RxDogTagRxJava 未处理错误的标识程序

RxDogTag 是一个用于标记 RxJava 2 观察者中的原始订阅点的实用程序，其目标是在出现未处理错误的情况下显示其订阅位置以便稍后进行错误报告/调查。 这仅适用于未实现onError（）的 RxJava 观察者。

请看如下经典的 RxJava 错误：

Observable.range(0, 10)
    .subscribeOn(Schedulers.io())
    .map(i -> null)
    .subscribe();

这是 RxJava 并发中非常常见的问题，没有标识的话，会产生如下的异常信息：

io.reactivex.exceptions.OnErrorNotImplementedException: The exception was not handled due to missing onError handler in the subscribe() method call. Further reading: https://github.com/ReactiveX/RxJava/wiki/Error-Handling | The mapper function returned a null value.
	at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:704)
	at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:701)
	at io.reactivex.internal.observers.LambdaObserver.onError(LambdaObserver.java:77)
	at io.reactivex.internal.observers.BasicFuseableObserver.onError(BasicFuseableObserver.java:100)
	at io.reactivex.internal.observers.BasicFuseableObserver.fail(BasicFuseableObserver.java:110)
	at io.reactivex.internal.operators.observable.ObservableMap$MapObserver.onNext(ObservableMap.java:59)
	at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeOnObserver.onNext(ObservableSubscribeOn.java:58)
	at io.reactivex.internal.operators.observable.ObservableScalarXMap$ScalarDisposable.run(ObservableScalarXMap.java:248)
	at io.reactivex.internal.operators.observable.ObservableJust.subscribeActual(ObservableJust.java:35)
	at io.reactivex.Observable.subscribe(Observable.java:12090)
	at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeTask.run(ObservableSubscribeOn.java:96)
	at io.reactivex.Scheduler$DisposeTask.run(Scheduler.java:578)
	at io.reactivex.internal.schedulers.ScheduledRunnable.run(ScheduledRunnable.java:66)
	at io.reactivex.internal.schedulers.ScheduledRunnable.call(ScheduledRunnable.java:57)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException: The mapper function returned a null value.
	at io.reactivex.internal.functions.ObjectHelper.requireNonNull(ObjectHelper.java:39)
	at io.reactivex.internal.operators.observable.ObservableMap$MapObserver.onNext(ObservableMap.java:57)
	... 14 more

你很难从这份报告中清楚的了解实际的状况。

而启用了 RxDogTag 后，相同的错误变成：

io.reactivex.exceptions.OnErrorNotImplementedException: The mapper function returned a null value.

Caused by: java.lang.NullPointerException: The mapper function returned a null value.
	at [[ Inferred subscribe point ]].(:0)
	at com.uber.anotherpackage.ReadMeExample.complex(ReadMeExample.java:55)
	at [[ Original trace ]].(:0)
	at io.reactivex.internal.functions.ObjectHelper.requireNonNull(ObjectHelper.java:39)
	at io.reactivex.internal.operators.observable.ObservableMap$MapObserver.onNext(ObservableMap.java:57)
	at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeOnObserver.onNext(ObservableSubscribeOn.java:58)
	at io.reactivex.internal.operators.observable.ObservableScalarXMap$ScalarDisposable.run(ObservableScalarXMap.java:248)
	at io.reactivex.internal.operators.observable.ObservableJust.subscribeActual(ObservableJust.java:35)
	at io.reactivex.Observable.subscribe(Observable.java:12090)
	at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeTask.run(ObservableSubscribeOn.java:96)
	at io.reactivex.Scheduler$DisposeTask.run(Scheduler.java:578)
	at io.reactivex.internal.schedulers.ScheduledRunnable.run(ScheduledRunnable.java:66)
	at io.reactivex.internal.schedulers.ScheduledRunnable.call(ScheduledRunnable.java:57)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

这样我们就知道这个错误对应的代码是 ReadMeExample.java:55. 这并非找出问题根源的银弹，但至少你知道错误在哪里。