Kotlin协程-try-catch基础
在 Kotlin 协程当中，我们通常把异常分为两大类，一类是取消异常（CancellationException），另一类是其他异常。在 Kotlin 协程当中，这两种异常的处理方式是不一样的。
一、cancel()无效？
当协程任务被取消的时候，它的内部是会产生一个 CancellationException 的。而协程的结构化并发，最大的优势就在于：如果我们取消了父协程，子协程也会跟着被取消。
1.cancel()不被响应
val job = launch(Dispatchers.Default) {
var i = 0
while (true) {
Thread.sleep(500L)
i++
println("i =$i")
}
}
delay(2000L)
job.cancel()
job.join()
println("END")
}
Log
i =1
i =2
i =3
i =4
i =5
i =6
i =7
i =8
i =9
i =10
i =11
i =12
......
程序无法停止上面的程序无法停止，协程任务的取消，需要互相协作。协程外部取消，协程内部需要做出响应才行。当我们调用 job.cancel() 以后，协程任务已经不是活跃状态了，但代码并没有把 isActive 作为循环条件，因此协程无法真正取消。
可以在协程体中加入状态判断：
runBlocking {
val job = launch(Dispatchers.Default) {
var i = 0
while (isActive) {
Thread.sleep(500L)
i++
println("i =$i")
}
}
delay(2000L)
job.cancel()
job.join()
println("END")
}
Log
i =1
i =2
i =3
i =4
END
Process finished with exit code 0把 while 循环的条件改成了 while (isActive)，这就意味着，只有协程处于活跃状态的时候，才会继续执行循环体内部的代码。协程的取消需要内部的配合。
2.结构被破坏
协程是结构化的，当我们取消父协程的时候，子协程也会跟着被取消。
但是特殊情况是嵌套创建的子协程并不会跟随父协程一起取消。
runBlocking {
val parentJob = launch(fixedDispatcher) {
launch(Job()) {
var i = 0
while (isActive) {
Thread.sleep(500L)
i++
println("First i:$i")
}
}
launch {
var i = 0
while (isActive) {
Thread.sleep(500L)
i++
println("Second i:$i")
}
}
}
delay(2000L)
parentJob.cancel()
parentJob.join()
println("End")
}
Log
Second i:1
First i:1
First i:2
Second i:2
Second i:3
First i:3
Second i:4
First i:4
End
First i:5
First i:6
First i:7
First i:8
First i:9
First i:10
First i:11
First i:12
First i:13
First i:14
First i:15
......可以发现，创建子协程的时候，使用了 launch(Job()){}，就打破了原有的协程结构。因为 launch(Job()){}创建的协程的父 Job 是在 launch 当中传入的 Job() 对象。所以调用 parentJob.cancel() 的时候，无法销毁该协程。
可以按如下修改：
runBlocking {
val parentJob = launch(fixedDispatcher) {
launch {
var i = 0
while (isActive) {
Thread.sleep(500L)
i++
println("First i:$i")
}
}
launch {
var i = 0
while (isActive) {
Thread.sleep(500L)
i++
println("Second i:$i")
}
}
}
delay(2000L)
parentJob.cancel()
parentJob.join()
println("End")
}
First i:1
Second i:1
First i:2
Second i:2
First i:3
Second i:3
First i:4
Second i:4
End
parentJob 与它内部的子协程之间都是父子关系，因此它们两个都是会响应协程取消的事件的。不要轻易打破协程的父子结构！
3.未正确处理 CancellationException
对于 Kotlin 提供的挂起函数，可以自动响应协程的取消。
例如：
runBlocking {
val parentJob = launch(Dispatchers.Default) {
launch {
var i = 0
while (true) {
delay(500L)
i++
println("First i = $i")
}
}
launch {
var i = 0
while (true) {
delay(500L)
i++
println("Second i = $i")
}
}
}
delay(2000L)
parentJob.cancel()
parentJob.join()
println("End")
}
First i = 1
Second i = 1
First i = 2
Second i = 2
First i = 3
Second i = 3
End
Process finished with exit code 0
delay() 函数可以自动检测当前的协程是否已经被取消，如果已经被取消的话，它会抛出一个 CancellationException，从而终止当前的协程。
runBlocking {
val parentJob = launch(Dispatchers.Default) {
launch {
var i = 0
while (true) {
try {
delay(500L)
} catch (e: CancellationException) {
println("Catch CancellationException")
throw e
}
i++
println("First i =$i")
}
}
launch {
var i = 0
while (true) {
delay(500L)
i++
println("Second i = $i")
}
}
}
delay(2000L)
parentJob.cancel()
parentJob.join()
println("END")
}
Log:
First i =1
Second i = 1
First i =2
Second i = 2
First i =3
Second i = 3
Catch CancellationException
END
Process finished with exit code 0try-catch 包裹了 delay() 以后，打印出“Catch CancellationException”，这就说明 delay() 确实可以自动响应协程的取消，并且产生 CancellationException 异常。
注意：捕获了 CancellationException 以后没有重新抛出去，就导致子协程无法正常取消。
runBlocking {
val parentJob = launch(Dispatchers.Default) {
launch {
var i = 0
while (true) {
try {
delay(500L)
} catch (e: CancellationException) {
println("Catch CancellationException")
//throw e
}
i++
println("First i =$i")
}
}
launch {
var i = 0
while (true) {
delay(500L)
i++
println("Second i = $i")
}
}
}
delay(2000L)
parentJob.cancel()
parentJob.join()
println("END")
}
......
