Kotlin协程createCoroutine和startCoroutine原理


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协程到底是怎么创建和启动的?本篇文章带你揭晓。

一、createCoroutine 和 startCoroutine

在Continuation.kt文件中,有2个基础API,这里单独提出来说一下,方便后面我们理解launch。

public fun <T> (suspend () -> T).createCoroutine(
 completion: Continuation<T>
): Continuation<Unit> =
 SafeContinuation(createCoroutineUnintercepted(completion).intercepted(), COROUTINE_SUSPENDED)
public fun <T> (suspend () -> T).startCoroutine(
 completion: Continuation<T>
) {
 createCoroutineUnintercepted(completion).intercepted().resume(Unit)
}

createCoroutine和startCoroutine就是用来创建和启动协程的基础API,launch、async等在底层一定程度上都使用了该基础API,launch和async只不过是封装而已。所以,我们先掌握它们。

这2个函数看起来差别不大,一个调用了resume开始了协程,一个没有调用,需要外部去调用resume(createCoroutine会把Continuation返回出去)。

既然launch和async可以用它们来创建和启动协程,那我们是否可以直接用它们来创建和启动协程?那当然可以。这里我举个startCoroutine的例子,仔细看它的函数声明,它其实是个扩展函数,扩展的是(suspend () -> T)这种类型。

(suspend () -> T):suspend函数+返回类型是T

它可以有2种写法:

//方式1-----------
val block = suspend {
 ...
 "云天明"
}
block.startCoroutine(continuation)
//方式2--------------
suspend fun getUserName(): String {
 ...
 return "云天明"
}
(::getUserName).startCoroutine(continuation)

一种是匿名的suspend函数,一种是正常的有名字的suspend函数。现在,我们简单写个demo来调一下startCoroutine。

//StartCoroutine.kt
fun main() {
 val continuation = object : Continuation<String> {
 override val context: CoroutineContext
 get() = EmptyCoroutineContext
 override fun resumeWith(result: Result<String>) {
 println("结果: ${result.getOrNull()}")
 }
 }
 block.startCoroutine(continuation)
 Thread.sleep(3000L)
}
val block = suspend {
 println("start")
 delay(2000L)
 println("end")
 "DX3906"
}

调起非常简单,startCoroutine是(suspend () -> T)的扩展函数,且需要传递一个Continuation参数。我们先反编译看一下,长什么样子。

public final class StartCoroutineKt {
 //block那块被转换成了一个类StartCoroutineKt$block$1,这里创建好一个实例对象,待会儿可以直接使用
 private static final Function1<Continuation<? super String>, Object> block = new StartCoroutineKt$block$1((Continuation<? super StartCoroutineKt$block$1>) null);
 public static final void main() {
 //调用扩展函数,将block和continuation参数传入。 
 ContinuationKt.startCoroutine(block, new StartCoroutineKt$main$continuation$1());
 Thread.sleep(3000);
 }
 public static final Function1<Continuation<? super String>, Object> getBlock() {
 return block;
 }
}
//对应block那块
final class StartCoroutineKt$block$1 extends SuspendLambda implements Function1<Continuation<? super String>, Object> {
 int label;
 StartCoroutineKt$block$1(Continuation<? super StartCoroutineKt$block$1> continuation) {
 super(1, continuation);
 }
 //创建StartCoroutineKt$block$1实例
 public final Continuation<Unit> create(Continuation<?> continuation) {
 return new StartCoroutineKt$block$1(continuation);
 }
 public final Object invoke(Continuation<? super String> continuation) {
 //创建StartCoroutineKt$block$1实例并执行invokeSuspend
 return ((StartCoroutineKt$block$1) create(continuation)).invokeSuspend(Unit.INSTANCE);
 }
 public final Object invokeSuspend(Object $result) {
 Object coroutine_suspended = IntrinsicsKt.getCOROUTINE_SUSPENDED();
 //状态机
 switch (this.label) {
 case 0:
 //label一开始是0
 ResultKt.throwOnFailure($result);
 System.out.println("start");
 this.label = 1;
 //这里正常情况会返回COROUTINE_SUSPENDED,label已经改成1了,下次走case 1的逻辑
 if (DelayKt.delay(2000, this) != coroutine_suspended) {
 break;
 } else {
 return coroutine_suspended;
 }
 case 1:
 //label为1,没有return,继续走最后的结束语句
 ResultKt.throwOnFailure($result);
 break;
 default:
 throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");
 }
 //结束
 System.out.println("end");
 return "云天明";
 }
}
//对应Continuation那块
public final class StartCoroutineKt$main$continuation$1 implements Continuation<String> {
 StartCoroutineKt$main$continuation$1() {
 }
 public CoroutineContext getContext() {
 return EmptyCoroutineContext.INSTANCE;
 }
 public void resumeWith(Object result) {
 //输出结果
 StringBuilder sb = new StringBuilder();
 sb.append("结果: ");
 sb.append((String) (Result.m29isFailureimpl(result) ? null : result));
 System.out.println(sb.toString());
 }
}

