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
React Native
Learn once, write anywhere:
Build mobile apps with React.
Getting Started · Learn the Basics · Showcase · Contribute · Community · Support
React Native brings React's declarative UI framework to iOS and Android. With React Native, you use native UI controls and have full access to the native platform.
- Declarative. React makes it painless to create interactive UIs. Declarative views make your code more predictable and easier to debug.
- Component-Based. Build encapsulated components that manage their own state, then compose them to make complex UIs.
- Developer Velocity. See local changes in seconds. Changes to JavaScript code can be live reloaded without rebuilding the native app.
- Portability. Reuse code across iOS, Android, and other platforms.
React Native is developed and supported by many companies and individual core contributors. Find out more in our ecosystem overview.
Contents
- Requirements
- Building your first React Native app
- Documentation
- Upgrading
- How to Contribute
- Code of Conduct
- License
📋 Requirements
React Native apps may target iOS 9.0 and Android 4.1 (API 16) or newer. You may use Windows, macOS, or Linux as your development operating system, though building and running iOS apps is limited to macOS. Tools like Expo can be used to work around this.
🎉 Building your first React Native app
Follow the Getting Started guide. The recommended way to install React Native depends on your project. Here you can find short guides for the most common scenarios:
📖 Documentation
The full documentation for React Native can be found on our website.
The React Native documentation discusses components, APIs, and topics that are specific to React Native. For further documentation on the React API that is shared between React Native and React DOM, refer to the React documentation.
The source for the React Native documentation and website is hosted on a separate repo, @facebook/react-native-website.
🚀 Upgrading
Upgrading to new versions of React Native may give you access to more APIs, views, developer tools and other goodies. See the Upgrading Guide for instructions.
React Native releases are discussed in the React Native Community, @react-native-community/react-native-releases.
👏 How to Contribute
The main purpose of this repository is to continue evolving React Native core. We want to make contributing to this project as easy and transparent as possible, and we are grateful to the community for contributing bugfixes and improvements. Read below to learn how you can take part in improving React Native.
Code of Conduct
Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.
Contributing Guide
Read our Contributing Guide to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to React Native.
Open Source Roadmap
You can learn more about our vision for React Native in the Roadmap.
Good First Issues
We have a list of good first issues that contain bugs which have a relatively limited scope. This is a great place to get started, gain experience, and get familiar with our contribution process.
Discussions
Larger discussions and proposals are discussed in @react-native-community/discussions-and-proposals.
📄 License
React Native is MIT licensed, as found in the LICENSE file.
React Native documentation is Creative Commons licensed, as found in the LICENSE-docs file.