Kotlin协程Dispatchers原理示例详解

Kotlin协程Dispatchers原理示例详解 目录 前置知识 demo startCoroutineCancellable intercepted()函数 DefaultScheduler中找dispatch函数 Runnable传入 Worker线程执行逻辑 小结 前置知识 Kotlin协程不是什么空中阁楼,Kotlin源代码会被编译成class字节码文件,最终会运行到虚拟机
目录
  • 前置知识
    • demo
    • startCoroutineCancellable
    • intercepted()函数
    • DefaultScheduler中找dispatch函数
    • Runnable传入
    • Worker线程执行逻辑
  • 小结

前置知识

Kotlin协程不是什么空中阁楼,Kotlin源代码会被编译成class字节码文件,最终会运行到虚拟机中。所以从本质上讲,Kotlin和Java是类似的,都是可以编译产生class的语言,但最终还是会受到虚拟机的限制,它们的代码最终会在虚拟机上的某个线程上被执行。

之前我们分析了launch的原理,但当时我们没有去分析协程创建出来后是如何与线程产生关联的,怎么被分发到具体的线程上执行的,本篇文章就带大家分析一下。

要想搞懂Dispatchers,我们先来看一下Dispatchers、CoroutineDispatcher、ContinuationInterceptor、CoroutineContext之间的关系

public actual object Dispatchers {
    @JvmStatic
    public actual val Default: CoroutineDispatcher = DefaultScheduler
    @JvmStatic
    public actual val Main: MainCoroutineDispatcher get() = MainDispatcherLoader.dispatcher
    @JvmStatic
    public actual val Unconfined: CoroutineDispatcher = kotlinx.coroutines.Unconfined
    @JvmStatic
    public val IO: CoroutineDispatcher = DefaultIoScheduler
}
public abstract class CoroutineDispatcher :
    AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
}
public interface ContinuationInterceptor : CoroutineContext.Element {}
public interface Element : CoroutineContext {}

Dispatchers中存放的是协程调度器(它本身是一个单例),有我们平时常用的IO、Default、Main等。这些协程调度器都是CoroutineDispatcher的子类,这些协程调度器其实都是CoroutineContext。

demo

我们先来看一个关于launch的demo:

fun main() {
    val coroutineScope = CoroutineScope(Job())
    coroutineScope.launch {
        println("Thread : ${Thread.currentThread().name}")
    }
    Thread.sleep(5000L)
}

在生成CoroutineScope时,demo中没有传入相关的协程调度器,也就是Dispatchers。那这个launch会运行到哪个线程之上?

运行试一下:

Thread : DefaultDispatcher-worker-1

居然运行到了DefaultDispatcher-worker-1线程上,这看起来明显是Dispatchers.Default协程调度器里面的线程。我明明没传Dispatchers相关的context,居然会运行到子线程上。说明运行到default线程是launch默认的。

它是怎么与default线程产生关联的?打开源码一探究竟:

public fun CoroutineScope.launch(
    context: CoroutineContext = EmptyCoroutineContext,
    start: CoroutineStart = CoroutineStart.DEFAULT,
    block: suspend CoroutineScope.() -> Unit
): Job {
    //代码1
    val newContext = newCoroutineContext(context)
    //代码2
    val coroutine = if (start.isLazy)
        LazyStandaloneCoroutine(newContext, block) else
        StandaloneCoroutine(newContext, active = true)
    //代码3
    coroutine.start(start, coroutine, block)
    return coroutine
}
  • 将传入的CoroutineContext构造出新的context
  • 启动模式,判断是否为懒加载,如果是懒加载则构建懒加载协程对象,否则就是标准的
  • 启动协程

我们重点关注代码1,这是与CoroutineContext相关的。

public actual fun CoroutineScope.newCoroutineContext(context: CoroutineContext): CoroutineContext {
    //从父协程那里继承过来的context+这次的context
    val combined = coroutineContext.foldCopiesForChildCoroutine() + context
    val debug = if (DEBUG) combined + CoroutineId(COROUTINE_ID.incrementAndGet()) else combined
    //combined可以简单的把它看成是一个map,它是CoroutineContext类型的
    //如果当前context不等于Dispatchers.Default,而且从map里面取ContinuationInterceptor(用于拦截之后分发线程的)值为空,说明没有传入协程应该在哪个线程上运行的相关参数
    return if (combined !== Dispatchers.Default && combined[ContinuationInterceptor] == null)
        debug + Dispatchers.Default else debug
}

