【Kotlin】协程的字节码原理

前言

协程是Koltin语言最重要的特性之一，也是最难理解的特性。网上关于kotlin协程的描述也是五花八门，有人说它是轻量级线程，有人说它是无阻塞式挂起，有人说它是一个异步框架等等，众说纷芸。甚至还有人出了书籍专门介绍kotlin协程。

笔者刚开始接触这个概念也是一头雾水：什么叫轻量级线程？难道它是一个操作系统级别的任务调度器吗？闻所未闻呀。

后来才知道协程它其实不是线程，它只是一个语言层面的东西，确切地讲它是一个轻量级的线程框架，主要功能是可以实现简洁的线程切换，避免了直接使用Thread导致的回调地狱。也可以说它用同步的方式实现异步操作。

看别人怎么介绍，还不如亲眼看一看协程的字节码，其实没那么难。

笔者原创，转载请注明来源：https://blog.csdn.net/devnn/article/details/135610313

协程字节码

在MainActivity的onCreate中写一段协程的代码：

lifecycleScope.launchWhenResumed {
      Log.i("MainActivity", "launchWhenResumed,isMainThread:${Thread.currentThread().id == Looper.getMainLooper().thread.id}")//这里打印true
      Log.i("MainActivity", "launchWhenResumed,threadId:${Thread.currentThread().id}")//这里threadId打印2也就是主线程
      //以下两个代码块是串行执行的
      withContext(Dispatchers.Main) {
          Thread.sleep(10000)
          Log.i("MainActivity", "launchWhenResumed,withContext1,threadId:${Thread.currentThread().id}")
      }
      withContext(Dispatchers.IO) {
          Log.i("MainActivity", "launchWhenResumed,withContext2,threadId:${Thread.currentThread().id}")
      }
 }

笔者用lifecycleScope创建了一个协程，用其它方式创建也是一样的，比如MainScope()、GlobalScope、viewModelScope等等，这些方式的区别不是本文介绍的重点。

以上代码的字节码内容如下：

 L33
    LINENUMBER 126 L33
    ALOAD 0
    CHECKCAST androidx/lifecycle/LifecycleOwner
    INVOKESTATIC androidx/lifecycle/LifecycleOwnerKt.getLifecycleScope (Landroidx/lifecycle/LifecycleOwner;)Landroidx/lifecycle/LifecycleCoroutineScope;
    NEW com/devnn/demo/MainActivity$onCreate$6
    DUP
    ACONST_NULL
    INVOKESPECIAL com/devnn/demo/MainActivity$onCreate$6.<init> (Lkotlin/coroutines/Continuation;)V
    CHECKCAST kotlin/jvm/functions/Function2
    INVOKEVIRTUAL androidx/lifecycle/LifecycleCoroutineScope.launchWhenResumed (Lkotlin/jvm/functions/Function2;)Lkotlinx/coroutines/Job;
    POP

协程代码块被封装成了一个匿名内部类，匿名内部类继承了SuspendLambda(它实现了Continuation接口)，同时实现了Function2接口，匿名内部类的构造方法需要一个Continuation参数，以上传null。

这个匿名内部类com/devnn/demo/MainActivity\$onCreate\$6.class内容如下：

final class com.devnn.demo.MainActivity$onCreate$6 extends kotlin.coroutines.jvm.internal.SuspendLambda implements kotlin.jvm.functions.Function2<kotlinx.coroutines.CoroutineScope, kotlin.coroutines.Continuation<? super kotlin.Unit>, java.lang.Object> {
  int label;

  com.devnn.demo.MainActivity$onCreate$6(kotlin.coroutines.Continuation<? super com.devnn.demo.MainActivity$onCreate$6>);
    Code:
       0: aload_0
       1: iconst_2
       2: aload_1
       3: invokespecial #13                 // Method kotlin/coroutines/jvm/internal/SuspendLambda."<init>":(ILkotlin/coroutines/Continuation;)V
       6: return

