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简要概括:
协程是 Kotlin 提供的一套线程 API 框架,可以很方便的做线程切换。 而且在不用关心线程调度的情况下,能轻松的做并发编程。也可以说协程就是一种并发设计模式。
下面是使用传统线程和协程执行任务:
Thread{ //执行耗时任务 }.start() val executors = Executors.newCachedThreadPool() executors.execute { //执行耗时任务 } GlobalScope.launch(Dispatchers.IO) { //执行耗时任务 }
在实际应用开发中,通常是在主线中去启动子线程执行耗时任务,等耗时任务执行完成,再将结果给主线程,然后刷新UI:
Thread{ //执行耗时任务 runOnMainThread { //获取耗时任务结果,刷新UI } }.start() val executors = Executors.newCachedThreadPool() executors.execute { //执行耗时任务 runOnMainThread { //获取耗时任务结果,刷新UI } } Observable.unsafeCreate<Unit> { //执行耗时任务 }.subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread()).subscribe { //获取耗时任务结果,刷新UI } GlobalScope.launch(Dispatchers.Main) { val result = withContext(Dispatchers.IO){ //执行耗时任务 } //直接拿到耗时任务结果,刷新UI refreshUI(result) }
从上面可以看到,使用Java 的 Thread
和 Executors
都需要手动去处理线程切换,这样的代码不仅不优雅,而且有一个重要问题,那就是要去处理与生命周期相关的上下文判断,这导致逻辑变复杂,而且容易出错。
RxJava 是一套优雅的异步处理框架,代码逻辑简化,可读性和可维护性都很高,很好的帮我们处理线程切换操作。这在 Java 语言环境开发下,是如虎添翼,但是在 Kotlin 语言环境中开发,如今的协程就比 RxJava 更方便,或者说更有优势。
下面看一个 Kotlin 中使用协程的例子:
GlobalScope.launch(Dispatchers.Main) { Log.d("TestCoroutine", "launch start: ${Thread.currentThread()}") val numbersTo50Sum = withContext(Dispatchers.IO) { //在子线程中执行 1-50 的自然数和 Log.d("TestCoroutine", "launch:numbersTo50Sum: ${Thread.currentThread()}") delay(1000) val naturalNumbers = generateSequence(0) { it + 1 } val numbersTo50 = naturalNumbers.takeWhile { it <= 50 } numbersTo50.sum() } val numbers50To100Sum = withContext(Dispatchers.IO) { //在子线程中执行 51-100 的自然数和 Log.d("TestCoroutine", "launch:numbers50To100Sum: ${Thread.currentThread()}") delay(1000) val naturalNumbers = generateSequence(51) { it + 1 } val numbers50To100 = naturalNumbers.takeWhile { it in 51..100 } numbers50To100.sum() } val result = numbersTo50Sum + numbers50To100Sum Log.d("TestCoroutine", "launch end:result=$result ${Thread.currentThread()}") } Log.d("TestCoroutine", "Hello World!,${Thread.currentThread()}")
控制台输出结果:
2023-01-02 16:05:45.846 10153-10153/com.wangjiang.example D/TestCoroutine: Hello World!,Thread[main,5,main]
2023-01-02 16:05:48.058 10153-10153/com.wangjiang.example D/TestCoroutine: launch start: Thread[main,5,main]
2023-01-02 16:05:48.059 10153-10322/com.wangjiang.example D/TestCoroutine: launch:numbersTo50Sum: Thread[DefaultDispatcher-worker-1,5,main]
2023-01-02 16:05:49.114 10153-10322/com.wangjiang.example D/TestCoroutine: launch:numbers50To100Sum: Thread[DefaultDispatcher-worker-1,5,main]
2023-01-02 16:05:50.376 10153-10153/com.wangjiang.example D/TestCoroutine: launch end:result=5050 Thread[main,5,main]
在上面的代码中:
launch
是一个函数,用于创建协程并将其函数主体的执行分派给相应的调度程序。
Dispatchers.MAIN
指示此协程应在为 UI 操作预留的主线程上执行。
Dispatchers.IO
指示此协程应在为 I/O 操作预留的线程上执行。
withContext(Dispatchers.IO)
将协程的执行操作移至一个 I/O 线程。
从控制台输出结果中,可以看出在计算 1-50 和 51-100 的自然数和的时候,线程是从主线程(Thread[main,5,main]
)切换到了协程的线程(DefaultDispatcher-worker-1,5,main
),这里计算 1-50 和 51-100 都是同一个子线程。
在这里有一个重要的现象,代码从逻辑上看起来是同步的,并且启动协程执行任务的时候,没有阻塞主线程继续执行相关操作,而且在协程中的异步任务执行完成之后,又自动切回了主线程。这就是 Kotlin 协程给开发做并发编程带来的好处。这也是有个概念的来源: Kotlin 协程同步非阻塞。
同步非阻塞”是真的“同步非阻塞” 吗?下面探究一下其中的猫腻,通过 Android Studio ,查看 .class 文件中的上面一段代码:
BuildersKt.launch$default((CoroutineScope)GlobalScope.INSTANCE, (CoroutineContext)Dispatchers.getMain(), (CoroutineStart)null, (Function2)(new Function2((Continuation)null) { int I$0; int label; @Nullable public final Object invokeSuspend(@NotNull Object $result) { Object var10000; int numbersTo50Sum; label17: { Object var5 = IntrinsicsKt.getCOROUTINE_SUSPENDED(); Function2 var10001; CoroutineContext var6; switch(this.label) { case 0: ResultKt.throwOnFailure($result); Log.d("TestCoroutine", "launch start: " + Thread.currentThread()); var6 = (CoroutineContext)Dispatchers.getIO(); var10001 = (Function2)(new Function2((Continuation)null) { int label; @Nullable public final Object invokeSuspend(@NotNull Object $result) { Object var4 = IntrinsicsKt.getCOROUTINE_SUSPENDED(); switch(this.label) { case 0: ResultKt.throwOnFailure($result); Log.d("TestCoroutine", "launch:numbersTo50Sum: " + Thread.currentThread()); this.label = 1; if (DelayKt.delay(1000L, this) == var4) { return var4; } break; case 1: ResultKt.throwOnFailure($result); break; default: throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine"); } Sequence naturalNumbers = SequencesKt.generateSequence(Boxing.boxInt(0), (Function1)null.INSTANCE); Sequence numbersTo50 = SequencesKt.takeWhile(naturalNumbers, (Function1)null.INSTANCE); return Boxing.boxInt(SequencesKt.sumOfInt(numbersTo50)); } @NotNull public final Continuation create(@Nullable Object value, @NotNull Continuation completion) { Intrinsics.checkNotNullParameter(completion, "completion"); Function2 var3 = new <anonymous constructor>(completion); return var3; } public final Object invoke(Object var1, Object var2) { return ((<undefinedtype>)this.create(var1, (Continuation)var2)).invokeSuspend(Unit.INSTANCE); } }); this.label = 1; var10000 = BuildersKt.withContext(var6, var10001, this); if (var10000 == var5) { return var5; } break; case 1: ResultKt.throwOnFailure($result); var10000 = $result; break; case 2: numbersTo50Sum = this.I$0; ResultKt.throwOnFailure($result); var10000 = $result; break label17; default: throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine"); } numbersTo50Sum = ((Number)var10000).intValue(); var6 = (CoroutineContext)Dispatchers.getIO(); var10001 = (Function2)(new Function2((Continuation)null) { int label; @Nullable public final Object invokeSuspend(@NotNull Object $result) { Object var4 = IntrinsicsKt.getCOROUTINE_SUSPENDED(); switch(this.label) { case 0: ResultKt.throwOnFailure($result); Log.d("TestCoroutine", "launch:numbers50To100Sum: " + Thread.currentThread()); this.label = 1; if (DelayKt.delay(1000L, this) == var4) { return var4; } break; case 1: ResultKt.throwOnFailure($result); break; default: throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine"); } Sequence naturalNumbers = SequencesKt.generateSequence(Boxing.boxInt(51), (Function1)null.INSTANCE); Sequence numbers50To100 = SequencesKt.takeWhile(naturalNumbers, (Function1)null.INSTANCE); return Boxing.boxInt(SequencesKt.sumOfInt(numbers50To100)); } @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); } }); this.I$0 = numbersTo50Sum; this.label = 2; var10000 = BuildersKt.withContext(var6, var10001, this); if (var10000 == var5) { return var5; } } int numbers50To100Sum = ((Number)var10000).intValue(); int result = numbersTo50Sum + numbers50To100Sum; Log.d("TestCoroutine", "launch end:result=" + result + ' ' + Thread.currentThread()); 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); } }), 2, (Object)null); Log.d("TestCoroutine", "Hello World!," + Thread.currentThread());
虽然上面 .class 文件中的代码比较复杂,但是从大体逻辑可以看出,Kotlin 协程也是通过回调接口来实现异步操作的,这也解释了 Kotlin 协程只是让代码逻辑是同步非阻塞,但是实际上并没有,只是 Kotlin 编译器为代码做了很多事情,这也是说 Kotlin 协程其实就是一套线程 API 框架的原因。
再看一个上面例子的变种:
GlobalScope.launch(Dispatchers.Main) { Log.d("TestCoroutine", "launch start: ${Thread.currentThread()}") val numbersTo50Sum = async { withContext(Dispatchers.IO) { Log.d("TestCoroutine", "launch:numbersTo50Sum: ${Thread.currentThread()}") delay(2000) val naturalNumbers = generateSequence(0) { it + 1 } val numbersTo50 = naturalNumbers.takeWhile { it <= 50 } numbersTo50.sum() } } val numbers50To100Sum = async { withContext(Dispatchers.IO) { Log.d("TestCoroutine", "launch:numbers50To100Sum: ${Thread.currentThread()}") delay(500) val naturalNumbers = generateSequence(51) { it + 1 } val numbers50To100 = naturalNumbers.takeWhile { it in 51..100 } numbers50To100.sum() } } // 计算 1-50 和 51-100 的自然数和是两个并发操作 val result = numbersTo50Sum.await() + numbers50To100Sum.await() Log.d("TestCoroutine", "launch end:result=$result ${Thread.currentThread()}") } Log.d("TestCoroutine", "Hello World!,${Thread.currentThread()}")
控制台输出结果:
2023-01-02 16:32:12.637 13303-13303/com.wangjiang.example D/TestCoroutine: Hello World!,Thread[main,5,main]
2023-01-02 16:32:13.120 13303-13303/com.wangjiang.example D/TestCoroutine: launch start: Thread[main,5,main]
2023-01-02 16:32:14.852 13303-13444/com.wangjiang.example D/TestCoroutine: launch:numbersTo50Sum: Thread[DefaultDispatcher-worker-2,5,main]
2023-01-02 16:32:14.853 13303-13443/com.wangjiang.example D/TestCoroutine: launch:numbers50To100Sum: Thread[DefaultDispatcher-worker-1,5,main]
2023-01-02 16:32:17.462 13303-13303/com.wangjiang.example D/TestCoroutine: launch end:result=5050 Thread[main,5,main]
async
创建了一个协程,它让计算 1-50 和 51-100 的自然数和是两个并发操作。上面控制台输出结果可以看到计算 1-50 的自然数和是在线程 Thread[DefaultDispatcher-worker-2,5,main]
中,而计算 51-100 的自然数和是在另一个线程Thread[DefaultDispatcher-worker-1,5,main]
中。
从上面的例子,协程在异步操作,也就是线程切换上:主线程启动子线程执行耗时操作,耗时操作执行完成将结果更新到主线程的过程中,代码逻辑简化,可读性高。
suspend 直译就是:挂起
suspend 是 Kotlin 语言中一个 关键字,用于修饰方法,当修饰方法时,表示这个方法只能被 suspend 修饰的方法调用或者在协程中被调用。
下面看一下将上面代码案例拆分成几个 suspend 方法:
fun getNumbersTo100Sum() { GlobalScope.launch(Dispatchers.Main) { Log.d("TestCoroutine", "launch start: ${Thread.currentThread()}") val result = calcNumbers1To100Sum() Log.d("TestCoroutine", "launch end:result=$result ${Thread.currentThread()}") } Log.d("TestCoroutine", "Hello World!,${Thread.currentThread()}") } private suspend fun calcNumbers1To100Sum(): Int { return calcNumbersTo50Sum() + calcNumbers50To100Sum() } private suspend fun calcNumbersTo50Sum(): Int { return withContext(Dispatchers.IO) { Log.d("TestCoroutine", "launch:numbersTo50Sum: ${Thread.currentThread()}") delay(1000) val naturalNumbers = generateSequence(0) { it + 1 } val numbersTo50 = naturalNumbers.takeWhile { it <= 50 } numbersTo50.sum() } } private suspend fun calcNumbers50To100Sum(): Int { return withContext(Dispatchers.IO) { Log.d("TestCoroutine", "launch:numbers50To100Sum: ${Thread.currentThread()}") delay(1000) val naturalNumbers = generateSequence(51) { it + 1 } val numbers50To100 = naturalNumbers.takeWhile { it in 51..100 } numbers50To100.sum() } }
控制台输出结果:
2023-01-03 14:47:57.047 11349-11349/com.wangjiang.example D/TestCoroutine: Hello World!,Thread[main,5,main]
2023-01-03 14:47:59.311 11349-11349/com.wangjiang.example D/TestCoroutine: launch start: Thread[main,5,main]
2023-01-03 14:47:59.312 11349-11537/com.wangjiang.example D/TestCoroutine: launch:numbersTo50Sum: Thread[DefaultDispatcher-worker-3,5,main]
2023-01-03 14:48:00.336 11349-11535/com.wangjiang.example D/TestCoroutine: launch:numbers50To100Sum: Thread[DefaultDispatcher-worker-1,5,main]
2023-01-03 14:48:01.339 11349-11349/com.wangjiang.example D/TestCoroutine: launch end:result=5050 Thread[main,5,main]
suspend 关键字标记方法时,其实是告诉 Kotlin 从协程内调用方法。所以这个“挂起”,并不是说方法或函数被挂起,也不是说线程被挂起。
假设一个非 suspend 修饰的方法调用 suspend 修饰的方法会怎么样呢?
