0.前言
Kotlin1.3開始，協程從experimental變成了release，前些日子看了看簡單的用法，今天就從原始碼的角度來看看Kotlin的協程究竟是怎樣形成的.
1.問題
看原始碼要帶著問題，我決定從以下三個問題來進行分析
1.1協程是如何建立的
1.2協程間是如何切換的
1.3協程是如何繫結到指定執行緒的
2.分析
2.1協程是如何建立的
啟動一個協程的方法
GlobalScope.launch { // launch new coroutine in background and continue
delay(1000L) // non-blocking delay for 1 second (default time unit is ms)
println("World!") // print after delay
}
複製程式碼這段程式碼就是啟動一個協程，並啟動，延遲1秒後列印world，就從這個launch方法進行切入
public fun CoroutineScope.launch(
context: CoroutineContext = EmptyCoroutineContext,
start: CoroutineStart = CoroutineStart.DEFAULT,
block: suspend CoroutineScope.() -> Unit
): Job {
val newContext = newCoroutineContext(context)
val coroutine = if (start.isLazy)
LazyStandaloneCoroutine(newContext, block) else
StandaloneCoroutine(newContext, active = true)
coroutine.start(start, coroutine, block)
return coroutine
}
複製程式碼程式碼很清楚，根據CoroutineStart是不是CoroutineStart.LAZY物件，建立不同的Job實現類，預設我們傳入的start引數為CoroutineStart.DEFAULT，這時我們建立的是一個StandaloneCoroutine物件，呼叫它的start方法啟動，然後對它進行返回。
2.2協程間是如何切換的
GlobalScope.launch(Dispatchers.Default){
println("Current thread is ${Thread.currentThread().name}")
launch {
delay(1000)
println("now")
}
println("next")
}
複製程式碼看一下這段程式碼，這段程式碼先列印出next，然後延遲1秒鐘後列印出now，有沒有一種感覺，這像是android裡handler的post和postDelay方法。首先看一下delay方法
@InternalCoroutinesApi
public interface Delay {
suspend fun delay(time: Long) {
if (time <= 0) return // don't delay
return suspendCancellableCoroutine { scheduleResumeAfterDelay(time, it) }
}
fun scheduleResumeAfterDelay(timeMillis: Long, continuation: CancellableContinuation<Unit>)
fun invokeOnTimeout(timeMillis: Long, block: Runnable): DisposableHandle =
DefaultDelay.invokeOnTimeout(timeMillis, block)
}
public suspend fun delay(timeMillis: Long) {
if (timeMillis <= 0) return // don't delay
return suspendCancellableCoroutine sc@ { cont: CancellableContinuation<Unit> ->
cont.context.delay.scheduleResumeAfterDelay(timeMillis, cont)
}
}
internal val CoroutineContext.delay: Delay get() = get(ContinuationInterceptor) as? Delay ?: DefaultDelay
複製程式碼delay方法在Delay.kt檔案裡，可以看到，這裡定義了一個Delay介面，scheduleResumeAfterDelay是用來重新把任務恢復排程的，invokeOnTimeout顯然是排程過程中發現時間到了以後要恢復執行的方法體。Delay是一個介面，看一它的實現類是如何實現scheduleResumeAfterDelay方法的。
internal abstract class EventLoopBase: CoroutineDispatcher(), Delay, EventLoop {
...
override fun scheduleResumeAfterDelay(timeMillis: Long, continuation: CancellableContinuation<Unit>) =
schedule(DelayedResumeTask(timeMillis, continuation))
...
複製程式碼先看DelayResumeTask
private inner class DelayedResumeTask(
timeMillis: Long,
private val cont: CancellableContinuation<Unit>
) : DelayedTask(timeMillis) {
init {
// Note that this operation isn't lock-free, but very short
cont.disposeOnCancellation(this)
}
override fun run() {
with(cont) { resumeUndispatched(Unit) }
}
}
複製程式碼這個類繼承自DelayTask，而DelayedTask實現了runnable介面，這裡複寫了run方法，呼叫了CancellableContinuation的resumeUndispatched方法。通過方法名可以看出經過等待時間後就會恢復執行。CancellableContinuation的實現類是CancellableContinuationImp跟進去看一看這個類
@PublishedApi
internal open class CancellableContinuationImpl<in T>(
delegate: Continuation<T>,
resumeMode: Int
) : AbstractContinuation<T>(delegate, resumeMode), CancellableContinuation<T>, Runnable {
...
override fun completeResume(token: Any) = completeStateUpdate(token as NotCompleted, state, resumeMode)
override fun CoroutineDispatcher.resumeUndispatched(value: T) {
val dc = delegate as? DispatchedContinuation
resumeImpl(value, if (dc?.dispatcher === this) MODE_UNDISPATCHED else resumeMode)
}
...