  public final java.lang.Object invokeSuspend(java.lang.Object);
    Code:
       0: invokestatic  #39                 // Method kotlin/coroutines/intrinsics/IntrinsicsKt.getCOROUTINE_SUSPENDED:()Ljava/lang/Object;
       3: astore_2
       4: aload_0
       5: getfield      #43                 // Field label:I
       8: tableswitch   { // 0 to 2
                     0: 36
                     1: 134
                     2: 176
               default: 186
          }
      36: aload_1
      37: invokestatic  #49                 // Method kotlin/ResultKt.throwOnFailure:(Ljava/lang/Object;)V
      40: ldc           #51                 // String MainActivity
      42: ldc           #53                 // String launchWhenResumed,isMainThread:
      44: invokestatic  #59                 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
      47: invokevirtual #63                 // Method java/lang/Thread.getId:()J
      50: invokestatic  #69                 // Method android/os/Looper.getMainLooper:()Landroid/os/Looper;
      53: invokevirtual #72                 // Method android/os/Looper.getThread:()Ljava/lang/Thread;
      56: invokevirtual #63                 // Method java/lang/Thread.getId:()J
      59: lcmp
      60: ifne          67
      63: iconst_1
      64: goto          68
      67: iconst_0
      68: invokestatic  #78                 // Method kotlin/coroutines/jvm/internal/Boxing.boxBoolean:(Z)Ljava/lang/Boolean;
      71: invokestatic  #84                 // Method kotlin/jvm/internal/Intrinsics.stringPlus:(Ljava/lang/String;Ljava/lang/Object;)Ljava/lang/String;
      74: invokestatic  #89                 // Method android/util/Log.i:(Ljava/lang/String;Ljava/lang/String;)I
      77: pop
      78: ldc           #51                 // String MainActivity
      80: ldc           #91                 // String launchWhenResumed,threadId:
      82: invokestatic  #59                 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
      85: invokevirtual #63                 // Method java/lang/Thread.getId:()J
      88: invokestatic  #95                 // Method kotlin/coroutines/jvm/internal/Boxing.boxLong:(J)Ljava/lang/Long;
      91: invokestatic  #84                 // Method kotlin/jvm/internal/Intrinsics.stringPlus:(Ljava/lang/String;Ljava/lang/Object;)Ljava/lang/String;
      94: invokestatic  #89                 // Method android/util/Log.i:(Ljava/lang/String;Ljava/lang/String;)I
      97: pop
      98: invokestatic  #101                // Method kotlinx/coroutines/Dispatchers.getMain:()Lkotlinx/coroutines/MainCoroutineDispatcher;
     101: checkcast     #103                // class kotlin/coroutines/CoroutineContext
     104: new           #105                // class com/devnn/demo/MainActivity$onCreate$6$1
     107: dup
     108: aconst_null
     109: invokespecial #107                // Method com/devnn/demo/MainActivity$onCreate$6$1."<init>":(Lkotlin/coroutines/Continuation;)V
     112: checkcast     #7                  // class kotlin/jvm/functions/Function2
     115: aload_0
     116: checkcast     #109                // class kotlin/coroutines/Continuation
     119: aload_0
     120: iconst_1
     121: putfield      #43                 // Field label:I
     124: invokestatic  #115                // Method kotlinx/coroutines/BuildersKt.withContext:(Lkotlin/coroutines/CoroutineContext;Lkotlin/jvm/functions/Function2;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
     127: dup
     128: aload_2
     129: if_acmpne     139
     132: aload_2
     133: areturn
     134: aload_1
     135: invokestatic  #49                 // Method kotlin/ResultKt.throwOnFailure:(Ljava/lang/Object;)V
     138: aload_1
     139: pop
     140: invokestatic  #119                // Method kotlinx/coroutines/Dispatchers.getIO:()Lkotlinx/coroutines/CoroutineDispatcher;
     143: checkcast     #103                // class kotlin/coroutines/CoroutineContext
     146: new           #121                // class com/devnn/demo/MainActivity$onCreate$6$2
     149: dup
     150: aconst_null
     151: invokespecial #122                // Method com/devnn/demo/MainActivity$onCreate$6$2."<init>":(Lkotlin/coroutines/Continuation;)V
     154: checkcast     #7                  // class kotlin/jvm/functions/Function2
     157: aload_0
     158: checkcast     #109                // class kotlin/coroutines/Continuation
     161: aload_0
     162: iconst_2
     163: putfield      #43                 // Field label:I
     166: invokestatic  #115                // Method kotlinx/coroutines/BuildersKt.withContext:(Lkotlin/coroutines/CoroutineContext;Lkotlin/jvm/functions/Function2;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
     169: dup
     170: aload_2
     171: if_acmpne     181
     174: aload_2
     175: areturn
     176: aload_1
     177: invokestatic  #49                 // Method kotlin/ResultKt.throwOnFailure:(Ljava/lang/Object;)V
     180: aload_1
     181: pop
     182: getstatic     #128                // Field kotlin/Unit.INSTANCE:Lkotlin/Unit;
     185: areturn
     186: new           #130                // class java/lang/IllegalStateException
     189: dup
     190: ldc           #132                // String call to 'resume' before 'invoke' with coroutine
     192: invokespecial #135                // Method java/lang/IllegalStateException."<init>":(Ljava/lang/String;)V
     195: athrow

