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1. Overview

In this article, we’ll be looking at coroutines from the Kotlin language. Simply put, coroutines allow us to create asynchronous programs in a very fluent way, and they’re based on the concept of Continuation-passing style programming.

The Kotlin language gives us basic constructs but can get access to more useful coroutines with the kotlinx-coroutines-core library. We’ll be looking at this library once we understand the basic building blocks of the Kotlin language.

2. Creating a Coroutine with BuildSequence

Let’s create a first coroutine using the buildSequence function.

And let’s implement a Fibonacci sequence generator using this function:

val fibonacciSeq = buildSequence {
    var a = 0
    var b = 1

    yield(1)

    while (true) {
        yield(a + b)

        val tmp = a + b
        a = b
        b = tmp
    }
}

The signature of a yield function is:

public abstract suspend fun yield(value: T)

The suspend keyword means that this function can be blocking. Such function can suspend a buildSequence coroutine.

Suspending functions can be created as standard Kotlin functions, but we need to be aware that we can only call them from within a coroutine. Otherwise, we’ll get a compiler error.

If we’ve suspended call within the buildSequence, that call will be transformed to the dedicated state in the state machine. A coroutine can be passed and assigned to a variable like any other function.

In the fibonacciSeq coroutine, we have two suspension points. First, when we’re calling yield(1) and second when we’re calling yield(a+b).

If that yield function results in some blocking call, the current thread will not block on it. It will be able to execute some other code. Once the suspended function finishes its execution, the thread can resume execution of the fibonacciSeq coroutine.

We can test our code by taking some elements from the Fibonacci sequence:

val res = fibonacciSeq
  .take(5)
  .toList()

assertEquals(res, listOf(1, 1, 2, 3, 5))

3. Adding the Maven Dependency for kotlinx-coroutines

Let’s look at the kotlinx-coroutines library which has useful constructs build on top of basic coroutines.

Let’s add the dependency to the kotlinx-coroutines-core library. Note that we also need to add the jcenter repository:

<dependency>
    <groupId>org.jetbrains.kotlinx</groupId>
    <artifactId>kotlinx-coroutines-core</artifactId>
    <version>0.16</version>
</dependency>

<repositories>
    <repository>
        <id>central</id>
        <url>http://jcenter.bintray.com</url>
     </repository>
</repositories>

4. Asynchronous Programming Using the launch() Coroutine

The kotlinx-coroutines library adds a lot of useful constructs that allow us to create asynchronous programs. Let’s say that we have an expensive computation function that is appending a String to the input list:

suspend fun expensiveComputation(res: MutableList<String>) {
    delay(1000L)
    res.add("word!")
}

We can use a launch coroutine that will execute that suspend function in a non-blocking way – we need to pass a thread pool as an argument to it.

The launch function is returning a Job instance on which we can call a join() method to wait for the results:

@Test
fun givenAsyncCoroutine_whenStartIt_thenShouldExecuteItInTheAsyncWay() {
    // given
    val res = mutableListOf<String>()

    // when
    runBlocking<Unit> {
        val promise = launch(CommonPool) { 
          expensiveComputation(res) 
        }
        res.add("Hello,")
        promise.join()
    }

    // then
    assertEquals(res, listOf("Hello,", "word!"))
}

To be able to test our code, we pass all logic into the runBlocking coroutine – which is a blocking call. Therefore our assertEquals() can be executed synchronously after the code inside of the runBlocking() method.

Note that in this example, although the launch() method is triggered first, it is a delayed computation. The main thread will proceed by appending the “Hello,” String to the result list.

After the one second delay that is introduced in the expensiveComputation() function, the “word!” String will be appended to the result.

5. Coroutines are Very Lightweight

Let’s imagine a situation in which we want to perform 100000 operations asynchronously. Spawning such a high number of threads will be very costly and will possibly yield an OutOfMemoryException.

Fortunately, when using the coroutines, this is not a case. We can execute as many blocking operations as we want. Under the hood, those operations will be handled by a fixed number of threads without excessive thread creation:

@Test
fun givenHugeAmountOfCoroutines_whenStartIt_thenShouldExecuteItWithoutOutOfMemory() {
    runBlocking<Unit> {
        // given
        val counter = AtomicInteger(0)
        val numberOfCoroutines = 100_000

        // when
        val jobs = List(numberOfCoroutines) {
            launch(CommonPool) {
                delay(1000L)
                counter.incrementAndGet()
            }
        }
        jobs.forEach { it.join() }

        // then
        assertEquals(counter.get(), numberOfCoroutines)
    }
}

Note that we’re executing 100,000 coroutines and each run adds a substantial delay. Nevertheless, there is no need to create too many threads because those operations are executed in an asynchronous way using thread from the CommonPool.