First i =656179
Catch CancellationException
First i =656180
Catch CancellationException
First i =656181
Catch CancellationException
First i =656182
Catch CancellationException
First i =656183
Catch CancellationException
.....
所以，捕获了 CancellationException 以后，要考虑是否应该重新抛出来。
二、 try-catch 不起作用？
runBlocking {
try {
launch {
delay(100L)
1 / 0
}
} catch (e: Exception) {
println("catch: $e")
}
delay(500L)
println("End")
}
Log
Exception in thread "main" java.lang.ArithmeticException: / by zero
at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch8$1$1.invokeSuspend(TestTryCatch.kt:225)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch8(TestTryCatch.kt:221)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:15)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
Process finished with exit code 1可以发现：try-catch 并没有成功捕获异常，程序等待了 100 毫秒左右，最终还是崩溃了。
使用async
runBlocking {
var deffered: Deferred? = null
try {
deffered = async {
delay(100L)
1 / 0
}
} catch (e: ArithmeticException) {
println("Catch:$e")
}
deffered?.await()
println("End")
}
Exception in thread "main" java.lang.ArithmeticException: / by zero
at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch9$1$1.invokeSuspend(TestTryCatch.kt:242)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch9(TestTryCatch.kt:237)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
Process finished with exit code 1 当协程体当中的“1/0”执行的时候，程序已经跳出 try-catch 的作用域了,所以 try-catch失效。
把 try-catch 挪到 launch{} 协程体内部。可以正常捕获到 ArithmeticException 这个异常了。
runBlocking {
var deffered: Deferred? = null
deffered = async {
try {
delay(100L)
1 / 0
} catch (e: ArithmeticException) {
println("Catch:$e")
}
}
deffered.await()
println("End")
}
Log
Catch:java.lang.ArithmeticException: / by zero
End
Process finished with exit code 0 注意：不要用 try-catch 直接包裹 launch、async。
使用 try-catch 包裹“deferred.await()”。
例：
runBlocking {
var deffered = async {
delay(100L)
1 / 0
}
try {
deffered.await()
} catch (e: Exception) {
println("Catch:$e")
}
println("End")
}
atch:java.lang.ArithmeticException: / by zero
End
Exception in thread "main" java.lang.ArithmeticException: / by zero
at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch11$1$deffered$1.invokeSuspend(TestTryCatch.kt:275)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch11(TestTryCatch.kt:272)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
Process finished with exit code 1await() 如果不调用的话，async 当中的异常是否发生？
runBlocking {
var deffered = async {
delay(100L)
1 / 0
}
delay(500L)
println("End")
}
Exception in thread "main" java.lang.ArithmeticException: / by zero
at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch12$1$deffered$1.invokeSuspend(TestTryCatch.kt:290)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch12(TestTryCatch.kt:287)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
Process finished with exit code 1可见，async 当中产生异常，即使不调用 await() 同样是会导致程序崩溃的。
三、SupervisorJob
使用 try-catch 包裹“deferred.await()”，需要配合 SupervisorJob 一起使用。实现“不调用 await() 就不会产生异常而崩溃”。
unBlocking {
val scope = CoroutineScope(SupervisorJob())
scope.async {
delay(100L)
1 / 0
}
delay(500L)
println("End")
}