  public final kotlin.coroutines.Continuation<kotlin.Unit> create(java.lang.Object, kotlin.coroutines.Continuation<?>);
    Code:
       0: new           #2                  // class com/devnn/demo/MainActivity$onCreate$6
       3: dup
       4: aload_2
       5: invokespecial #145                // Method "<init>":(Lkotlin/coroutines/Continuation;)V
       8: checkcast     #109                // class kotlin/coroutines/Continuation
      11: areturn

  public final java.lang.Object invoke(kotlinx.coroutines.CoroutineScope, kotlin.coroutines.Continuation<? super kotlin.Unit>);
    Code:
       0: aload_0
       1: aload_1
       2: aload_2
       3: invokevirtual #151                // Method create:(Ljava/lang/Object;Lkotlin/coroutines/Continuation;)Lkotlin/coroutines/Continuation;
       6: checkcast     #2                  // class com/devnn/demo/MainActivity$onCreate$6
       9: getstatic     #128                // Field kotlin/Unit.INSTANCE:Lkotlin/Unit;
      12: invokevirtual #153                // Method invokeSuspend:(Ljava/lang/Object;)Ljava/lang/Object;
      15: areturn

  public java.lang.Object invoke(java.lang.Object, java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: checkcast     #159                // class kotlinx/coroutines/CoroutineScope
       5: aload_2
       6: checkcast     #109                // class kotlin/coroutines/Continuation
       9: invokevirtual #161                // Method invoke:(Lkotlinx/coroutines/CoroutineScope;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
      12: areturn
}

这个匿名内部类只有一个字段就是int类型的label。同时它的主要业务逻辑代码在invokeSuspend函数中，这个函数里面有一个tableswitch的判断，根据label的值，判断跳转到哪一块代码执行。看这个有点类似有限状态机。

Function2接口内容：

public interface Function2<in P1, in P2, out R> : Function<R> {
   /** Invokes the function with the specified arguments. */
  public operator fun invoke(p1: P1, p2: P2): R
}

MainActivity的协程中有两段withContext{}代码块，它们也被封装成了匿名内部类，这个匿名内部类跟外部的协程代码块一样，继承了SuspendLambda类，同时实现了Function2接口。

第一个withContext代码块的匿名内部类是com/devnn/demo/MainActivity\$onCreate\$6\$1.class

第二个withContext代码块的匿名内部类是com/devnn/demo/MainActivity\$onCreate\$6\$2.class

转载请注明来源：
https://blog.csdn.net/devnn/article/details/135610313

com/devnn/demo/MainActivity\$onCreate\$6\$1.class的内容如下：

final class com.devnn.demo.MainActivity$onCreate$6$1 extends kotlin.coroutines.jvm.internal.SuspendLambda implements kotlin.jvm.functions.Function2<kotlinx.coroutines.CoroutineScope, kotlin.coroutines.Continuation<? super java.lang.Integer>, java.lang.Object> {
  int label;