}
複製程式碼resumeUndispatched方法裡呼叫了resumeImp方法，這是繼承自AbstractContinuation的方法
protected fun resumeImpl(proposedUpdate: Any?, resumeMode: Int) {
loopOnState { state ->
when (state) {
is NotCompleted -> {
if (updateStateToFinal(state, proposedUpdate, resumeMode)) return
}
is CancelledContinuation -> {
/*
* If continuation was cancelled, then all further resumes must be
* ignored, because cancellation is asynchronous and may race with resume.
* Racy exception are reported so no exceptions are lost
*
* :todo: we should somehow remember the attempt to invoke resume and fail on the second attempt.
*/
if (proposedUpdate is CompletedExceptionally) {
handleException(proposedUpdate.cause)
}
return
}
else -> error("Already resumed, but proposed with update $proposedUpdate")
}
}
}
複製程式碼這裡會根據不同的狀態呼叫不同的方法.
private fun updateStateToFinal(expect: NotCompleted, proposedUpdate: Any?, mode: Int): Boolean {
...
completeStateUpdate(expect, proposedUpdate, mode)
return true
}
protected fun completeStateUpdate(expect: NotCompleted, update: Any?, mode: Int) {
...
dispatchResume(mode)
}
private fun dispatchResume(mode: Int) {
if (tryResume()) return // completed before getResult invocation -- bail out
// otherwise, getResult has already commenced, i.e. completed later or in other thread
dispatch(mode)
}
internal fun <T> DispatchedTask<T>.dispatch(mode: Int = MODE_CANCELLABLE) {
val delegate = this.delegate
if (mode.isDispatchedMode && delegate is DispatchedContinuation<*> && mode.isCancellableMode == resumeMode.isCancellableMode) {
// dispatch directly using this instance's Runnable implementation
val dispatcher = delegate.dispatcher
val context = delegate.context
if (dispatcher.isDispatchNeeded(context)) {
dispatcher.dispatch(context, this)
} else {
UndispatchedEventLoop.resumeUndispatched(this)
}
} else {
resume(delegate, mode)
}
}
複製程式碼刪掉了不相關的程式碼，只保留dispatch這條主線，相信很容易個看明白最終又把這個任務放回到Dispatcher裡面去了。那個else分支的resume其實內部呼叫的是Continuation.resume擴充套件方法，最終一樣要呼叫到resumeImpl中，又回到上面已經分析的流程裡了，這是處理有Continuation代理的情況。以上就是當delay時間到達後協程是如何重新恢復的。
接下來看一看延時是如何實現的，協程裡有個預設的DefaultExecutor執行緒用來執行協程程式碼
override fun run() {
timeSource.registerTimeLoopThread()
try {
var shutdownNanos = Long.MAX_VALUE
if (!notifyStartup()) return
while (true) {
Thread.interrupted() // just reset interruption flag
var parkNanos = processNextEvent()
if (parkNanos == Long.MAX_VALUE) {
// nothing to do, initialize shutdown timeout
if (shutdownNanos == Long.MAX_VALUE) {
val now = timeSource.nanoTime()
if (shutdownNanos == Long.MAX_VALUE) shutdownNanos = now + KEEP_ALIVE_NANOS
val tillShutdown = shutdownNanos - now
if (tillShutdown <= 0) return // shut thread down
parkNanos = parkNanos.coerceAtMost(tillShutdown)
} else
parkNanos = parkNanos.coerceAtMost(KEEP_ALIVE_NANOS) // limit wait time anyway
}
if (parkNanos > 0) {
// check if shutdown was requested and bail out in this case
if (isShutdownRequested) return
timeSource.parkNanos(this, parkNanos)
}
}
} finally {
_thread = null // this thread is dead
acknowledgeShutdownIfNeeded()
timeSource.unregisterTimeLoopThread()
// recheck if queues are empty after _thread reference was set to null (!!!)
