JarCastingFaux Pas: Error handling in Functional Programming
Faux pas noun, /fəʊ pɑː/: blunder; misstep, false step
Faux Pas is a library that simplifies error handling for Functional Programming in Java. It fixes the issue that none of the functional interfaces in the Java Runtime by default is allowed to throw checked exceptions.
- Technology stack: Java 8+, functional interfaces
- Status: 0.x, originally ported from Riptide, used in production
Example
interface Client {
User read(final String name) throws IOException;
}
Function<String, User> readUser = throwingFunction(client::read);
readUser.apply("Bob"); // may throw IOException directly
Features
- Checked exceptions for functional interfaces
- Compatible with the JDK types
Dependencies
- Java 8 or higher
- Lombok (no runtime dependency)
Installation
Add the following dependency to your project:
<dependency>
<groupId>org.zalando</groupId>
<artifactId>faux-pas</artifactId>
<version>${faux-pas.version}</version>
</dependency>
Usage
Throwing functional interfaces
Faux Pas has a variant of every major functional interface from the Java core:
ThrowingRunnableThrowingSupplierThrowingConsumerThrowingFunctionThrowingUnaryOperatorThrowingPredicateThrowingBiConsumerThrowingBiFunctionThrowingBinaryOperatorThrowingBiPredicate
The followings statements apply to each of them:
- extends the official interface, i.e. they are 100% compatible
- sneakily throws the original exception
Creation
The way the Java runtime implemented functional interfaces always requires additional type information, either by using a cast or a local variable:
// compiler error
client::read.apply(name);
// too verbose
((ThrowingFunction<String, User, IOException>) client::read).apply(name);
// local variable may not always be desired
ThrowingFunction<String, User, IOException> readUser = client::read;
readUser.apply(name);
As a workaround there is a static factory method for every interface type inFauxPas. All of them are called throwingRunnable, throwingSupplier and so forth. It allows for concise one-line statements:
List<User> users = names.stream()
.map(throwingFunction(client::read))
.collect(toList());
Try-with-resources alternative
Traditional try-with-resources statements are compiled into byte code that includes unreachable parts and unfortunately JaCoCo has no support for filtering yet. That's why we came up with an alternative implementation. The official example for the try-with-resources statement looks like this:
try (BufferedReader br =
new BufferedReader(new FileReader(path))) {
return br.readLine();
}
Compared to ours:
return tryWith(new BufferedReader(new FileReader(path)), br ->
br.readLine()
);
CompletableFuture.exceptionally(Function)
CompletableFuture.exceptionally(..) is a very powerful but often overlooked tool. It allows to inject partial exception handling into a CompletableFuture:
future.exceptionally(e -> {
Throwable t = e instanceof CompletionException ? e.getCause() : e;
if (t instanceof NoRouteToHostException) {
return fallbackValueFor(e);
}
throw e instanceof CompletionException ? e : new CompletionException(t);
})
Unfortunately it has a contract that makes it harder to use than it needs to:
- It takes a
Throwableas an argument, but doesn't allow to re-throw it as-is. This can be circumvented by optionally wrapping it in aCompletionExceptionbefore rethrowing it. - The throwable argument is sometimes wrapped inside a
CompletionExceptionand sometimes it's not, depending on whether there is any other computation step before theexceptionally(..)call or not.
In order to use the operation correctly one needs to follow these rules:
- Unwrap given throwable if it's an instance of
CompletionException. - Wrap checked exceptions in a
CompletionExceptionbefore throwing.
FauxPas.partially(..) relives some of the pain by changing the interface and contract a bit to make it more usable. The following example is functionally equivalent to the one from above:
future.exceptionally(partially(e -> {
if (e instanceof NoRouteToHostException) {
return fallbackValueFor(e);
}
throw e;
}))
- Takes a
ThrowingFunction<Throwable, T, Throwable>, i.e. it allows clients to- directly re-throw the throwable argument
- throw any exception during exception handling as-is
- Will automatically unwrap a
CompletionExceptionbefore passing it to the given function. I.e. the supplied function will never have to deal withCompletionExceptiondirectly. Except for the rare occasion that theCompletionExceptionhas no cause, in which case it will be passed to the given function. - Will automatically wrap any thrown
Exceptioninside aCompletionException, if needed.
The last example is actually so common, that there is an overloaded version of partially that caters for this use particular case:
future.exceptionally(partially(NoRouteToHostException.class, this::fallbackValueFor))
CompletableFuture.whenComplete(BiConsumer)
future.whenComplete(failedWith(TimeoutException.class, e -> {
request.cancel();
}))
Other missing pieces in CompletableFuture's API are exceptionallyCompose and handleCompose. Both can be seen as a combination of exceptionally + compose and handle + compose respectively. They basically allow to supply another CompletableFuture rather than concrete values directly. This is allows for asynchronous fallbacks:
exceptionallyCompose(users.find(name), e -> archive.find(name))
Getting Help
If you have questions, concerns, bug reports, etc., please file an issue in this repository's Issue Tracker.
Getting Involved/Contributing
To contribute, simply make a pull request and add a brief description (1-2 sentences) of your addition or change. For more details, check the contribution guidelines.
