Exceptions have been with us for 25 years in Java but have we learned to use them properly? Are checked exceptions a mistake? Should you use throw runtime or checked exceptions? And what to do with a checked exception, when you get one? And how to slay the boss of all exceptions: the NullPointerException. Let’s put these old questions in the context of Java8+ lambdas, Vavr monads, Lombok, Spring, JAX-RS and other modern frameworks, and see the best practices of handling errors in Java today.Let’s put these old questions in the context of Java8+ lambdas, Vavr monads, Lombok, Spring, JAX-RS and other modern frameworks, and see the best practices of handling errors in Java today.

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The Definitive Guide to
Working with Exceptions in Java
victor.rentea@gmail.com ♦ ♦ @victorrentea ♦ VictorRentea.ro
Code: https://github.com/victorrentea/exceptions-guide.git

2. Victor Rentea
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A bit of history

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At the beginning
There were no functions

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At the beginning
There were no exceptions

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40 years ago, in C:
int errno = f(...);
The Age of Libraries
25y, Java: Checked vs Runtime
35y, C++: Invisible exceptions

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We don't Recover
In web apps today, when handling exceptions,This talk is NOT
about library development

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Checked Exceptions

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code

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Diaper Anti-Pattern
} catch (Exception e) {
// TODO waste nights
} a.k.a. Exception Swallowing
Shawarma-style
Exception-Handling
Including Runtime bugs= catches all the sh*t

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throws
try
catch (Exception t) {/*surprise!*/}
RuntimeExceptions won the War !
(except for recoverable?
errors thrown by libraries)
because we don’t see them
Why should I know
about that IOException ?

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} catch (SomeCheckedException e) {
throw new SomeRuntimeException(e);
}
Ignore recurrent exception?
(eg. a file poller)
log.trace(e.getMessage())

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} catch (SomeCheckedException e) {
throw new SomeRuntimeException(e);
}
} catch (SomeCheckedException e) {
throw new SomeRuntimeException(e);
}
e.printStackTrace();log.error(e.getMessage(), e);
System.err
might NOT be logged!

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} catch (SomeCheckedException e) {
throw new SomeRuntimeException(e);
}
TERROR
What if no one logs it?
Same exception logged multiple times
Log-Rethrow Anti-Pattern
Real Solution
log.error(e.getMessage(), e);

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Legacy Code
Global Exception Handler
Logs and reports any unhandled exceptions
Core of Your App
Only RuntimeExceptions allowed
Checked -> Runtime
old lib

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Presenting Errors to Users
What to show them?
Exception's message
int
enum
How to unit-test that?
How about MVC?
Not developer-friendly
Finite-set
Never a Stack Trace: Security

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code

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class YourException extends RuntimeException {
public enum ErrorCode {
GENERAL,
PHONE_ALREADY_REGISTERED,
...
}
private final ErrorCode code;
private final Object[] params;
...
}
Error Messages
Any case not required to report
messages.properties
GENERAL = Oups!
PHONE_ALREADY_REGISTERED = The phone {0} is assigned to another user
Checkatstartup
_fr _ro _es

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Global Exception Handler
Logs and reports any unhandled exceptions
selectively catch only
recoverable cases (x-rare)
Checked -> Runtime
= the only valid reason for creating more exception types
Keep Stack Traces from reaching the UI/API

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Catch-Rethrow-with-Debug Pattern
➢ Searching for hints in the log above?
➢ Breakpoints?
Why? To find the value of some variable.
} catch (AEx e) {
log.error("Debug info " + id);
throw new BEx(e);
}
} catch (AEx e) {
throw new BEx("Info " + id, e);
}
Must-Have
Never decapitate the exceptions!
Debugging an Exception

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4 Reasons to Catch-rethrow
User-visible exceptions
(code)
Tests
(code)
Developers
(+debug info)
Wrapping a Checked
(into a runtime)
@SneakyThrows
(Lombok lib)

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The Queen Of All Exceptions

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I've never seen you looking so lovely as you did tonight,
I've never seen you shine so bright,
I've never seen so many men ask you if you wanted to dance,
Looking for a little romance,
given half a chance,
And I have never seen that dress you're wearing,
Or the highlights in your hair that catch your eyes,
I have been blind;
= P1 production Incident
The Lady In Redby Chris De Burgh
= on-call bug
= unsolved for years
= spend the night with you
= although they lack the knowledge/skills
= 50-lines stack-trace
= the DEBUG logs before it
N P E

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➔ Useful message in Java 15

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NPE
How do you fight it ?
Throw early
Optional

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Optional<>
For functions that might not return anything

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You, defeating the Null Monster
Customer.getMemberCard(): Optional<MemberCard>
Entity Getters returning Optional for NULLABLE fields of an Entity

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Java 8

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Java 8 Functional Interfaces
(eg Function, Predicate, ...)
don't declare any checked exceptions.
Methods throwing checked are painful to use with Stream API

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code

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Mmm..
Higher-order
Functions!
Convert functions to
Rethrow as Runtime
Function<..> Unchecked.function(CheckedFunction<..> f)
(jOOL lib)

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Try<>(vavr lib)
Can hold either a result or an exception
Use to collect both results and exceptions in one pass

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Key Points
• Use Runtime; @SneakyThrows; Unchecked (jool)
• Anti-Patterns: Diaper and Log-Rethrow
• Enum error codes for users or tests
• Global Exception Handler
• Catch-Rethrow-with-Debug
• Defeating NPE with early-throw, or Optional
• Try (vavr)
➔ my 'Functional Design Patterns' talk at Devoxx Belgium

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Left-Over
• Async exceptions (Executors, @Async, CompletableFuture, Reactive-X)
• finally {throw}
• AutoCloseable.close() {throw} –> Suppressed Exceptions – link
• throw before mutating object state
• Don't use exceptions for flow control (as a GOTO statement)
• InterruptedException – link
• Sealed classes + records + Switch Template Matching Java 16+

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