Kotlin launch原理全面分析
作者：且听真言
在Android开发中，launch是我们经常用的，今天来看看它是什么原理，文中通过示例代码介绍的非常详细，对大家的学习或者工作具有一定的参考学习价值，需要的朋友们下面随着小编来一起学习吧
一、协程是如何创建的
launch、async 可以创建、启动新的协程,那么协程到底是如何创建的？
runBlocking {
println(Thread.currentThread().name)
launch {
println(Thread.currentThread().name)
delay(100L)
}
Thread.sleep(1000L)
}
Log
main @coroutine#1
main @coroutine#2
Process finished with exit code 0runBlocking{} 启动了第一个协程，launch{} 启动了第二个协程。
协程启动的基础API
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{}、startCoroutine{}就是 Kotlin 协程当中最基础的两个创建协程的 API。启动协程有三种常见的方式：launch、runBlocking、async。它们其实属于协程中间层提供的 API，而它们的底层都调用了“基础层”的协程 API。
createCoroutine{}、startCoroutine{}是扩展函数，其扩展接收者类型是一个函数类型：suspend () -> T，代表了“无参数，返回值为 T 的挂起函数或者 Lambda”。而对于函数本身，它们两个都接收一个 Continuation<T> 类型的参数，其中一个函数，还会返回一个 Continuation<Unit> 类型的返回值。
val block = suspend {
println("Hello")
delay(1000L)
println("World!")
"Result"
}
fun testLaunch2() {
val continuation = object : Continuation<String> {
override val context: CoroutineContext
get() = EmptyCoroutineContext
override fun resumeWith(result: Result<String>) {
println("Result:" + result.getOrNull())
}
}
block.startCoroutine(continuation)
}
fun main() {
testLaunch2()
Thread.sleep(2000L)
}
Log
Hello
World!
Result:Result
Process finished with exit code 0类型为suspend () -> T的函数或者Lambda 表达式可以用 block.startCoroutine() 来启动协程了。
Continuation 有两种用法，一种是在实现挂起函数的时候，用于传递挂起函数的执行结果；另一种是在调用挂起函数的时候，以匿名内部类的方式，用于接收挂起函数的执行结果。
使用 createCoroutine() 这个方法其实上面代码的逻辑：
fun testLaunch3() {
val continuation = object : Continuation<String> {
override val context: CoroutineContext
get() = EmptyCoroutineContext
override fun resumeWith(result: Result<String>) {
println("Result:" + result.getOrNull())
}
}
val coroutinue = block.createCoroutine(continuation)
coroutinue.resume(Unit)
}
val block = suspend {
println("Hello")
delay(1000L)
println("World!")
"Result"
}
fun main() {
testLaunch3()
Thread.sleep(2000L)
}
Log
Hello
World!
Result:Result
Process finished with exit code 0createCoroutine() 创建一个协程，先不启动。调用 resume() 才能启动。createCoroutine()、startCoroutine() 的源代码差别也并不大，只是前者没有调用 resume()，而后者调用了 resume()。startCoroutine() 之所以可以创建并同时启动协程的原因就在于，它在源码中直接调用了 resume(Unit)。
将 startCoroutine()转换为Java：
package com.example.myapplication.testcoroutinue;
import kotlin.Metadata;
import kotlin.Result;
import kotlin.ResultKt;
import kotlin.Unit;
import kotlin.coroutines.Continuation;
import kotlin.coroutines.ContinuationKt;
import kotlin.coroutines.CoroutineContext;
import kotlin.coroutines.EmptyCoroutineContext;
import kotlin.coroutines.intrinsics.IntrinsicsKt;
import kotlin.jvm.functions.Function1;
import kotlin.jvm.internal.Intrinsics;
import kotlinx.coroutines.DelayKt;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
@Metadata(
mv = {1, 6, 0},
k = 2,
d1 = {"\u0000\u001e\n\u0000\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\u0010\u000e\n\u0002\u0010\u0000\n\u0002\b\u0004\n\u0002\u0010\u0002\n\u0002\b\u0002\u001a\u0006\u0010\b\u001a\u00020\t\u001a\u0006\u0010\n\u001a\u00020\t\",\u0010\u0000\u001a\u0018\b\u0001\u0012\n\u0012\b\u0012\u0004\u0012\u00020\u00030\u0002\u0012\u0006\u0012\u0004\u0018\u00010\u00040\u0001ø\u0001\u0000¢\u0006\n\n\u0002\u0010\u0007\u001a\u0004\b\u0005\u0010\u0006\u0082\u0002\u0004\n\u0002\b\u0019¨\u0006\u000b"},