  com.devnn.demo.MainActivity$onCreate$6$1(kotlin.coroutines.Continuation<? super com.devnn.demo.MainActivity$onCreate$6$1>);
    Code:
       0: aload_0
       1: iconst_2
       2: aload_1
       3: invokespecial #13                 // Method kotlin/coroutines/jvm/internal/SuspendLambda."<init>":(ILkotlin/coroutines/Continuation;)V
       6: return

  public final java.lang.Object invokeSuspend(java.lang.Object);
    Code:
       0: invokestatic  #37                 // Method kotlin/coroutines/intrinsics/IntrinsicsKt.getCOROUTINE_SUSPENDED:()Ljava/lang/Object;
       3: pop
       4: aload_0
       5: getfield      #41                 // Field label:I
       8: tableswitch   { // 0 to 0
                     0: 28
               default: 61
          }
      28: aload_1
      29: invokestatic  #47                 // Method kotlin/ResultKt.throwOnFailure:(Ljava/lang/Object;)V
      32: ldc2_w        #48                 // long 10000l
      35: invokestatic  #55                 // Method java/lang/Thread.sleep:(J)V
      38: ldc           #57                 // String MainActivity
      40: ldc           #59                 // String launchWhenResumed,withContext1,threadId:
      42: invokestatic  #63                 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
      45: invokevirtual #67                 // Method java/lang/Thread.getId:()J
      48: invokestatic  #73                 // Method kotlin/coroutines/jvm/internal/Boxing.boxLong:(J)Ljava/lang/Long;
      51: invokestatic  #79                 // Method kotlin/jvm/internal/Intrinsics.stringPlus:(Ljava/lang/String;Ljava/lang/Object;)Ljava/lang/String;
      54: invokestatic  #84                 // Method android/util/Log.i:(Ljava/lang/String;Ljava/lang/String;)I
      57: invokestatic  #88                 // Method kotlin/coroutines/jvm/internal/Boxing.boxInt:(I)Ljava/lang/Integer;
      60: areturn
      61: new           #90                 // class java/lang/IllegalStateException
      64: dup
      65: ldc           #92                 // String call to 'resume' before 'invoke' with coroutine
      67: invokespecial #95                 // Method java/lang/IllegalStateException."<init>":(Ljava/lang/String;)V
      70: athrow

  public final kotlin.coroutines.Continuation<kotlin.Unit> create(java.lang.Object, kotlin.coroutines.Continuation<?>);
    Code:
       0: new           #2                  // class com/devnn/demo/MainActivity$onCreate$6$1
       3: dup
       4: aload_2
       5: invokespecial #102                // Method "<init>":(Lkotlin/coroutines/Continuation;)V
       8: checkcast     #104                // class kotlin/coroutines/Continuation
      11: areturn

  public final java.lang.Object invoke(kotlinx.coroutines.CoroutineScope, kotlin.coroutines.Continuation<? super java.lang.Integer>);
    Code:
       0: aload_0
       1: aload_1
       2: aload_2
       3: invokevirtual #110                // Method create:(Ljava/lang/Object;Lkotlin/coroutines/Continuation;)Lkotlin/coroutines/Continuation;
       6: checkcast     #2                  // class com/devnn/demo/MainActivity$onCreate$6$1
       9: getstatic     #116                // Field kotlin/Unit.INSTANCE:Lkotlin/Unit;
      12: invokevirtual #118                // Method invokeSuspend:(Ljava/lang/Object;)Ljava/lang/Object;
      15: areturn

  public java.lang.Object invoke(java.lang.Object, java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: checkcast     #124                // class kotlinx/coroutines/CoroutineScope
       5: aload_2
       6: checkcast     #104                // class kotlin/coroutines/Continuation
       9: invokevirtual #126                // Method invoke:(Lkotlinx/coroutines/CoroutineScope;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
      12: areturn
}

com.devnn.demo.MainActivity$onCreate$6$2的内容如下：

final class com.devnn.demo.MainActivity$onCreate$6$2 extends kotlin.coroutines.jvm.internal.SuspendLambda implements kotlin.jvm.functions.Function2<kotlinx.coroutines.CoroutineScope, kotlin.coroutines.Continuation<? super kotlin.Unit>, java.lang.Object> {
  int label;

