Module kotlinx-coroutines-debug

Debugging facilities for kotlinx.coroutines on JVM.

Overview

This module provides a debug JVM agent that allows to track and trace existing coroutines. The main entry point to debug facilities is DebugProbes API. Call to DebugProbes.install installs debug agent via ByteBuddy and starts spying on coroutines when they are created, suspended and resumed.

After that, you can use DebugProbes.dumpCoroutines to print all active (suspended or running) coroutines, including their state, creation and suspension stacktraces. Additionally, it is possible to process the list of such coroutines via DebugProbes.dumpCoroutinesInfo or dump isolated parts of coroutines hierarchy referenced by a Job or CoroutineScope instances using DebugProbes.printJob and DebugProbes.printScope respectively.

Using in your project

Add kotlinx-coroutines-debug to your project test dependencies:

dependencies {
    testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-debug:1.3.3'
}

Using in unit tests

For JUnit4 debug module provides special test rule, CoroutinesTimeout, for installing debug probes and to dump coroutines on timeout to simplify tests debugging.

Its usage is better demonstrated by the example (runnable code is here):

class TestRuleExample {
    @get:Rule
    public val timeout = CoroutinesTimeout.seconds(1)

    private suspend fun someFunctionDeepInTheStack() {
        withContext(Dispatchers.IO) {
            delay(Long.MAX_VALUE) // Hang method
        }  
    }

    @Test
    fun hangingTest() = runBlocking {
        val job = launch {
            someFunctionDeepInTheStack()
        }
        job.join() // Join will hang
    }
}

After 1 second, test will fail with TestTimeoutException and all coroutines (runBlocking and launch) and their stacktraces will be dumped to the console.

Using as JVM agent

It is possible to use this module as a standalone JVM agent to enable debug probes on the application startup. You can run your application with an additional argument: -javaagent:kotlinx-coroutines-debug-1.3.3.jar. Additionally, on Linux and Mac OS X you can use kill -5 $pid command in order to force your application to print all alive coroutines.

Example of usage

Capabilities of this module can be demonstrated by the following example (runnable code is here):

suspend fun computeValue(): String = coroutineScope {
    val one = async { computeOne() }
    val two = async { computeTwo() }
    combineResults(one, two)
}

suspend fun combineResults(one: Deferred<String>, two: Deferred<String>): String =
    one.await() + two.await()

suspend fun computeOne(): String {
    delay(5000)
    return "4"
}

suspend fun computeTwo(): String {
    delay(5000)
    return "2"
}

fun main() = runBlocking {
    DebugProbes.install()
    val deferred = async { computeValue() }
    // Delay for some time
    delay(1000)
    // Dump running coroutines
    DebugProbes.dumpCoroutines()
    println("\nDumping only deferred")
    DebugProbes.printJob(deferred)
}

Printed result will be:

Coroutines dump 2018/11/12 21:44:02

Coroutine "coroutine#2":DeferredCoroutine{Active}@289d1c02, state: SUSPENDED
	at kotlinx.coroutines.DeferredCoroutine.await$suspendImpl(Builders.common.kt:99)
	at ExampleKt.combineResults(Example.kt:11)
	at ExampleKt$computeValue$2.invokeSuspend(Example.kt:7)
	at ExampleKt$main$1$deferred$1.invokeSuspend(Example.kt:25)
	(Coroutine creation stacktrace)
	at kotlin.coroutines.intrinsics.IntrinsicsKt__IntrinsicsJvmKt.createCoroutineUnintercepted(IntrinsicsJvm.kt:116)
	at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:25)
	at kotlinx.coroutines.BuildersKt.async$default(Unknown Source)
	at ExampleKt$main$1.invokeSuspend(Example.kt:25)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:32)
	at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:233)
	at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
	at ExampleKt.main(Example.kt:23)
	at ExampleKt.main(Example.kt)

... More coroutines here ...

Dumping only deferred
"coroutine#2":DeferredCoroutine{Active}, continuation is SUSPENDED at line kotlinx.coroutines.DeferredCoroutine.await$suspendImpl(Builders.common.kt:99)
			"coroutine#3":DeferredCoroutine{Active}, continuation is SUSPENDED at line ExampleKt.computeOne(Example.kt:14)
		"coroutine#4":DeferredCoroutine{Active}, continuation is SUSPENDED at line ExampleKt.computeTwo(Example.kt:19)

Status of the API

API is purely experimental and it is not guaranteed that it won't be changed (while it is marked as @ExperimentalCoroutinesApi). Do not use this module in production environment and do not rely on the format of the data produced by DebugProbes.

Debug agent and Android

Unfortunately, Android runtime does not support Instrument API necessary for kotlinx-coroutines-debug to function, triggering java.lang.NoClassDefFoundError: Failed resolution of: Ljava/lang/management/ManagementFactory;.

Nevertheless, it will be possible to support debug agent on Android as soon as GradleAspectJ-Android will support android-gradle 3.3