d2 = {"block", "Lkotlin/Function1;", "Lkotlin/coroutines/Continuation;", "", "", "getBlock", "()Lkotlin/jvm/functions/Function1;", "Lkotlin/jvm/functions/Function1;", "main", "", "testLaunch2", "My_Application.app.main"}
)
public final class TestCoroutinue888Kt {
// Kotlin 为 block 变量生成的静态变量
@NotNull
private static final Function1 block;
public static final void main() {
testLaunch2();
Thread.sleep(2000L);
}
// $FF: synthetic method
public static void main(String[] var0) {
main();
}
// Kotlin 为 block 变量生成的静态变量以及方法
@NotNull
public static final Function1 getBlock() {
return block;
}
public static final void testLaunch2() {
//continuation 变量对应的匿名内部类
<undefinedtype> continuation = new Continuation() {
@NotNull
public CoroutineContext getContext() {
return (CoroutineContext)EmptyCoroutineContext.INSTANCE;
}
public void resumeWith(@NotNull Object result) {
String var2 = "Result:" + (String)(Result.isFailure-impl(result) ? null : result);
System.out.println(var2);
}
};
//block.startCoroutine(continuation) 转换成了ContinuationKt.startCoroutine(block, (Continuation)continuation)
ContinuationKt.startCoroutine(block, (Continuation)continuation);
}
static {
//实现了 Continuation 接口
Function1 var0 = (Function1)(new Function1((Continuation)null) {
int label;
//invokeSuspend()为协程状态机逻辑
@Nullable
public final Object invokeSuspend(@NotNull Object $result) {
Object var3 = IntrinsicsKt.getCOROUTINE_SUSPENDED();
String var2;
switch(this.label) {
case 0:
ResultKt.throwOnFailure($result);
var2 = "Hello";
System.out.println(var2);
this.label = 1;
if (DelayKt.delay(1000L, this) == var3) {
return var3;
}
break;
case 1:
ResultKt.throwOnFailure($result);
break;
default:
throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");
}
var2 = "World!";
System.out.println(var2);
return "Result";
}
@NotNull
public final Continuation create(@NotNull Continuation completion) {
Intrinsics.checkNotNullParameter(completion, "completion");
Function1 var2 = new <anonymous constructor>(completion);
return var2;
}
public final Object invoke(Object var1) {
return ((<undefinedtype>)this.create((Continuation)var1)).invokeSuspend(Unit.INSTANCE);
}
});
block = var0;
}
}public fun <T> (suspend () -> T).startCoroutine(
completion: Continuation<T>
) {
createCoroutineUnintercepted(completion).intercepted().resume(Unit)
}在 startCoroutine() 当中，首先会调用 createCoroutineUnintercepted() 方法。
public expect fun <T> (suspend () -> T).createCoroutineUnintercepted(
completion: Continuation<T>
): Continuation<Unit>代码中的 expect，一种声明，由于 Kotlin 是面向多个平台的，具体的实现，就需要在特定的平台实现。
public actual fun <T> (suspend () -> T).createCoroutineUnintercepted(
completion: Continuation<T>
): Continuation<Unit> {
val probeCompletion = probeCoroutineCreated(completion)
return if (this is BaseContinuationImpl)
create(probeCompletion)
else
createCoroutineFromSuspendFunction(probeCompletion) {
(this as Function1<Continuation<T>, Any?>).invoke(it)
}
}actual，代表了 createCoroutineUnintercepted() 在 JVM 平台的实现。
createCoroutineUnintercepted() 是一个扩展函数，this代表了 block 变量。(this is BaseContinuationImpl) 条件为ture，就会调用 create(probeCompletion)。
