I'm trying to use the inline function use with a FileInputStream instead of the classic try/catch IOException so that
try {
val is = FileInputStream(file)
// file handling...
}
catch (e: IOException) {
e.printStackTrace()
}
becomes
FileInputStream(file).use { fis ->
// do stuff with file
}
My question is, why use the function use if it stills throws exception? Do I have to wrap use in a try/catch? This seems ridiculous.
From Kotlin documentation:
Executes the given block function on this resource and then closes it
down correctly whether an exception is thrown or not.
When you use an object that implements the Closeable interface, you need to call the close() method when you are done with it, so it releases any system resources associated with the object.
You need to be careful and close it even when an exception is thrown. In this kind of situation that is error prone, cause you might not know or forget to handle it properly, it is better to automate this pattern. That's exactly what the use function does.
Your try-catch does not close the resource so you are comparing apples to oranges. If you close the resource in finally block:
val is = FileInputStream(file)
try {
...
}
catch (e: IOException) {
...
}
finally {
is.close()
}
is definitely more verbose than use which handles closing the resource.
Related
Not sure how to handle checked exception in the Mono flow.
return Mono.when(monoPubs)
.zipWhen((monos) -> repository.findById(...))
.map((tuple) -> tuple.getT2())
.zipWhen((org) -> createMap(org))
.map((tuple) -> tuple.getT2())
.zipWhen((map) -> emailService.sendEmail(...))
.flatMap(response -> {
return Mono.just(userId);
});
Here, the sendEmail method is declared with throws Exception.
public Mono<Boolean> sendEmail(...)
throws MessagingException, IOException
So, How to handle this checked exception in the zipWhen flow.
Also, How to handle
.zipWhen((map) -> emailService.sendEmail(...))
if the method returns void.
You need to review implementation of the sendEmail. You cannot throw checked exceptions from the publisher and need to wrap any checked exception into an unchecked exception.
The Exceptions class provides a propagate method that could be used to wrap any checked exception into an unchecked exception.
try {
...
}
catch (SomeCheckedException e) {
throw Exceptions.propagate(e);
}
As an alternative, you could use lombok #SneakyThrows to wrap non-reactive method.
Exceptions are thrown from Mono method, you can use onError* methods to handle the exception the way you like
var result = Mono.just("test")
.zipWhen((map) -> sendEmail())
.onErrorMap(SendEmailException.class, e -> new RuntimeException(e.getMessage()))
.flatMap(Mono::just);
Also it is not very clear from your post that if sendEmail takes param or not, sendEmail is not taking any input, I would just use doOnNext as it is a void method.
I want to run cleanup code after a certain block of code completes, regardless of exceptions. This is not a closeable resource and I cannot use try-with-resources (or Kotlin's use).
In Java, I could do the following:
try {
// ... Run some code
} catch(Exception ex) {
// ... Handle exception
} finally {
// ... Cleanup code
}
Is the following Kotlin code equivalent?
runCatching {
// ... Run some code
}.also {
// ... Cleanup code
}.onFailure {
// ... Handle exception
}
Edit: added boilerplate exception handling - my concern is with ensuring the cleanup code runs, and maintainability.
There is one important difference, where the code inside runCatching contains an early return. A finally block will be executed even after a return, whereas also has no such magic.
This code, when run, will print nothing:
fun test1()
runCatching {
return
}.also {
println("test1")
}
}
This code, when run, will print "test2":
fun test2() {
try {
return
} finally {
println("test2")
}
}
There is one big difference between both code samples. try...finally propagates exceptions while runCatching().also() catches/consumes them. To make it similar you would have to throw the result at the end:
runCatching {
// ... Run some code
}.also {
// ... Cleanup code
}.getOrThrow()
But still, it is not really 1:1 equivalent. It catches all exceptions just to rethrow them. For this reason, it is probably less performant than simple try...finally.
Also, I think this is less clear for the reader. try...finally is a standard way of dealing with exceptions. By using runCatching() just to immediately rethrow, you actually confuse people reading this code later.
Your question sounded a little like you believed Kotlin does not have try...finally and you need to search for alternatives. If this is the case, then of course Kotlin has try...finally and I think you should use it instead of runCatching().
As per Kotlin's doc for runCatching:
Calls the specified function block and returns its encapsulated result if invocation was successful, catching any Throwable exception that was thrown from the block function execution and encapsulating it as a failure.
Even if finally always runs after a try block and also always runs after a runCatching, they do not serve the same purpose.
finally doesn't receive any argument and cannot operate on the values of the try block, while also receives the Result of the runCatching block.