6. Cancellation and Timeouts

Sometimes, after we have triggered some long-running asynchronous computation, we want to cancel it because we’re no longer interested in the result.

When we start our asynchronous action with the launch() coroutine, we can examine the isActive flag. This flag is set to false whenever the main thread invokes the cancel() method on the instance of the Job:

@Test
fun givenCancellableJob_whenRequestForCancel_thenShouldQuit() {
    runBlocking<Unit> {
        // given
        val job = launch(CommonPool) {
            while (isActive) {
                println("is working")
            }
        }

        delay(1300L)

        // when
        job.cancel()

        // then cancel successfully

    }
}

This is a very elegant and easy way to use the cancellation mechanism. In the asynchronous action, we only need to check if the isActive flag is equal to false and cancel our processing.

When we’re requesting some processing and are not sure how much time that computation will take, it’s advisable to set the timeout on such an action. If the processing does not finish within the given timeout, we’ll get an exception, and we can react to it appropriately.

For example, we can retry the action:

@Test(expected = CancellationException::class)
fun givenAsyncAction_whenDeclareTimeout_thenShouldFinishWhenTimedOut() {
    runBlocking<Unit> {
        withTimeout(1300L) {
            repeat(1000) { i ->
                println("Some expensive computation $i ...")
                delay(500L)
            }
        }
    }
}

If we do not define a timeout, it’s possible that our thread will be blocked forever because that computation will hang. We cannot handle that case in our code if the timeout is not defined.

7. Running Asynchronous Actions Concurrently

Let’s say that we need to start two asynchronous actions concurrently and wait for their results afterward. If our processing takes one second and we need to execute that processing twice, the runtime of synchronous blocking execution will be two seconds.

It would be better if we could run both those actions in separate threads and wait for those results in the main thread.

We can leverage the async() coroutine to achieve this by starting processing in two separate threads concurrently:

@Test
fun givenHaveTwoExpensiveAction_whenExecuteThemAsync_thenTheyShouldRunConcurrently() {
    runBlocking<Unit> {
        val delay = 1000L
        val time = measureTimeMillis {
            // given
            val one = async(CommonPool) { 
                someExpensiveComputation(delay) 
            }
            val two = async(CommonPool) { 
                someExpensiveComputation(delay) 
            }

            // when
            runBlocking {
                one.await()
                two.await()
            }
        }

        // then
        assertTrue(time < delay * 2)
    }
}

After we submit the two expensive computations, we suspend the coroutine by executing the runBlocking() call. Once results one and two are available, the coroutine will resume, and the results are returned. Executing two tasks in this way should take around one second.

We can pass CoroutineStart.LAZY as the second argument to the async() method, but this will mean the asynchronous computation will not be started until requested. Because we are requesting computation in the runBlocking coroutine, it means the call to two.await() will be made only once the one.await() has finished:

@Test
fun givenTwoExpensiveAction_whenExecuteThemLazy_thenTheyShouldNotConcurrently() {
    runBlocking<Unit> {
        val delay = 1000L
        val time = measureTimeMillis {
            // given
            val one 
              = async(CommonPool, CoroutineStart.LAZY) {
                someExpensiveComputation(delay) 
              }
            val two 
              = async(CommonPool, CoroutineStart.LAZY) { 
                someExpensiveComputation(delay) 
            }

            // when
            runBlocking {
                one.await()
                two.await()
            }
        }

        // then
        assertTrue(time > delay * 2)
    }
}

The laziness of the execution in this particular example causes our code to run synchronously. That happens because when we call await(), the main thread is blocked and only after task one finishes task two will be triggered.

We need to be aware of performing asynchronous actions in a lazy way as they may run in a blocking way.

8. Conclusion

In this article, we looked at basics of Kotlin coroutines.

We saw that buildSequence is the main building block of every coroutine. We described how the flow of execution in this Continuation-passing programming style looks.

Finally, we looked at the kotlinx-coroutines library that ships a lot of very useful constructs for creating asynchronous programs.

The implementation of all these examples and code snippets can be found in the GitHub project – this is a Maven project, so it should be easy to import and run as it is.

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