本节内容涵盖了异常处理与在异常上取消。我们已经知道取消协程会在挂起点抛出 CancellationException 并且它会被协程的机制所忽略。在这⾥我们会看看在取消过程中抛出异常或同 ⼀个协程的多个⼦协程抛出异常时会发⽣什么。
异常的传播
协程构建器有两种形式：⾃动传播异常（launch 与 actor）或向⽤⼾暴露异常（async 与 produce）。当 这些构建器⽤于创建⼀个根协程时，即该协程不是另⼀个协程的⼦协程，前者这类构建器将异常视为未 捕获异常，类似 Java 的 Thread.uncaughtExceptionHandler，⽽后者则依赖⽤⼾来最终消费异 常，例如通过 await 或 receive（produce 与 receive 的相关内容包含于通道章节）。 可以通过⼀个使⽤ GlobalScope 创建根协程的简单⽰例来进⾏演⽰：
import kotlinx.coroutines.* fun main() = runBlocking { val job = GlobalScope.launch { // launch 根协程 println("Throwing exception from launch") throw IndexOutOfBoundsException() // 我们将在控制台打印 Thread.defaultUncaughtExceptionHandler } job.join() println("Joined failed job") val deferred = GlobalScope.async { // async 根协程 println("Throwing exception from async") throw ArithmeticException() // 没有打印任何东西，依赖⽤⼾去调⽤等待 } try { deferred.await() println("Unreached") } catch (e: ArithmeticException) { println("Caught ArithmeticException") } }这段代码的输出如下（调试）：
Throwing exception from launch Exception in thread "DefaultDispatcher-worker-2 @coroutine#2" java.lang.IndexOutOfBoundsException Joined failed job Throwing exception from async Caught ArithmeticExceptionCoroutineExceptionHandler
It is possible to customize the default behavior of printing uncaught exceptions to the console. CoroutineExceptionHandler context element on a root coroutine can be used as generic catch block for this root coroutine and all its children where custom exception handling may take place. It is similar to Thread.uncaughtExceptionHandler. You cannot recover from the exception in the CoroutineExceptionHandler . The coroutine had already completed with the corresponding exception when the handler is called. Normally, the handler is used to log the exception, show some kind of error message, terminate, and/or restart the application. 在 JVM 中可以重定义⼀个全局的异常处理者来将所有的协程通过 ServiceLoader 注册到 CoroutineExceptionHandler。全局异常处理者就如同 Thread.defaultUncaughtExceptionHandler ⼀样，在没有更多的指定的异常处理者被注册的 时候被使⽤。在 Android 中，uncaughtExceptionPreHandler 被设置在全局协程异常处理者中。
CoroutineExceptionHandler is invoked only on uncaught exceptions — exceptions that were not handled in any other way. In particular, all children coroutines (coroutines created in the context of another Job) delegate handling of their exceptions to their parent coroutine, which also delegates to the parent, and so on until the root, so the CoroutineExceptionHandler installed in their context is never used. In addition to that, async builder always catches all exceptions and represents them in the resulting Deferred object, so its CoroutineExceptionHandler has no effect either
Coroutines running in supervision scope do not propagate exceptions to their parent and are excluded from this rule. A further Supervision section of this document gives more details.val handler = CoroutineExceptionHandler { _, exception -> println("CoroutineExceptionHandler got $exception") } val job = GlobalScope.launch(handler) { // root coroutine, running in GlobalScope throw AssertionError() } val deferred = GlobalScope.async(handler) { // also root, but async instead of launch throw ArithmeticException() // 没有打印任何东西，依赖⽤⼾去调⽤ deferred.await() } joinAll(job, deferred)这段代码的输出如下：
CoroutineExceptionHandler got java.lang.AssertionError取消与异常
取消与异常紧密相关。协程内部使⽤ CancellationException 来进⾏取消，这个异常会被所有的 处理者忽略，所以那些可以被 catch 代码块捕获的异常仅仅应该被⽤来作为额外调试信息的资源。当 ⼀个协程使⽤ Job.cancel 取消的时候，它会被终⽌，但是它不会取消它的⽗协程。
val job = launch { val child = launch { try { delay(Long.MAX_VALUE) } finally { println("Child is cancelled") } } yield() println("Cancelling child") child.cancel() child.join() yield() println("Parent is not cancelled") } job.join()这段代码的输出如下：
Cancelling child Child is cancelled Parent is not cancelled如果⼀个协程遇到了 CancellationException 以外的异常，它将使⽤该异常取消它的⽗协程。这 个⾏为⽆法被覆盖，并且⽤于为结构化的并发（structured concurrency）提供稳定的协程层级结构。 CoroutineExceptionHandler 的实现并不是⽤于⼦协程。
在本例中，CoroutineExceptionHandler 总是被设置在由 GlobalScope 启动的协程中。将异常处理者设置在 runBlocking 主作⽤域内启动的协程中是没有意义的，尽管⼦协程已经设置了异常处理者，但是主协程也总是会被取消的。当⽗协程的所有⼦协程都结束后，原始的异常才会被⽗协程处理，⻅下⾯这个例⼦
