Introduction: Static checker for Rx Threading Effects, based on the Checker Framework
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The RxThreadEffectChecker is a static analysis tool for preventing UI access off the main thread in stream-based (RxJava/RxAndroid) Android applications.

We are publishing this code as a companion artifact for our ASE 2018 paper:

Safe Stream-Based Programming with Refinement Types. Benno Stein, Lazaro Clapp, Manu Sridharan, Bor-Yuh Evan Chang. ASE 2018 (arxiv: https://arxiv.org/abs/1808.02998 )

This checker builds heavily on the Checker Framework, and particularly it works in tandem with their guieffect checker. Besides the code in this repository, we have worked to upstream changes to the Checker Framework that make this checker possible. Note that running this checker requires using javac from JDK 8, and hence it cannot be run alongside Error Prone in the same build.

From the ASE 2018 paper's abstract

In stream-based programming, data sources are abstracted as a stream of values that can be manipulated via callback functions. Stream-based programming is exploding in popularity, as it provides a powerful and expressive paradigm for handling asynchronous data sources in interactive software. However, high-level stream abstractions can also make it difficult for developers to reason about control- and data-flow relationships in their programs. This is particularly impactful when asynchronous stream-based code interacts with thread-limited features such as UI frameworks that restrict UI access to a single thread, since the threading behavior of streaming constructs is often non-intuitive and insufficiently documented. In this paper, we present a type-based approach that can statically prove the thread-safety of UI accesses in stream-based software. Our key insight is that the fluent APIs of stream-processing frameworks enable the tracking of threads via type-refinement, making it possible to reason automatically about what thread a piece of code runs on -- a difficult problem in general.

We implement the system as an annotation-based Java typechecker for Android programs built upon the popular ReactiveX framework and evaluate its efficacy by annotating and analyzing 8 open-source apps, where we find 33 instances of unsafe UI access while incurring an annotation burden of only one annotation per 186 source lines of code. We also report on our experience applying the typechecker to two much larger apps from the Uber Technologies Inc. codebase, where it currently runs on every code change and blocks changes that introduce potential threading bugs.

Building and running basic unit tests

./gradlew build

Analyzing your Android application

After successfully building with ./gradlew build, there will be a jar containing the checker at ./rx-thread-checker/build/libs/rx-thread-checker.jar, another jar containing the annotation classes at ./rx-thread-checker-qual/build/libs/rx-thread-checker-qual.jar, and annotation stub files in ./rx-thread-checker/src/main/resources/stub

First, add the following dependencies to your build.gradle: (or equivalent imports in other build systems)

annotationProcessor files('<path to rx-thread-checker.jar>', '<path to rx-thread-checker-qual.jar>')
annotationProcessor 'org.checkerframework:checker:2.5.5'
implementation files('<path to rx-threaad-checker-qual.jar>')
implementation 'org.checkerframework:checker:2.5.5'

Next, configure your build to run the checker by adding the following snippet to your build.gradle (or add equivalent javac flags in other build systems):

tasks.withType(JavaCompile) {
  options.compilerArgs << "-processor" << "com.ubercab.rxthreadchecker.RxThreadchecker" << "-Astubs=<path to stubs directory>"

To run the checker without compiling the application, add the flag -proc:only to the above.

See our sample app for a simple but complete application with the RxThreadEffectChecker integrated into its build according to these instructions.

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