还是比较清晰的,

  • 首先object : Continuation<String>是肯定会生成一个匿名内部类,在该类中,简单在resumeWith里面输出了一下结果
  • block那块代码,也会生成一个匿名内部类。需要注意的是,它继承自SuspendLambda,这个没见过,待会儿分析,里面有几个方法:create、invoke、invokeSuspend。其中create是创建该类的实例,invoke是调用create方法并执行invokeSuspend,invokeSuspend里面是状态机相关的逻辑。
  • main里面执行了ContinuationKt.startCoroutine(block, continuation),调起了扩展方法(扩展方法的原理就是这样的)

反编译出来的代码大致结构我们是了解了,现在需要分析一下startCoroutine具体是怎么走的了,看它是怎么利用这些反编译出来的代码的。

createCoroutineUnintercepted

public fun <T> (suspend () -> T).startCoroutine(
 completion: Continuation<T>
) {
 createCoroutineUnintercepted(completion).intercepted().resume(Unit)
}
//这个函数是expect的,没有函数体
public expect fun <T> (suspend () -> T).createCoroutineUnintercepted(
 completion: Continuation<T>
): Continuation<Unit>

startCoroutine首先是调用了createCoroutineUnintercepted函数,而createCoroutineUnintercepted是expect的,它是一种声明。因为Kotlin是跨平台的,所以部分逻辑与平台相关,这个createCoroutineUnintercepted就是这种。它没有函数体,我们只关心JVM平台,所以需要到JVM平台上找该函数的实现。在Kotlin源码地图文章中,我们提到协程源码,分为2个仓库,一个是Kotlin仓库,一个是Kotlin协程仓库。这个createCoroutineUnintercepted是在Kotlin仓库中,具体位置是:kotlin/libraries/stdlib/jvm/src/kotlin/coroutines/intrinsics/IntrinsicsJvm.kt

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)
 }
}

咦,createCoroutineUnintercepted居然也是(suspend () -> T)的扩展函数,所以if那里的this指的就是block,也就是StartCoroutineKt$block$1。它继承自SuspendLambda。

internal abstract class SuspendLambda(
 public override val arity: Int,
 completion: Continuation<Any?>?
) : ContinuationImpl(completion), FunctionBase<Any?>, SuspendFunction {
 constructor(arity: Int) : this(arity, null)
 public override fun toString(): String =
 if (completion == null)
 Reflection.renderLambdaToString(this) // this is lambda
 else
 super.toString() // this is continuation
}
internal abstract class ContinuationImpl(
 completion: Continuation<Any?>?,
 private val _context: CoroutineContext?
) : BaseContinuationImpl(completion) {
 ......
}
//BaseContinuationImpl实现了Continuation接口
internal abstract class BaseContinuationImpl(
 public val completion: Continuation<Any?>?
) : Continuation<Any?>, CoroutineStackFrame, Serializable {
 ...
}