TLDR; .runCatching{}.also{} is a more advanced try{}finally{}
There is also a difference in what is the result of evaluating the expression.
Consider the following code:
fun main() {
val foo = try {
throw Exception("try")
} catch(e: Exception) {
"catch"
} finally {
"finally"
}
val bar = runCatching{
throw Exception("runCatching")
}.also{
"also"
}.onFailure {
"onFailure"
}
println(foo)
println(bar)
}
The output will be:
catch
Failure(java.lang.Exception: runCatching)
https://pl.kotl.in/a0aByS5l1
EDIT:
An interesting article that points out some differences as well:
https://medium.com/#mattia23r/a-take-on-functional-error-handling-in-kotlin-515b67b4212b
Now let’s give a second look at the implementation of runCatching in the gist above. What does it do? It catches everything.
In this case, it goes even further: it catches all Throwables. For those not knowing, Throwable is everything that can go after a throw keyword; it has two descendants: Exceptions and Errors. We haven’t mentioned Errors so far; Errors usually represent something wrong that happened at a lower level than your business logic, something that can’t usually be recovered with a simple catch.
As per kotest docs: https://github.com/kotest/kotest/blob/master/doc/nondeterministic.md
You can tell eventually to ignore specific exceptions and any others will immediately fail the test.
I want to pass multiple exceptions to eventually that I know would be thrown by my block so that I can explicitly skip them.
Right now I only see a way to pass one, how do I pass more than one exception to eventually to skip it in case the block throws those exceptions?
You may use superclass for all your exceptions like
eventually(200.milliseconds, exceptionClass = RuntimeException::class) {
throw IllegalStateException()
}
or wrap exceptions
eventually(200.milliseconds, exceptionClass = IllegalStateException::class) {
runCatching { throw UnknownError() }
.onFailure { throw IllegalStateException(it) }
}
In 4.4.3 there are no features with collection of Exception
I'm working on a kotlin web backend and have something like this:
try {
val uuid = UUID.fromString(someString)
} catch (e: IllegalArgumentException) {
throw BadRequestException("invalid UUID")
}
doSomething(uuid)
The code above doesn't compile since uuid is unresolved outside the try block.
Alternatives I can imagine are:
move doSomething(uuid) inside the try block, but I'd rather avoid that so I don't accidentally catch some other potential IllegalArgumentException thrown by doSomething (if that happens for whatever reason I want things to fail and get a 500 in my logs so I can investigate)
use a nullable var instead and initialize it to null but that seems a bit ugly?
This throw BadRequestException pattern is working well otherwise so I don't want to change the return type of the method or something like that in order to avoid throwing.
Is there a better / more elegant / recommended pattern for this in Kotlin?
In Kotlin, try/catch can be used as an expression. Branches that throw don't affect the resolved type. So you can write:
val uuid = try {
UUID.fromString(someString)
} catch (e: IllegalArgumentException) {
throw BadRequestException("invalid UUID")
}
I'm still learning Kotlin and I just learned about the "use" and how it is a replacement for a try, catch and finally block.
However I am curious if it is possible to customize it's exception handling for example:
var connection: Connection? = null
try {
connection = dataSource.connection
connection.prepareStatement(query).execute()
} catch (e: SQLException) {
logger.log("Specific error for that query")
e.printStackTrace()
} finally {
if (connection != null && !connection.isClosed) {
connection.close()
}
}
That code is my current one, I have a specific error I would like to display on the catch, would that be possible using use?
This is my current use code:
dataSource.connection.use { connection ->
connection.prepareStatement(query).execute()
}
As commented by #Tenfour04, and from the documentation
[use] Executes the given block function on this resource and then closes it down correctly whether an exception is thrown or not.
In particular it is implemented like this:
public inline fun <T : AutoCloseable?, R> T.use(block: (T) -> R): R {
var exception: Throwable? = null
try {
return block(this)
} catch (e: Throwable) {
exception = e
throw e
} finally {
this.closeFinally(exception)
}
}
That piece of code should look familiar if you're a Java developer, but basically it executes block passing this (i.e. the receiver object) as an argument to your block of code. At the end it closes the AutoCloseable resource. If at any point an exception is thrown (either inside block or while closing the resource), that exception is thrown back to the caller, i.e. your code.
As an edge case you could have 2 exceptions, one when executing block and one when closing the resource. This is handled by closeFinally (whose source is available in the same file linked above) and the exception thrown while closing the resource is added as a suppressed exception to the one thrown from block – that's because only up to 1 exception can be thrown by a method, so they had to choose which one to throw. The same actually applies to the try-with-resources statement in Java.