val handler = CoroutineExceptionHandler { _, exception -> println("CoroutineExceptionHandler got $exception") } val job = GlobalScope.launch(handler) { launch { // 第⼀个⼦协程 try { delay(Long.MAX_VALUE) } finally { withContext(NonCancellable) { println("Children are cancelled, but exception is not handled until all children terminate") delay(100) println("The first child finished its non cancellable block") } } } launch { // 第⼆个⼦协程 delay(10) println("Second child throws an exception") throw ArithmeticException() } } job.join()这段代码的输出如下：
Second child throws an exception异常聚合
当协程的多个⼦协程因异常⽽失败时，⼀般规则是“取第⼀个异常”，因此将处理第⼀个异常。在第⼀个 异常之后发⽣的所有其他异常都作为被抑制的异常绑定⾄第⼀个异常。
import kotlinx.coroutines.* import java.io.* fun main() = runBlocking { val handler = CoroutineExceptionHandler { _, exception -> println("CoroutineExceptionHandler got $exception with suppressed ${exception.suppressed.contentToString()}") } val job = GlobalScope.launch(handler) { launch { try { delay(Long.MAX_VALUE) // 当另⼀个同级的协程因 IOException 失败时，它将被取消 } finally { throw ArithmeticException() // 第⼆个异常 } } launch { delay(100) throw IOException() // ⾸个异常 } delay(Long.MAX_VALUE) } job.join() }这段代码的输出如下：
CoroutineExceptionHandler got java.io.IOException with suppressed [java.lang.ArithmeticException]取消异常是透明的，默认情况下是未包装的：
val handler = CoroutineExceptionHandler { _, exception -> println("CoroutineExceptionHandler got $exception") } val job = GlobalScope.launch(handler) { val inner = launch { // 该栈内的协程都将被取消 launch { launch { throw IOException() // 原始异常 } } } try { inner.join() } catch (e: CancellationException) { println("Rethrowing CancellationException with original cause") throw e // 取消异常被重新抛出，但原始 IOException 得到了处理 } } job.join()这段代码的输出如下：
Rethrowing CancellationException with original cause CoroutineExceptionHandler got java.io.IOException监督
正如我们之前研究的那样，取消是在协程的整个层次结构中传播的双向关系。让我们看⼀下需要单向取消的情况。
此类需求的⼀个良好⽰例是在其作⽤域内定义作业的 UI 组件。如果任何⼀个 UI 的⼦作业执⾏失败了， 它并不总是有必要取消（有效地杀死）整个 UI 组件，但是如果 UI 组件被销毁了（并且它的作业也被取消 了），由于它的结果不再被需要了，它有必要使所有的⼦作业执⾏失败。
另⼀个例⼦是服务进程孵化了⼀些⼦作业并且需要 监督 它们的执⾏，追踪它们的故障并在这些⼦作业 执⾏失败的时候重启。
监督作业
SupervisorJob 可以被⽤于这些⽬的。它类似于常规的 Job，唯⼀的不同是：SupervisorJob 的取消只 会向下传播。这是⾮常容易从⽰例中观察到的：
import kotlinx.coroutines.* fun main() = runBlocking { val supervisor = SupervisorJob() with(CoroutineScope(coroutineContext + supervisor)) { // 启动第⼀个⼦作业——这个⽰例将会忽略它的异常（不要在实践中这么做！） val firstChild = launch(CoroutineExceptionHandler { _, _ -> }) { println("First child is failing") throw AssertionError("First child is cancelled") } // 启动第两个⼦作业 val secondChild = launch { firstChild.join() // 取消了第⼀个⼦作业且没有传播给第⼆个⼦作业 println("First child is cancelled: ${firstChild.isCancelled}, but second one is still active") try { delay(Long.MAX_VALUE) } finally { // 但是取消了监督的传播 println("Second child is cancelled because supervisor is cancelled") } } // 等待直到第⼀个⼦作业失败且执⾏完成 firstChild.join() println("Cancelling supervisor") supervisor.cancel() secondChild.join() } }这段代码的输出如下：
First child is failing First child is cancelled: true, but second one is still active Cancelling supervisor Second child is cancelled because supervisor is cancelled监督作用域
对于作⽤域的并发，supervisorScope 可以被⽤来替代 coroutineScope 来实现相同的⽬的。它只会单 向的传播并且当作业⾃⾝执⾏失败的时候将所有⼦作业全部取消。作业⾃⾝也会在所有的⼦作业执⾏ 结束前等待，就像 coroutineScope 所做的那样。
import kotlin.coroutines.* import kotlinx.coroutines.* fun main() = runBlocking { try { supervisorScope { val child = launch { try { println("Child is sleeping") delay(Long.MAX_VALUE) } finally { println("Child is cancelled") } } // 使⽤ yield 来给我们的⼦作业⼀个机会来执⾏打印 yield() println("Throwing exception from scope") throw AssertionError() } } catch(e: AssertionError) { println("Caught assertion error") } }这段代码的输出如下：
Child is sleeping Throwing exception from scope Child is cancelled Caught assertion error监督协程中的异常
常规的作业和监督作业之间的另⼀个重要区别是异常处理。监督协程中的每⼀个⼦作业应该通过异常 处理机制处理⾃⾝的异常。这种差异来⾃于⼦作业的执⾏失败不会传播给它的⽗作业的事实。这意味 着在 supervisorScope 内部直接启动的协程确实使⽤了设置在它们作⽤域内的 CoroutineExceptionHandler，与⽗协程的⽅式相同（查看 CoroutineExceptionHandler ⼩节以获知 更多细节）。
import kotlin.coroutines.* import kotlinx.coroutines.* fun main() = runBlocking { val handler = CoroutineExceptionHandler { _, exception -> println("CoroutineExceptionHandler got $exception") } supervisorScope { val child = launch(handler) { println("Child throws an exception") throw AssertionError() } println("Scope is completing") } println("Scope is completed") }这段代码的输出如下：
Scope is completing Child throws an exception CoroutineExceptionHandler got java.lang.AssertionError Scope is completed