调用launch的时候,我们没有传入context,默认参数是EmptyCoroutineContext。这里的combined,它其实是CoroutineContext类型的,可以简单的看成是map(其实不是,只是类似)。

通过combined[ContinuationInterceptor]可以将传入的线程调度相关的参数给取出来,这里如果取出来为空,是给该context添加了一个Dispatchers.Default,然后把新的context返回出去了。所以launch默认情况下,会走到default线程去执行。

补充一点:CoroutineContext能够通过+连接是因为它内部有个public operator fun plus函数。能够通过combined[ContinuationInterceptor]这种方式访问元素是因为有个public operator fun get函数。

public interface CoroutineContext {
     /**
     * Returns the element with the given [key] from this context or `null`.
     */
    public operator fun <E : Element> get(key: Key<E>): E?
     /**
     * Returns a context containing elements from this context and elements from  other [context].
     * The elements from this context with the same key as in the other one are dropped.
     */
    public operator fun plus(context: CoroutineContext): CoroutineContext {
        ......
    }
}

startCoroutineCancellable

上面我们分析了launch默认情况下,context中会增加Dispatchers.Default的这个协程调度器,到时launch的Lambda会在default线程上执行,其中具体流程是怎么样的,我们分析一下。

在之前的文章 Kotlin协程之launch原理 中我们分析过,launch默认情况下会最终执行到startCoroutineCancellable函数。

public fun <T> (suspend () -> T).startCoroutineCancellable(completion: Continuation<T>): Unit = runSafely(completion) {
    //构建ContinuationImpl
    createCoroutineUnintercepted(completion).intercepted().resumeCancellableWith(Result.success(Unit))
}
public actual fun <T> (suspend () -> T).createCoroutineUnintercepted(
    completion: Continuation<T>
): Continuation<Unit> {
    val probeCompletion = probeCoroutineCreated(completion)
    return if (this is BaseContinuationImpl)
        //走这里
        create(probeCompletion)
    else
        createCoroutineFromSuspendFunction(probeCompletion) {
            (this as Function1<Continuation<T>, Any?>).invoke(it)
        }
}

在Kotlin协程之launch原理 文章中,咱们分析过create(probeCompletion)这里创建出来的是launch的那个Lambda,编译器会产生一个匿名内部类,它继承自SuspendLambda,而SuspendLambda是继承自ContinuationImpl。

所以 createCoroutineUnintercepted(completion)一开始构建出来的是一个ContinuationImpl,接下来需要去看它的intercepted()函数。

intercepted()函数

internal abstract class ContinuationImpl(
    completion: Continuation<Any?>?,
    private val _context: CoroutineContext?
) : BaseContinuationImpl(completion) {
    constructor(completion: Continuation<Any?>?) : this(completion, completion?.context)
    public override val context: CoroutineContext
        get() = _context!!
    @Transient
    private var intercepted: Continuation<Any?>? = null
    public fun intercepted(): Continuation<Any?> =
        intercepted
            ?: (context[ContinuationInterceptor]?.interceptContinuation(this) ?: this)
                .also { intercepted = it }
}

第一次走到intercepted()函数时,intercepted肯定是为null的,还没初始化。此时会通过context[ContinuationInterceptor]取出Dispatcher对象,然后调用该Dispatcher对象的interceptContinuation()函数。这个Dispatcher对象在demo这里其实就是Dispatchers.Default。

public actual object Dispatchers {
    @JvmStatic
    public actual val Default: CoroutineDispatcher = DefaultScheduler
}

可以看到,Dispatchers.Default是一个CoroutineDispatcher对象,interceptContinuation()函数就在CoroutineDispatcher中。

public abstract class CoroutineDispatcher :
    AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
    public final override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> =
        DispatchedContinuation(this, continuation)
}
public fun <T> (suspend () -> T).startCoroutineCancellable(completion: Continuation<T>): Unit = runSafely(completion) {
    createCoroutineUnintercepted(completion).intercepted().resumeCancellableWith(Result.success(Unit))
}

