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Ramanpreet Nara
7756114250
Summary:
## Summary
When an exception is raised by Java in a synchronous call from JS to Java, the Java portion of the stack trace is simply ignored when the exception is forwarded to JS. This problem doesn't exist for async calls. I did some digging to figure out why this works with async calls, but not sync calls, to get to the bottom of T52585087.
In T52585087, `TurboModuleRegistry.get<Spec>('I18nAssets')` fails because of a `NullPointerException` in Java. However, since there's no stack trace associated with the `NullPointerException`, it's hard to diagnose the problem.
## Asynchronous calls
ReactNative's NativeModules thread is a background thread on which we schedule asynchronous NativeModule method invocations. We spawn our background threads in Java using the [`MessageQueueThreadImpl`](https://fburl.com/diffusion/u0vdm5k3). Each thread is associated with a queue on which you can schedule some work. These `MessageQueueThread`s have a [C++ API](https://fburl.com/diffusion/596740d8) that we can use to schedule some work from C++.
NativeModule method invocations in JS produce a C++/Java boundry, because our JS executes C++, which executes the Java NativeModule method. But since the method is asynchronous, instead of invoking it immediately, we wrap it in a C++ lambda and use the C++ API of `MessageQueueThread` to schedule it to be invoked later. Therefore, when it actually invokes, we'll get Java invoking C++, which invokes Java.
When the NativeModule method (implemented in Java) throws an exception, fbjni will convert that exception into a `jni::JniException` and start unwinding the C++ stack. Eventually, this exception will reach the outermost Java/C++ boundry. Typically, at this point, the program would crash, but because we used fbjni to register all our native functions, our `jni::JniException` will automatically be converted into a regular Java exception. This exception will be caught by MessageQueueThreadHandler [here](https://fburl.com/diffusion/c4thoca7), and handled using our ExceptionHandler NativeModule.
## Synchronous calls
In synchronous execution, JS uses a `JSI::HostObject` to call the Java method directly. If the Java method throws an error, because we're using fbjni, that Java exception will be converted to a `jni::JniException` object, which will contain the stack tract of the Java object. However, from what I could gather, Hermes doesn't know about `jni::JniException`. So, simply ignore the stack trace associated with the Java exception, understanding only the exception message. Hence, all synchronous calls into Java only display the JS stack trace. What we really want is to build an platform-agnostic abstraction that understands `jni::JniException` (and whatever its analogue is in iOS, if any) and use that to bridge between Native and JS.
## Temporary Solution
We know that when NativeModules are created, we do a synchronous call from JS to C++ to Java. This synchronous call happens when you do a property access on the `NativeModules` object. Therefore, at the very least, to get to the bottom of T52585087, we could log all exceptions that are thrown whenever a NativeModule is created. This should help us get to the bottom of T52585087.
Reviewed By: mdvacca
Differential Revision: D17126667
fbshipit-source-id: a6fb27aaea094b9559939ddcc260d3a2c6e71492
Building React Native for Android
See the docs on the website.
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