public open fun create(completion: Continuation<*>): Continuation<Unit> {
throw UnsupportedOperationException("create(Continuation) has not been overridden")
}在默认情况下，这个 create() 方法是会抛出异常的。
@NotNull
public final Continuation create(@NotNull Continuation completion) {
Intrinsics.checkNotNullParameter(completion, "completion");
Function1 var2 = new <anonymous constructor>(completion);
return var2;
}返回了Continuation 对象。
public fun <T> (suspend () -> T).startCoroutine(
completion: Continuation<T>
) {
createCoroutineUnintercepted(completion).intercepted().resume(Unit)
}intercepted() 在JVM 实现如下：
public actual fun <T> Continuation<T>.intercepted(): Continuation<T> =
(this as? ContinuationImpl)?.intercepted() ?: this将 Continuation 强转成了 ContinuationImpl，调用了它的 intercepted()。
ContinuationImpl 的源代码:
internal abstract class ContinuationImpl(
completion: Continuation<Any?>?,
private val _context: CoroutineContext?
) : BaseContinuationImpl(completion) {
@Transient
private var intercepted: Continuation<Any?>? = null
public fun intercepted(): Continuation<Any?> =
intercepted
?: (context[ContinuationInterceptor]?.interceptContinuation(this) ?: this)
.also { intercepted = it }
}通过 ContinuationInterceptor，对 Continuation 进行拦截，从而将程序的执行逻辑派发到特定的线程之上。
resume(Unit)：
public fun <T> (suspend () -> T).startCoroutine(
completion: Continuation<T>
) {
createCoroutineUnintercepted(completion).intercepted().resume(Unit)
}resume(Unit)，作用其实就相当于启动了协程。
二、launch 是如何启动协程的
fun main() {
testLaunch11()
Thread.sleep(2000L)
}
fun testLaunch11() {
val coroutineScope = CoroutineScope(Job())
coroutineScope.launch {
println("Hello")
delay(1000L)
println("World!")
}
}
Log
Hello
World!
Process finished with exit code 0转Java
package com.example.myapplication.testcoroutinue;
import kotlin.Metadata;
import kotlin.ResultKt;
import kotlin.Unit;
import kotlin.coroutines.Continuation;
import kotlin.coroutines.CoroutineContext;
import kotlin.coroutines.intrinsics.IntrinsicsKt;
import kotlin.jvm.functions.Function2;
import kotlin.jvm.internal.Intrinsics;
import kotlinx.coroutines.BuildersKt;
import kotlinx.coroutines.CoroutineScope;
import kotlinx.coroutines.CoroutineScopeKt;
import kotlinx.coroutines.CoroutineStart;
import kotlinx.coroutines.DelayKt;
import kotlinx.coroutines.Job;
import kotlinx.coroutines.JobKt;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
@Metadata(
mv = {1, 6, 0},
k = 2,
d1 = {"\u0000\n\n\u0000\n\u0002\u0010\u0002\n\u0002\b\u0002\u001a\u0006\u0010\u0000\u001a\u00020\u0001\u001a\u0006\u0010\u0002\u001a\u00020\u0001¨\u0006\u0003"},
d2 = {"main", "", "testLaunch11", "My_Application.app.main"}
)
public final class TestCoroutinue999Kt {
public static final void main() {
testLaunch11();
Thread.sleep(2000L);
}
// $FF: synthetic method
public static void main(String[] var0) {
main();
}
public static final void testLaunch11() {
CoroutineScope coroutineScope = CoroutineScopeKt.CoroutineScope((CoroutineContext)JobKt.Job$default((Job)null, 1, (Object)null));
//对应 launch 当中的 Lambda。
BuildersKt.launch$default(coroutineScope, (CoroutineContext)null, (CoroutineStart)null, (Function2)(new Function2((Continuation)null) {