这个方法非常简单,就是新建并且返回了一个DispatchedContinuation对象,将this和continuation给传入进去。这里的this是Dispatchers.Default。

所以,最终我们发现走完startCoroutineCancellable的前2步之后,也就是走完intercepted()之后,创建的是DispatchedContinuation对象,最后是调用的DispatchedContinuation的resumeCancellableWith函数。最后这步比较关键,这是真正将协程的具体执行逻辑放到线程上执行的部分。

internal class DispatchedContinuation<in T>(
    //这里传入的dispatcher在demo中是Dispatchers.Default
    @JvmField val dispatcher: CoroutineDispatcher,
    @JvmField val continuation: Continuation<T>
) : DispatchedTask<T>(MODE_UNINITIALIZED), CoroutineStackFrame, Continuation<T> by continuation {
    inline fun resumeCancellableWith(
        result: Result<T>,
        noinline onCancellation: ((cause: Throwable) -> Unit)?
    ) {
        val state = result.toState(onCancellation)
        //代码1
        if (dispatcher.isDispatchNeeded(context)) {
            _state = state
            resumeMode = MODE_CANCELLABLE
            //代码2
            dispatcher.dispatch(context, this)
        } else {
            //代码3
            executeUnconfined(state, MODE_CANCELLABLE) {
                if (!resumeCancelled(state)) {
                    resumeUndispatchedWith(result)
                }
            }
        }
    }
}
internal abstract class DispatchedTask<in T>(
    @JvmField public var resumeMode: Int
) : SchedulerTask() {
    ......
}
internal actual typealias SchedulerTask = Task
internal abstract class Task(
    @JvmField var submissionTime: Long,
    @JvmField var taskContext: TaskContext
) : Runnable {
    ......
}
public abstract class CoroutineDispatcher :
    AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
    public abstract fun dispatch(context: CoroutineContext, block: Runnable)
    public open fun isDispatchNeeded(context: CoroutineContext): Boolean = true
}

从DispatchedContinuation的继承结构来看,它既是一个Continuation(通过委托给传入的continuation参数),也是一个Runnable。

  • 首先看代码1:这个dispatcher在demo中其实是Dispatchers.Default ,然后调用它的isDispatchNeeded(),这个函数定义在CoroutineDispatcher中,默认就是返回true,只有Dispatchers.Unconfined返回false
  • 代码2:调用Dispatchers.Default的dispatch函数,将context和自己(DispatchedContinuation,也就是Runnable)传过去了
  • 代码3:对应Dispatchers.Unconfined的情况,它的isDispatchNeeded()返回false

现在我们要分析代码2之后的执行逻辑,也就是将context和Runnable传入到dispatch函数之后是怎么执行的。按道理,看到Runnable,那可能这个与线程执行相关,应该离我们想要的答案不远了。回到Dispatchers,我们发现Dispatchers.Default是DefaultScheduler类型的,那我们就去DefaultScheduler中或者其父类中去找dispatch函数。

DefaultScheduler中找dispatch函数

public actual object Dispatchers {
    @JvmStatic
    public actual val Default: CoroutineDispatcher = DefaultScheduler
}
internal object DefaultScheduler : SchedulerCoroutineDispatcher(
    CORE_POOL_SIZE, MAX_POOL_SIZE,
    IDLE_WORKER_KEEP_ALIVE_NS, DEFAULT_SCHEDULER_NAME
) {
    ......
}
internal open class SchedulerCoroutineDispatcher(
    private val corePoolSize: Int = CORE_POOL_SIZE,
    private val maxPoolSize: Int = MAX_POOL_SIZE,
    private val idleWorkerKeepAliveNs: Long = IDLE_WORKER_KEEP_ALIVE_NS,
    private val schedulerName: String = "CoroutineScheduler",
) : ExecutorCoroutineDispatcher() {
    private var coroutineScheduler = createScheduler()
    private fun createScheduler() =
        CoroutineScheduler(corePoolSize, maxPoolSize, idleWorkerKeepAliveNs, schedulerName)
     override fun dispatch(context: CoroutineContext, block: Runnable): Unit = coroutineScheduler.dispatch(block)
}

