Finalizers –The not so Good, the Bad and the Ugly

A Garbage Collection Saga

Gautam Singh www.livemykarma.com @singhkgautam

Fun Day

Work Day

Analyzing Thread Dump

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

Finalizable Object life Cycle

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

Problem Class

Load Test Result (Before)

Load Test Result (After)

Problem with Finalizers

Finalizers are unpredictable, often dangerous, and generally unnecessary

Effective Java – Joshua Bloch

Dead Object Resurrection(Bad)

Dead Object Resurrection(Bad)

Increase Allocation Cost(Bad)

Allocation(Bytes) Per Instance

Around 3X

Cause more GCs(Ugly)

No Guarantee, when called (Ugly)

No Guarantee (Example)

Make object concurrent(Ugly)

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

How to avoid Finalizers Pitfalls

Do you really need finalizer?

Utilize Lifecycle methods, if available

Provide explicit termination method.

Reduce data overhead(Before)

Reduce data overhead(After)

Utilize Cache or Object pool

Image: http://www.birdsandblooms.com/birding/birding-basics/bird-behavior-caching-food/

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

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

Questions?