private fun calcNumbersTo100Sum(): Int { return calcNumbersTo50Sum() + calcNumbers50To100Sum() }
此时,编译器会提示:
Suspend function 'calcNumbersTo50Sum' should be called only from a coroutine or another suspend function
Suspend function 'calcNumbers50To100' should be called only from a coroutine or another suspend function
下面查看 .class 文件中的上面方法 calcNumbers50To100Sum 代码:
private final Object calcNumbers50To100Sum(Continuation $completion) { return BuildersKt.withContext((CoroutineContext)Dispatchers.getIO(), (Function2)(new Function2((Continuation)null) { int label; @Nullable public final Object invokeSuspend(@NotNull Object $result) { Object var4 = IntrinsicsKt.getCOROUTINE_SUSPENDED(); switch(this.label) { case 0: ResultKt.throwOnFailure($result); Log.d("TestCoroutine", "launch:numbers50To100Sum: " + Thread.currentThread()); this.label = 1; if (DelayKt.delay(1000L, this) == var4) { return var4; } break; case 1: ResultKt.throwOnFailure($result); break; default: throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine"); } Sequence naturalNumbers = SequencesKt.generateSequence(Boxing.boxInt(51), (Function1)null.INSTANCE); Sequence numbers50To100 = SequencesKt.takeWhile(naturalNumbers, (Function1)null.INSTANCE); return Boxing.boxInt(SequencesKt.sumOfInt(numbers50To100)); } @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); } }), $completion); }
可以看到 private suspend fun calcNumbers50To100Sum()
经过 Kotlin 编译器编译后变成了private final Object calcNumbers50To100Sum(Continuation $completion)
, suspend
消失了,方法多了一个参数 Continuation $completion
,所以 suspend
修饰 Kotlin 的方法或函数,编译器会对此方法做特殊处理。
另外,suspend
修饰的方法,也预示着这个方法是 耗时方法,告诉方法调用者要使用协程。当执行 suspend
方法,也预示着要切换线程,此时主线程依然可以继续执行,而协程里面的代码可能被挂起了。
下面再稍为修改 calcNumbers50To100Sum
方法:
private suspend fun calcNumbers50To100Sum(): Int { Log.d("TestCoroutine", "launch:numbers50To100Sum:start: ${Thread.currentThread()}") val sum= withContext(Dispatchers.Main) { Log.d("TestCoroutine", "launch:numbers50To100Sum: ${Thread.currentThread()}") delay(1000) val naturalNumbers = generateSequence(51) { it + 1 } val numbers50To100 = naturalNumbers.takeWhile { it in 51..100 } numbers50To100.sum() } Log.d("TestCoroutine", "launch:numbers50To100Sum:end: ${Thread.currentThread()}") return sum }
控制台输出结果:
2023-01-03 15:28:04.349 15131-15131/com.bilibili.studio D/TestCoroutine: Hello World!,Thread[main,5,main]
2023-01-03 15:28:04.803 15131-15131/com.bilibili.studio D/TestCoroutine: launch start: Thread[main,5,main]
2023-01-03 15:28:04.804 15131-15266/com.bilibili.studio D/TestCoroutine: launch:numbersTo50Sum: Thread[DefaultDispatcher-worker-3,5,main]
2023-01-03 15:28:06.695 15131-15131/com.bilibili.studio D/TestCoroutine: launch:numbers50To100Sum:start: Thread[main,5,main]
2023-01-03 15:28:06.696 15131-15131/com.bilibili.studio D/TestCoroutine: launch:numbers50To100Sum: Thread[main,5,main]
2023-01-03 15:28:07.700 15131-15131/com.bilibili.studio D/TestCoroutine: launch:numbers50To100Sum:end: Thread[main,5,main]
2023-01-03 15:28:07.700 15131-15131/com.bilibili.studio D/TestCoroutine: launch end:result=5050 Thread[main,5,main]
主线程不受协程线程的影响。
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