SuspendLambda是ContinuationImpl的子类,而ContinuationImpl是BaseContinuationImpl的子类。所以上面的if (this is BaseContinuationImpl)判断是ok的,会走到create(probeCompletion)。也就是StartCoroutineKt$block$1的create方法,在里面会创建StartCoroutineKt$block$1实例。

public final Continuation<Unit> create(Continuation<?> continuation) {
 return new StartCoroutineKt$block$1(continuation);
}

走到这里相当于startCoroutine中的createCoroutineUnintercepted(completion)这一步就走完了,它最终返回的是StartCoroutineKt$block$1的实例,也就是一个Continuation。它标志着协程被创建好了。再来看下intercepted是什么逻辑

intercepted

public fun <T> (suspend () -> T).startCoroutine(
 completion: Continuation<T>
) {
 createCoroutineUnintercepted(completion).intercepted().resume(Unit)
}
//好家伙,intercepted也是expect的
public expect fun <T> Continuation<T>.intercepted(): Continuation<T>

发现这里的intercepted扩展函数也是expect的,又得去kotlin仓库里面找jvm相关的实现。我找了下,路径在这里:kotlin/libraries/stdlib/jvm/src/kotlin/coroutines/intrinsics/IntrinsicsJvm.kt

public actual fun <T> Continuation<T>.intercepted(): Continuation<T> =
 (this as? ContinuationImpl)?.intercepted() ?: this

intercepted是一个扩展函数,这里的this也就是前面createCoroutineUnintercepted(completion)创建出来的StartCoroutineKt$block$1实例,它本身是SuspendLambda的子类,而SuspendLambda就是ContinuationImpl的子类。所以这里的as?会转换成功,转换出来的不是null。也就是说走到了ContinuationImpl的intercepted()


internal abstract class ContinuationImpl(
 completion: Continuation<Any?>?,
 private val _context: CoroutineContext?
) : BaseContinuationImpl(completion) {
 constructor(completion: Continuation<Any?>?) : this(completion, completion?.context)
 //这个context其实就是传入的Continuation中的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 }
}
@Transient
private var intercepted: Continuation<Any?>? = null
public fun intercepted(): Continuation<Any?> =
 intercepted
 ?: (context[ContinuationInterceptor]?.interceptContinuation(this) ?: this)
 .also { intercepted = it }

第一次执行这里时intercepted是null,那么会从context中取ContinuationInterceptor,而context就是Continuation传入的context,我们传入的是EmptyCoroutineContext,取出来是null(ContinuationInterceptor会对Continuation进行拦截,然后将执行逻辑指派到对应的线程之上去,这块的逻辑后面再细说,就不详细展开了。),所以这里intercepted()最终执行结果就是返回this,this也就是StartCoroutineKt$block$1(block函数生成的类)。

intercepted()走完后再回到startCoroutine:

public fun <T> (suspend () -> T).startCoroutine(
 completion: Continuation<T>
) {
 createCoroutineUnintercepted(completion).intercepted().resume(Unit)
}

resume

就差最后一个resume(Unit)了,前面createCoroutineUnintercepted(completion).intercepted()创建出来的是StartCoroutineKt$block$1实例,所以我们需要到这个类里面去找resume函数。

再提一下类的继承关系:

StartCoroutineKt$block$1 extends SuspendLambda implements Function1
internal abstract class SuspendLambda(
 public override val arity: Int,
 completion: Continuation<Any?>?
) : ContinuationImpl(completion), FunctionBase<Any?>, SuspendFunction 
internal abstract class ContinuationImpl(
 completion: Continuation<Any?>?,
 private val _context: CoroutineContext?
) : BaseContinuationImpl(completion) 
internal abstract class BaseContinuationImpl(
 public val completion: Continuation<Any?>?
) : Continuation<Any?>, CoroutineStackFrame, Serializable
public interface Continuation<in T> {
 public val context: CoroutineContext
 public fun resumeWith(result: Result<T>)
}