if (!isEmpty) thread() // recreate thread if it is needed
}
}
複製程式碼override fun processNextEvent(): Long {
if (!isCorrectThread()) return Long.MAX_VALUE
// queue all delayed tasks that are due to be executed
val delayed = _delayed.value
if (delayed != null && !delayed.isEmpty) {
val now = timeSource.nanoTime()
while (true) {
// make sure that moving from delayed to queue removes from delayed only after it is added to queue
// to make sure that 'isEmpty' and `nextTime` that check both of them
// do not transiently report that both delayed and queue are empty during move
delayed.removeFirstIf {
if (it.timeToExecute(now)) {
enqueueImpl(it)
} else
false
} ?: break // quit loop when nothing more to remove or enqueueImpl returns false on "isComplete"
}
}
// then process one event from queue
dequeue()?.run()
return nextTime
}
複製程式碼DefaultExecutor不斷獲取task並執行，而這些task事件就是儲存在_delayed裡的，這裡可以將_delayed理解為一個佇列。簡述這兩段程式碼做的事情就是就是死迴圈遍歷task佇列該執行的就執行並出隊，沒到執行時間的就留在佇列。 總結一下，協程就是維持了一個類似android Looper和MessageQueuen的東西，將要執行的程式碼封裝成Coroutine放入佇列，然後通過迴圈並根據一定條件不停的取出執行。
2.3協程是如何繫結到指定執行緒的
回到launch方法
public fun CoroutineScope.launch(
context: CoroutineContext = EmptyCoroutineContext,
start: CoroutineStart = CoroutineStart.DEFAULT,
block: suspend CoroutineScope.() -> Unit
): Job {
val newContext = newCoroutineContext(context)
val coroutine = if (start.isLazy)
LazyStandaloneCoroutine(newContext, block) else
StandaloneCoroutine(newContext, active = true)
coroutine.start(start, coroutine, block)
return coroutine
}
複製程式碼看一下StandaloneCoroutine的start方法
public fun <R> start(start: CoroutineStart, receiver: R, block: suspend R.() -> T) {
initParentJob()
start(block, receiver, this)
}
複製程式碼start(block, receiver, this)呼叫的就是CoroutineStart裡的invoke方法，這裡其實是CoroutineStart對操作符進行了複寫，並不是遞迴呼叫，這個start就是launch方法傳進來的，預設是CoroutineStart.DEFAULT，這是一個列舉物件
@InternalCoroutinesApi
public operator fun <R, T> invoke(block: suspend R.() -> T, receiver: R, completion: Continuation<T>) =
when (this) {
CoroutineStart.DEFAULT -> block.startCoroutineCancellable(receiver, completion)
CoroutineStart.ATOMIC -> block.startCoroutine(receiver, completion)
CoroutineStart.UNDISPATCHED -> block.startCoroutineUndispatched(receiver, completion)
CoroutineStart.LAZY -> Unit // will start lazily
}
internal fun <T> (suspend () -> T).startCoroutineCancellable(completion: Continuation<T>) =
createCoroutineUnintercepted(completion).intercepted().resumeCancellable(Unit)
internal fun <T> Continuation<T>.resumeCancellable(value: T) = when (this) {
is DispatchedContinuation -> resumeCancellable(value)
else -> resume(value)
}
@Suppress("NOTHING_TO_INLINE") // we need it inline to save us an entry on the stack
inline fun resumeCancellable(value: T) {
if (dispatcher.isDispatchNeeded(context)) {
_state = value
resumeMode = MODE_CANCELLABLE
dispatcher.dispatch(context, this)
} else {
UndispatchedEventLoop.execute(this, value, MODE_CANCELLABLE) {
if (!resumeCancelled()) {
resumeUndispatched(value)
}
}
}
}
複製程式碼總之到了這裡，就是通過 dispatcher.dispatch(...)把這個任務分發給執行緒/執行緒池去執行了，分發方式根據CoroutineStart物件有關。
3.總結一下
上面說了很多原始碼上的東西，畫張圖，方便理解