  com.devnn.demo.MainActivity$onCreate$6$2(kotlin.coroutines.Continuation<? super com.devnn.demo.MainActivity$onCreate$6$2>);
    Code:
       0: aload_0
       1: iconst_2
       2: aload_1
       3: invokespecial #13                 // Method kotlin/coroutines/jvm/internal/SuspendLambda."<init>":(ILkotlin/coroutines/Continuation;)V
       6: return

  public final java.lang.Object invokeSuspend(java.lang.Object);
    Code:
       0: invokestatic  #37                 // Method kotlin/coroutines/intrinsics/IntrinsicsKt.getCOROUTINE_SUSPENDED:()Ljava/lang/Object;
       3: pop
       4: aload_0
       5: getfield      #41                 // Field label:I
       8: tableswitch   { // 0 to 0
                     0: 28
               default: 56
          }
      28: aload_1
      29: invokestatic  #47                 // Method kotlin/ResultKt.throwOnFailure:(Ljava/lang/Object;)V
      32: ldc           #49                 // String MainActivity
      34: ldc           #51                 // String launchWhenResumed,withContext2,threadId:
      36: invokestatic  #57                 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
      39: invokevirtual #61                 // Method java/lang/Thread.getId:()J
      42: invokestatic  #67                 // Method kotlin/coroutines/jvm/internal/Boxing.boxLong:(J)Ljava/lang/Long;
      45: invokestatic  #73                 // Method kotlin/jvm/internal/Intrinsics.stringPlus:(Ljava/lang/String;Ljava/lang/Object;)Ljava/lang/String;
      48: invokestatic  #78                 // Method android/util/Log.i:(Ljava/lang/String;Ljava/lang/String;)I
      51: pop
      52: getstatic     #84                 // Field kotlin/Unit.INSTANCE:Lkotlin/Unit;
      55: areturn
      56: new           #86                 // class java/lang/IllegalStateException
      59: dup
      60: ldc           #88                 // String call to 'resume' before 'invoke' with coroutine
      62: invokespecial #91                 // Method java/lang/IllegalStateException."<init>":(Ljava/lang/String;)V
      65: athrow

  public final kotlin.coroutines.Continuation<kotlin.Unit> create(java.lang.Object, kotlin.coroutines.Continuation<?>);
    Code:
       0: new           #2                  // class com/devnn/demo/MainActivity$onCreate$6$2
       3: dup
       4: aload_2
       5: invokespecial #98                 // Method "<init>":(Lkotlin/coroutines/Continuation;)V
       8: checkcast     #100                // class kotlin/coroutines/Continuation
      11: areturn

  public final java.lang.Object invoke(kotlinx.coroutines.CoroutineScope, kotlin.coroutines.Continuation<? super kotlin.Unit>);
    Code:
       0: aload_0
       1: aload_1
       2: aload_2
       3: invokevirtual #106                // Method create:(Ljava/lang/Object;Lkotlin/coroutines/Continuation;)Lkotlin/coroutines/Continuation;
       6: checkcast     #2                  // class com/devnn/demo/MainActivity$onCreate$6$2
       9: getstatic     #84                 // Field kotlin/Unit.INSTANCE:Lkotlin/Unit;
      12: invokevirtual #108                // Method invokeSuspend:(Ljava/lang/Object;)Ljava/lang/Object;
      15: areturn

  public java.lang.Object invoke(java.lang.Object, java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: checkcast     #114                // class kotlinx/coroutines/CoroutineScope
       5: aload_2
       6: checkcast     #100                // class kotlin/coroutines/Continuation
       9: invokevirtual #116                // Method invoke:(Lkotlinx/coroutines/CoroutineScope;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
      12: areturn
}

每次执行新的匿名内部类代码(就是withContext代码块)时，就把当前这个匿名内部类传递给了新的匿名内部类。注意每个匿名内部类都是Continuation的实现。看到这里就知道其实Continuation就是一个回调。这其实就跟Java的回调一样，只是Kotlin隐式地实现了回调，并且加了状态机机制。每次执行一段suspend代码之后，将状态值修改成新值，执行回调时就知道走哪一段代码块。

转载请注明来源：
https://blog.csdn.net/devnn/article/details/135610313