so-called assembly tracking feature. This shims a custom Observable, Single and Completable into their chains which captures the current stacktrace when those operators were instantiated (assembly-time). Whenever an error is signalled via onError, these middle components attach this assembly-time stacktraces as last causes of that exception. This may help locating the problematic sequence in a codebase where there are too many similar flows and the plain exception itself doesn't tell which one failed in your codebase.

Example:

RxJavaHooks.enableAssemblyTracking();
try {
    Observable.empty().single()
    .subscribe(System.out::println, Throwable::printStackTrace);
} finally {
   RxJavaHooks.resetAssemblyTracking();
}

This will print something like this:

java.lang.NoSuchElementException
at rx.internal.operators.OnSubscribeSingle(OnSubscribeSingle.java:57)
...
Assembly trace:
at com.example.TrackingExample(TrackingExample:10)

The stacktrace string is also available in a field to support debugging and discovering the status of various operators in a running chain.

The stacktrace is filtered by removing irrelevant entries such as Thread entry points, unit test runners and the entries of the tracking system itself to reduce noise.

RxJavaSchedulersHook

Deprecated

This plugin allows you to override the default computation, i/o, and new thread Schedulers with Schedulers of your choosing. To do this, extend the class RxJavaSchedulersHook and override these methods:

• Scheduler getComputationScheduler( )
• Scheduler getIOScheduler( )
• Scheduler getNewThreadScheduler( )
• Action0 onSchedule(action)

Then follow these steps:

1. Create an object of the new RxJavaDefaultSchedulers subclass you have implemented.
2. Obtain the global RxJavaPlugins instance via RxJavaPlugins.getInstance( ).
3. Pass your default schedulers object to the registerSchedulersHook( ) method of that instance.

When you do this, RxJava will begin to use the Schedulers returned by your methods rather than its built-in defaults.

RxJavaErrorHandler

Deprecated

This plugin allows you to register a function that will handle errors that are passed to Subscriber.onError(Throwable). To do this, extend the class RxJavaErrorHandler and override this method:

• void handleError(Throwable e)

Then follow these steps:

1. Create an object of the new RxJavaErrorHandler subclass you have implemented.
2. Obtain the global RxJavaPlugins instance via RxJavaPlugins.getInstance( ).
3. Pass your error handler object to the registerErrorHandler( ) method of that instance.

When you do this, RxJava will begin to use your error handler to field errors that are passed to Subscriber.onError(Throwable).

RxJavaObservableExecutionHook

Deprecated

This plugin allows you to register functions that RxJava will call upon certain regular RxJava activities, for instance for logging or metrics-collection purposes. To do this, extend the class RxJavaObservableExecutionHook and override any or all of these methods:

method	when invoked
onCreate( )	during Observable.create( )
onSubscribeStart( )	immediately before Observable.subscribe( )
onSubscribeReturn( )	immediately after Observable.subscribe( )
onSubscribeError( )	upon a failed execution of Observable.subscribe( )
onLift( )	during Observable.lift( )

Then follow these steps:

1. Create an object of the new RxJavaObservableExecutionHook subclass you have implemented.
2. Obtain the global RxJavaPlugins instance via RxJavaPlugins.getInstance( ).
3. Pass your execution hooks object to the registerObservableExecutionHook( ) method of that instance.

When you do this, RxJava will begin to call your functions when it encounters the specific conditions they were designed to take note of.