最后发现dispatch函数在其父类SchedulerCoroutineDispatcher中,在这里构建了一个CoroutineScheduler,直接调用了CoroutineScheduler对象的dispatch,然后将Runnable(也就是上面的DispatchedContinuation对象)传入。

Runnable传入

internal class CoroutineScheduler(
    @JvmField val corePoolSize: Int,
    @JvmField val maxPoolSize: Int,
    @JvmField val idleWorkerKeepAliveNs: Long = IDLE_WORKER_KEEP_ALIVE_NS,
    @JvmField val schedulerName: String = DEFAULT_SCHEDULER_NAME
) : Executor, Closeable {
    override fun execute(command: Runnable) = dispatch(command)
    fun dispatch(block: Runnable, taskContext: TaskContext = NonBlockingContext, tailDispatch: Boolean = false) {
        trackTask() // this is needed for virtual time support
        //代码1:构建Task,Task实现了Runnable接口
        val task = createTask(block, taskContext)
        //代码2:取当前线程转为Worker对象,Worker是一个继承自Thread的类
        val currentWorker = currentWorker()
        //代码3:尝试将Task提交到本地队列并根据结果执行相应的操作
        val notAdded = currentWorker.submitToLocalQueue(task, tailDispatch)
        if (notAdded != null) {
            //代码4:notAdded不为null,则再将notAdded(Task)添加到全局队列中
            if (!addToGlobalQueue(notAdded)) {
                throw RejectedExecutionException("$schedulerName was terminated")
            }
        }
        val skipUnpark = tailDispatch && currentWorker != null
        // Checking 'task' instead of 'notAdded' is completely okay
        if (task.mode == TASK_NON_BLOCKING) {
            if (skipUnpark) return
            //代码5: 创建Worker并开始执行该线程
            signalCpuWork()
        } else {
            // Increment blocking tasks anyway
            signalBlockingWork(skipUnpark = skipUnpark)
        }
    }
    private fun currentWorker(): Worker? = (Thread.currentThread() as? Worker)?.takeIf { it.scheduler == this }
    internal inner class Worker private constructor() : Thread() {
        .....
    }
}

观察发现,原来CoroutineScheduler类实现了java.util.concurrent.Executor接口,同时实现了它的execute方法,这个方法也会调用dispatch()。

  • 代码1:首先是通过Runnable构建了一个Task,这个Task其实也是实现了Runnable接口,只是把传入的Runnable包装了一下
  • 代码2:将当前线程取出来转换成Worker,当然第一次时,这个转换不会成功,这个Worker是继承自Thread的一个类
  • 代码3:将task提交到本地队列中,这个本地队列待会儿会在Worker这个线程执行时取出Task,并执行Task
  • 代码4:如果task提交到本地队列的过程中没有成功,那么会添加到全局队列中,待会儿也会被Worker取出来Task并执行
  • 代码5:创建Worker线程,并开始执行

开始执行Worker线程之后,我们需要看一下这个线程的run方法执行的是啥,也就是它的具体执行逻辑。

Worker线程执行逻辑

internal inner class Worker private constructor() : Thread() {
    override fun run() = runWorker()
    private fun runWorker() {
        var rescanned = false
        while (!isTerminated && state != WorkerState.TERMINATED) {
            //代码1
            val task = findTask(mayHaveLocalTasks)
            if (task != null) {
                rescanned = false
                minDelayUntilStealableTaskNs = 0L
                //代码2
                executeTask(task)
                continue
            } else {
                mayHaveLocalTasks = false
            }
            if (minDelayUntilStealableTaskNs != 0L) {
                if (!rescanned) {
                    rescanned = true
                } else {
                    rescanned = false
                    tryReleaseCpu(WorkerState.PARKING)
                    interrupted()
                    LockSupport.parkNanos(minDelayUntilStealableTaskNs)
                    minDelayUntilStealableTaskNs = 0L
                }
                continue
            }
            tryPark()
        }
        tryReleaseCpu(WorkerState.TERMINATED)
    }
    fun findTask(scanLocalQueue: Boolean): Task? {
        if (tryAcquireCpuPermit()) return findAnyTask(scanLocalQueue)
        // If we can't acquire a CPU permit -- attempt to find blocking task
        val task = if (scanLocalQueue) {
            localQueue.poll() ?: globalBlockingQueue.removeFirstOrNull()
        } else {
            globalBlockingQueue.removeFirstOrNull()
        }
        return task ?: trySteal(blockingOnly = true)
    }
    private fun executeTask(task: Task) {
        val taskMode = task.mode
        idleReset(taskMode)
        beforeTask(taskMode)
        runSafely(task)
        afterTask(taskMode)
    }
    fun runSafely(task: Task) {
        try {
            task.run()
        } catch (e: Throwable) {
            val thread = Thread.currentThread()
            thread.uncaughtExceptionHandler.uncaughtException(thread, e)
        } finally {
            unTrackTask()
        }
    }
}

