A Pragmatic approach to work with exceptions in Java. Talk recording: https://www.youtube.com/watch?v=LRwCE7GreSM&feature=youtu.be Discussing checked exceptions, Vavr, Lombok, and JooL library, Spring Framework and many more. Supporting in-depth article series: https://victorrentea.ro/blog/exception-handling-guide-in-java/

1. 118 © VictorRentea.ro
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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
I wrote a series of articles with further details on this topic: https://victorrentea.ro/blog/exception-handling-guide-in-java/

2. Victor Rentea
Blog, Talks, Goodies:
VictorRentea.ro
Independent Trainer
Founder of
Bucharest Software Craftsmanship Community
Java Champion
❤️ Simple Design, Refactoring, Unit Testing ❤️

3. Technical Training
400 days
(100+ online)2000 devs8 years
Training for you or your company: VictorRentea.ro
40 companies
Posting
Good Stuff on:
HibernateSpring Functional Prog
Java PerformanceReactive
Design Patterns
Clean Code
Refactoring
Unit Testing any
lang

4. Clean Code
Refactoring
Clean Exception Handling

5. VictorRentea.ro122
A bit of history

6. 123 © VictorRentea.ro
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At the beginning
There were no functions

7. 124 © VictorRentea.ro
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At the beginning
There were no exceptions

8. 125 © VictorRentea.ro
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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

9. 126 © VictorRentea.ro
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We don't Recover
In web apps today, when handling exceptions,This talk is NOT
about library development

10. 127 © VictorRentea.ro
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Checked Exceptions,
today

11. 128 © VictorRentea.ro
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code

12. 129 © VictorRentea.ro
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Diaper Anti-Pattern
} catch (Exception e) {
// TODO waste nights
}
a.k.a. Swallowing Exceptions
Shawarma-style
Exception-Handling
+ Runtime bugs= catches all the sh*t

13. 130 © VictorRentea.ro
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} catch (Exception e) {
// TODO waste nights
}
e.printStackTrace();IDE default code block
System.err
might NOT be captured in your log file!

14. 131 © VictorRentea.ro
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throws
try
catch (Exception t) {/*surprise!*/}
RuntimeExceptions won the War !
because they don’t annoy us
Why should I know
about that IOException ?

15. 132 © VictorRentea.ro
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Ignoring recurrent exception?
(eg. a poller)
log.trace(e.getMessage())
What to do with Checked Exceptions?
} catch (SomeCheckedException e) {
}
throw new SomeRuntimeException(e);
Wrap them
in Runtimes
Dynamicall lower log level via
Spring Actuator or Logback JMX Bean

16. 133 © VictorRentea.ro
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} catch (SomeCheckedException e) {
}
} catch (SomeCheckedException e) {
log.error(e.getMessage(), e);
}
TERROR
What if it slips unlogged ?
Same exception logged multiple times
Log-Rethrow Anti-Pattern
Real Solution
throw new SomeRuntimeException(e);

17. 134 © VictorRentea.ro
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Legacy Code
Global Exception Handler
Logs and reports any unhandled exceptions
Core Logic
Only RuntimeExceptions allowed
Checked -> Runtime
old lib

18. 135 © VictorRentea.ro
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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 values set
Never a Stack Trace: Security
(OK for small apps)

19. 136 © VictorRentea.ro
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code

20. 137 © VictorRentea.ro
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class MyException extends RuntimeException {
public enum ErrorCode {
GENERAL,
PHONE_ALREADY_REGISTERED,
...
}
private final ErrorCode code;
private final Object[] params;
...
}
Translating Exceptions
Any case not useful to users
messages.properties
GENERAL = Oups!
PHONE_ALREADY_REGISTERED = The phone {0} is assigned to another user
Checkatstartup
_fr _ro _es

21. 138 © VictorRentea.ro
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Global Exception Handler
only catch
recoverable exceptions (if any)
Checked -> Runtime

22. 139 © VictorRentea.ro
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Catch-Rethrow-with-Debug Pattern
➢ Check the logs above?
➢ Breakpoints?
Why? To find the value of some key variable
} catch (AEx e) {
throw new BEx("Info " + id, e);
}
Must-Have
Debugging an Exception
AEx(

23. 140 © VictorRentea.ro
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Never Decapitate Your Exceptions !
} catch (SomeEx e) {
throw new AnotherEx("Oops");
}

24. 141 © VictorRentea.ro
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4 Reasons to Catch-rethrow
User-friendly message
(code)
Tests
(code)
Developers
(rethrow with debug message)
Rethrow as Runtime @SneakyThrows
(Lombok)
for use-case fatal errors
} catch (SomeEx e) {
throw new RuntimeException(e);
}

25. 142 © VictorRentea.ro
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The Boss Of All Exceptions

26. 143 © VictorRentea.ro
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27. 145 © VictorRentea.ro
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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

28. 147 © VictorRentea.ro
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We CHARGE against it!
We don't defend against NULL

29. 148 © VictorRentea.ro
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We CHARGE against it!
We don't defend against NULL
Throw early
for constraints
return Optional
for "valid" nulls

30. 149 © VictorRentea.ro
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code

31. 151 © VictorRentea.ro
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Optional<>
Customer.getMemberCard(): Optional<MemberCard>
entity for every nullable column

32. 152 © VictorRentea.ro
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You, defeating the Null
Customer.getMemberCard(): Optional<MemberCard>

33. 153 © VictorRentea.ro
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Java 8

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Java 8 Functional Interfaces
(eg Function, Predicate, ...)
+
checked exceptions
= pain

35. 155 © VictorRentea.ro
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code

36. 156 © VictorRentea.ro
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Higher-order
Functions!
Wrap checked exceptions in lambdas
Function<> Unchecked.function(CheckedFunction<> f)
<dependency>
<groupId>org.jooq</groupId>
<artifactId>jool</artifactId>
</dependency>

37. 157 © VictorRentea.ro
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Try<>
Can hold either a result or an exception
Collect both results and exceptions in a single pass
<dependency>
<groupId>io.vavr</groupId>
<artifactId>vavr</artifactId>
</dependency>

38. 158 © VictorRentea.ro
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Key Points
• Use Runtime; @SneakyThrows; Unchecked for lambdas (jool)
• Anti-Patterns: Diaper, Decapitation, Log-Rethrow
• Global Exception Handler
• Enum error codes for users or tests
• Catch-Rethrow-with-Debug
• Defeating NPE with early-throw, or Optional
• Try (vavr)
Further Reading, details and comments on this topic: https://victorrentea.ro/blog/exception-handling-guide-in-java/

39. 159 © VictorRentea.ro
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Further Reading, details and comments on this topic:
https://victorrentea.ro/blog/exception-handling-guide-in-java/
VictorRentea.ro
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victorrentea@gmail.com
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