int label;
@Nullable
public final Object invokeSuspend(@NotNull Object $result) {
Object var3 = IntrinsicsKt.getCOROUTINE_SUSPENDED();
String var2;
switch(this.label) {
case 0:
ResultKt.throwOnFailure($result);
var2 = "Hello";
System.out.println(var2);
this.label = 1;
if (DelayKt.delay(1000L, this) == var3) {
return var3;
}
break;
case 1:
ResultKt.throwOnFailure($result);
break;
default:
throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");
}
var2 = "World!";
System.out.println(var2);
return Unit.INSTANCE;
}
@NotNull
public final Continuation create(@Nullable Object value, @NotNull Continuation completion) {
Intrinsics.checkNotNullParameter(completion, "completion");
Function2 var3 = new <anonymous constructor>(completion);
return var3;
}
public final Object invoke(Object var1, Object var2) {
return ((<undefinedtype>)this.create(var1, (Continuation)var2)).invokeSuspend(Unit.INSTANCE);
}
}), 3, (Object)null);
}
}launch源码
public fun CoroutineScope.launch(
context: CoroutineContext = EmptyCoroutineContext,
start: CoroutineStart = CoroutineStart.DEFAULT,
block: suspend CoroutineScope.() -> Unit
): Job {
//launch 会根据传入的 CoroutineContext 创建出新的 Context。
val newContext = newCoroutineContext(context)
//launch 会根据传入的启动模式来创建对应的协程对象。这里有两种，一种是标准的，一种是懒加载的。
val coroutine = if (start.isLazy)
LazyStandaloneCoroutine(newContext, block) else
StandaloneCoroutine(newContext, active = true)
//启动协程。
coroutine.start(start, coroutine, block)
return coroutine
}coroutine.start() :
public abstract class AbstractCoroutine<in T>(
parentContext: CoroutineContext,
initParentJob: Boolean,
active: Boolean
) : JobSupport(active), Job, Continuation<T>, CoroutineScope {
public fun <R> start(start: CoroutineStart, receiver: R, block: suspend R.() -> T) {
start(block, receiver, this)
}
}AbstractCoroutine.kt 对应协程的抽象逻辑。AbstractCoroutine 的start() 方法，用于启动协程。
public enum class CoroutineStart {
public operator fun <T> invoke(block: suspend () -> T, completion: Continuation<T>): Unit =
when (this) {
DEFAULT -> block.startCoroutineCancellable(completion)
ATOMIC -> block.startCoroutine(completion)
UNDISPATCHED -> block.startCoroutineUndispatched(completion)
LAZY -> Unit // will start lazily
}
}start(block, receiver, this)，进入 CoroutineStart.invoke()。
invoke() 方法当中，根据 launch 传入的启动模式，以不同的方式启动协程。当启动模式是 ATOMIC 的时候，就会调用 block.startCoroutine(completion)。startCoroutineUndispatched(completion) 和 startCoroutineCancellable(completion)，只是在 startCoroutine() 的基础上增加了一些额外的功能而已。前者代表启动协程以后就不会被分发，后者代表启动以后可以响应取消。
startCoroutineCancellable(completion)
public fun <T> (suspend () -> T).startCoroutineCancellable(completion: Continuation<T>): Unit = runSafely(completion) {
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)
create(probeCompletion)
else
createCoroutineFromSuspendFunction(probeCompletion) {
(this as Function1<Continuation<T>, Any?>).invoke(it)
}
}startCoroutineCancellable() 的源代码，会调用 createCoroutineUnintercepted()，然后调用 create(probeCompletion)，然后最终会调用create() 方法。launch 这个 API，只是对协程的基础元素 startCoroutine() 等方法进行了一些封装而已。
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