run方法直接调用的runWorker(),在里面是一个while循环,不断从队列中取Task来执行。

  • 代码1:从本地队列或者全局队列中取出Task
  • 代码2:执行这个task,最终其实就是调用这个Runnable的run方法。

也就是说,在Worker这个线程中,执行了这个Runnable的run方法。还记得这个Runnable是谁么?它就是上面我们看过的DispatchedContinuation,这里的run方法执行的就是协程任务,那这块具体的run方法的实现逻辑,我们应该到DispatchedContinuation中去找。

internal class DispatchedContinuation<in T>(
    @JvmField val dispatcher: CoroutineDispatcher,
    @JvmField val continuation: Continuation<T>
) : DispatchedTask<T>(MODE_UNINITIALIZED), CoroutineStackFrame, Continuation<T> by continuation {
    ......
}
internal abstract class DispatchedTask<in T>(
    @JvmField public var resumeMode: Int
) : SchedulerTask() {
    public final override fun run() {
        assert { resumeMode != MODE_UNINITIALIZED } // should have been set before dispatching
        val taskContext = this.taskContext
        var fatalException: Throwable? = null
        try {
            val delegate = delegate as DispatchedContinuation<T>
            val continuation = delegate.continuation
            withContinuationContext(continuation, delegate.countOrElement) {
                val context = continuation.context
                val state = takeState() // NOTE: Must take state in any case, even if cancelled
                val exception = getExceptionalResult(state)
                /*
                 * Check whether continuation was originally resumed with an exception.
                 * If so, it dominates cancellation, otherwise the original exception
                 * will be silently lost.
                 */
                val job = if (exception == null && resumeMode.isCancellableMode) context[Job] else null
                //非空,且未处于active状态
                if (job != null && !job.isActive) {
                    //开始之前,协程已经被取消,将具体的Exception传出去
                    val cause = job.getCancellationException()
                    cancelCompletedResult(state, cause)
                    continuation.resumeWithStackTrace(cause)
                } else {
                    //有异常,传递异常
                    if (exception != null) {
                        continuation.resumeWithException(exception)
                    } else {
                        //代码1
                        continuation.resume(getSuccessfulResult(state))
                    }
                }
            }
        } catch (e: Throwable) {
            // This instead of runCatching to have nicer stacktrace and debug experience
            fatalException = e
        } finally {
            val result = runCatching { taskContext.afterTask() }
            handleFatalException(fatalException, result.exceptionOrNull())
        }
    }
}

我们主要看一下代码1处,调用了resume开启协程。前面没有异常,才开始启动协程,这里才是真正的开始启动协程,开始执行launch传入的Lambda表达式。这个时候,协程的逻辑是在Worker这个线程上执行的了,切到某个线程上执行的逻辑已经完成了。

ps: rusume会走到BaseContinuationImpl的rusumeWith,然后走到launch传入的Lambda匿名内部类的invokeSuspend方法,开始执行状态机逻辑。前面的文章 Kotlin协程createCoroutine和startCoroutine原理 我们分析过这里,这里就只是简单提一下。

到这里,Dispatchers的执行流程就算完了,前后都串起来了。

小结

Dispatchers是协程框架中与线程交互的关键。底层会有不同的线程池,Dispatchers.Default、IO,协程任务来了的时候会封装成一个个的Runnable,丢到线程中执行,这些Runnable的run方法中执行的其实就是continuation.resume,也就是launch的Lambda生成的SuspendLambda匿名内部类,也就是开启协程状态机,开始协程的真正执行。

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