Slides from my talk @ DevoxxUS 2017. Finalizer (the Object.finalize() method) is a construct in Java which is as old as Java itself, still only few are aware of its huge side effects. In this talk we will discuss why finalizers are not always a good choice, its overhead on Garbage Collector and how it can be avoided. Using simple JMH benchmark I will demonstrate the performance overhead of finalizers. We will also talk about my firsthand experience with finalizers where just by removing finalizers I was able to fix major performance issue in a webapp for a large retailer.

1. Finalizers –The not so Good, the Bad
and the Ugly
A Garbage Collection Saga
Gautam Singh
www.livemykarma.com
@singhkgautam

2. Fun Day 

3. Work Day 

4. Analyzing Thread Dump

5. What is Finalizer?
• Method on Object class.
protected void finalize() throws Throwable
• Used to perform postmortem cleanup.
• Called by GC after an object is dead.
• Called by Finalizer thread

6. Finalizable Object life Cycle

7. Benchmark Results
Avg. time(ns) per
Object creation
7X – 600X slow
Avg. Time(ms) spend
in GC
50X – 200X more
Allocation(Bytes) Per
Instance
Around 3X
Benchmarking library: JMH
http://openjdk.java.net/projects/code-tools/jmh/
GitHub Repo: https://github.com/LiveMyKarma/Finalizer

8. Problem Class

9. Load Test Result (Before)

10. Load Test Result (After)

11. Problem with Finalizers
Finalizers are unpredictable, often
dangerous, and generally unnecessary
Effective Java – Joshua Bloch

12. Dead Object Resurrection(Bad)

13. Dead Object Resurrection(Bad)

14. Increase Allocation Cost(Bad)
Allocation(Bytes) Per
Instance
Around 3X

15. Cause more GCs(Ugly)

16. No Guarantee, when called (Ugly)

17. No Guarantee (Example)

18. Make object concurrent(Ugly)

19. When to use Finalizer(Not so Good)
• De-allocation of non-memory resources for
objects
– With hard to predict lifetimes
– For which cleanup is not time-critical
• Safety net when client forget to call explicit
clean-up method.
– Logging

20. How to avoid Finalizers Pitfalls

21. Do you really need finalizer?

22. Utilize Lifecycle methods, if available

23. Provide explicit termination method.

24. Reduce data overhead(Before)

25. Reduce data overhead(After)

26. Utilize Cache or Object pool
Image: http://www.birdsandblooms.com/birding/birding-basics/bird-behavior-caching-food/

27. References
• Effective Java (2nd Edition)
By Joshua Bloch
https://www.amazon.com/Effective-Java-2nd-Joshua-
Bloch/dp/0321356683
• The Garbage Collection Handbook: The Art of
Automatic Memory Management
By Richard Jones, Antony Hosking, Eliot Moss
http://gchandbook.org/
• Benchmark Source
https://github.com/LiveMyKarma/Finalizer

28. I think my eyes are getting better. Instead of a
big dark blur, I see a big light blur.
HAN SOLO, Star Wars Episode VI: Return of the Jedi

29. Questions?