Log
End
Process finished with exit code 0
使用 SupervisorJob 创建一个 scope 以后，用 scope.async{}启动协程后，只要不调用“deferred.await()”，程序就不会因为异常而崩溃。
runBlocking {
val coroutineScope = CoroutineScope(SupervisorJob())
val deferred = coroutineScope.async {
delay(100L)
1 / 0
}
try {
deferred.await()
} catch (e: Exception) {
println("Catch:$e")
}
delay(500L)
println("End")
}
Log
Catch:java.lang.ArithmeticException: / by zero
End
Process finished with exit code 0使用“coroutineScope.async {}”创建了协程，同时也用 try-catch 包裹“deferred.await()”，这样一来，异常就成功地被捕获了。
public fun SupervisorJob(parent: Job? = null) : CompletableJob
= SupervisorJobImpl(parent)
public interface CompletableJob : Job {
public fun complete(): Boolean
public fun completeExceptionally(exception: Throwable): Boolean
}SupervisorJob() 不是构造函数，它只是一个普通的顶层函数。这个方法返回的对象，是 Job 的子类。SupervisorJob 与 Job 最大的区别就在于，当它的子 Job 发生异常的时候，其他的子 Job 不会受到牵连。
对于普通 Job， 出现异常时的应对策略是：由于 parentJob 是一个普通的 Job 对象，当 job1 发生异常之后，它会导致 parentJob 取消，进而导致 job2、job3 也受到牵连。
如果把 parentJob 改为 SupervisorJob，job1 发生异常的的话，就不会影响到其他的 Job 了。
注意：灵活使用 SupervisorJob，控制异常传播的范围。
四、CoroutineExceptionHandler
runBlocking {
val coroutineScope = CoroutineScope(coroutineContext)
coroutineScope.launch {
async {
delay(100L)
}
launch {
delay(100L)
1/0
}
}
delay(1000L)
println("END")
}
Exception in thread "main" java.lang.ArithmeticException: / by zero
at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch15$1$1$2.invokeSuspend(TestTryCatch.kt:338)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch15(TestTryCatch.kt:329)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
Process finished with exit code 1
使用CoroutineExceptionHandler 处理上述代码中的异常。
runBlocking {
val coroutineExceptionHandler = CoroutineExceptionHandler { _, throwable ->
println("Catch: $throwable")
}
val coroutineScope = CoroutineScope(coroutineContext + Job() + coroutineExceptionHandler)
coroutineScope.launch {
async {
delay(100L)
}
launch {
delay(100L)
1 / 0
}
}
delay(1000L)
println("END")
}
Log
Catch: java.lang.ArithmeticException: / by zero
END
Process finished with exit code 0定义了一个 CoroutineExceptionHandler，然后把它传入了 scope 当中，就可以捕获其中所有的异常了。
注意点：在特定场景，为什么 CoroutineExceptionHandler 不起作用？
runBlocking {
val coroutineExceptionHandler = CoroutineExceptionHandler { _, throwable ->
println("Catch: $throwable")
}
val coroutineScope = CoroutineScope(coroutineContext)
coroutineScope.launch {
async {
delay(100L)
}
launch(coroutineExceptionHandler) {
delay(100L)
1 / 0
}
}
delay(1000L)
println("END")
}
Exception in thread "main" java.lang.ArithmeticException: / by zero
at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch17$1$1$2.invokeSuspend(TestTryCatch.kt:383)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch17(TestTryCatch.kt:371)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
Process finished with exit code 1把自定义的 myExceptionHandler，放到出现异常的 launch 那里传了进去。myExceptionHandler 并不会起作用，异常不会被它捕获。注意：myExceptionHandler 直接定义在发生异常的位置反而不生效，而定义在最顶层却可以生效。因为 只在顶层的协程当中才会起作用。也就是说，当子协程当中出现异常以后，它们都会统一上报给顶层的父协程，然后顶层的父协程才会去调用 CoroutineExceptionHandler，来处理对应的异常。所以需要记住：使用 CoroutineExceptionHandler 处理复杂结构的协程异常，它仅在顶层协程中起作用。