StartCoroutineKt$block$1中没有该resume函数,其父类SuspendLambda也没有该函数,再到SuspendLambda的父类ContinuationImpl中,发现也没有。再到ContinuationImpl的父类BaseContinuationImpl中,也没有该函数,只有一个resumeWith,奇了怪了。后来,我发现这个resume函数是一个扩展函数:

public inline fun <T> Continuation<T>.resume(value: T): Unit =
 resumeWith(Result.success(value))

而resume这个扩展函数最终是调用的resumeWith,resumeWidth的实现在BaseContinuationImpl中。

public final override fun resumeWith(result: Result<Any?>) {
 var current = this
 var param = result
 while (true) {
 probeCoroutineResumed(current)
 with(current) {
 val completion = completion!! // fail fast when trying to resume continuation without completion
 val outcome: Result<Any?> =
 try {
 val outcome = invokeSuspend(param)
 if (outcome === COROUTINE_SUSPENDED) return
 Result.success(outcome)
 } catch (exception: Throwable) {
 Result.failure(exception)
 }
 releaseIntercepted() // this state machine instance is terminating
 if (completion is BaseContinuationImpl) {
 current = completion
 param = outcome
 } else {
 //label等于1时走这里
 completion.resumeWith(outcome)
 return
 }
 }
 }
}

这个开了个while(true)循环,不断地执行invokeSuspend(),如果遇到invokeSuspend返回结果是COROUTINE_SUSPENDED则退出while(true)循环。

public final Object invokeSuspend(Object $result) {
 Object coroutine_suspended = IntrinsicsKt.getCOROUTINE_SUSPENDED();
 //状态机
 switch (this.label) {
 case 0:
 //label一开始是0
 ResultKt.throwOnFailure($result);
 System.out.println("start");
 this.label = 1;
 //这里正常情况会返回COROUTINE_SUSPENDED,label已经改成1了,下次走case 1的逻辑
 if (DelayKt.delay(2000, this) != coroutine_suspended) {
 break;
 } else {
 return coroutine_suspended;
 }
 case 1:
 //label为1,没有return,继续走最后的结束语句
 ResultKt.throwOnFailure($result);
 break;
 default:
 throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");
 }
 //结束
 System.out.println("end");
 return "云天明";
}

invokeSuspend实际上就是我们的demo中的StartCoroutineKt$block$1里的invokeSuspend函数。在demo中,这个invokeSuspend第一次的时候状态机那里,label是0,所以会随即走到DelayKt.delay(2000, this),它是一个挂起函数,此时会拿到结果:COROUTINE_SUSPENDED。resumeWith遇到COROUTINE_SUSPENDED就不会继续往下走了,等到delay执行完成之后,会回调这个resumeWith函数,再继续走invokeSuspend,此时label已经是1了,走到状态机逻辑那里,返回结果“云天明”。

这个结果会被resumeWidth的outcome接收住,resumeWidth中的这个completion其实就是我们demo中的StartCoroutineKt$main$continuation$1(实现Continuation<String>的那个类,是通过构造函数传进来的),最终会走到completion.resumeWith(outcome),也就是来到了输出结果的地方:println("结果: ${result.getOrNull()}")。整个流程就走完了。

二、小结

createCoroutine用来创建协程,startCoroutine用来创建并启动协程。它们内部的原理是类似的,只是一个没有调用resume启动协程,另一个调用了resume启动协程。编译的时候,会生成一个SuspendLambda的实现类,该类invokeSuspend用于执行状态机的逻辑,调用resume后该状态机会被触发,状态机走完,协程也就走完了。

三、小练习

大家可以在学习完本篇文章之后,试着去分析一下launch的原理,看看launch与startCoroutine是什么关系。

作者:潇风寒月

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