This document discusses an approach called Incinerator for detecting and eliminating stale references in Java applications. Stale references occur when an object is no longer accessible but still referenced. Incinerator transforms stale references to null references during garbage collection to avoid memory leaks. It was implemented in 1000 lines of C++ and tested on an open source OSGi framework, detecting and resolving 10 scenarios of stale references with less than 4% performance overhead.

1. Incinerator
Detecting & Resolving
Stale References
in Java
Koutheir ATTOUCHI1,2, Gaël THOMAS1, André BOTTARO2, Gilles MULLER1
1 Laboratoire d’Informatique de Paris 6, France.
2 Orange Labs, Grenoble, France.
Wednesday 29th of October 2014 1

2. The Smart Home
Multiple Application Domains
§ Security
§ Energy
§ Healthcare
§ Comfort
§ Well being
§ Multimedia
§ Content sharing
§ Games
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3. The Smart Home
Connected devices
Devices are
§ Connected
§ Easy to install
§ Affordable
§ Fridge
§ Thermostat
§ Fork
§ Oven
§ etc.
Connected devices
§ Toothbrush
§ Washing machine
§ Scale
§ Locks
§ Dishwasher
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4. Home Automation Gateway
Offered by a Smart Home Operator
One common embedded platform
§ Hosting applications delivered by various tiers1
Objectives
§ Reduce service
deployment costs
§ Share connected
devices between
tiers
1 Condry et al. (IECON’99); Royon and Frénot (2007)
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5. Hypothesis
Unreliable and long-running applications
§ Applications can be
– Buggy
– Malicious
§ The platform runs for long duration
§ Abrupt restarts of the platform can be dangerous for
– Devices
– Inhabitants
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6. Addressed Issues
Resolve stale references in Java applications
Stale reference
§ A reference to a stale object: an object that became unusable
– Many objects become unusable after updates and uninstallation of
applications
StaRlef Rereefnecreen ce
Object in App1 O1 Object in App2
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O2
App2 uninstalled ! O2 stale

7. Smart Home Alarm Application
Motivation by an example
Gas Sensor
1
Stale
Reference Siren Driver 2.0
GasSensor Driver Siren Driver 1.0
Alarm Application
Siren Configuration
(Format: Simple Text)
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Alarm Siren
(Format: XML)
3
1
2
3
Update Siren Driver è version 2.0
Convert Siren configuration format è XML
Forget to update the reference to Siren Driver 1.0
3
2
1
2
Memory leak
Data corruption
Physical hazard

8. Significant Memory Leak
Caused by Stale References1
O1
C2 C4
ClassLoader(S)
S
1 Lindholm and Yellin (1999); Johnson and Dawson (2014)
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C3
Class(S)
Stale reference
Stale object

9. Significant Memory Leak
Caused by Stale References
Memory leaks caused by stale references appearing after
multiple updates of a buggy application
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10. Incinerator
Approach
§ Detect and eliminate* stale
references
§ Incinerator transforms stale
references to null references
during garbage collection cycle
* The Devil is in the Details…
Incinerator
O1
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O2
S2
S1
O3
O4
Stale reference Stale object

11. Cleaning up resources
in Object.finalize()
Objective:
§ Allow objects to cleanup their
resources
– Execute Object.finalize()
without risking exceptions due
to stale references
§ Incinerator allows unreachable
finalizable objects to hold stale
references
– Defer stale references
elimination to the following
garbage collection cycle
Incinerator
Defer
Unreachable finalizable object
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S1
O3
O4
F1
Stale reference Stale object

12. Synchronization Issues
when Eliminating Stale References
Incinerator
S1
T1 T2 T3
T1, T2 and T3 synchronize and T2 blocked on forever
the monitor of S1
Stale reference Stale object Active thread Blocked thread
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13. Synchronization Handling
when Eliminating Stale References
MMoonniittoorr((SS11)) "= S Sttaalele
S1
Incinerator
Wake up!
T1 T2 T3
! Wake up!
NPE
!
NPE
!
NPE
Stale reference Stale object Active thread Blocked thread
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14. Implementation
§ 1000 lines of C++ in the J3/VMKit1 JVM
§ Tested on Knopflerfish2, an open source recognized
implementation of OSGi
– 10 lines added to Knopflerfish
§ Independent of the garbage collector algorithm
§ Open source
– http://llvm.org/svn/llvm-project/vmkit/branches/incinerator/
1 Geoffray et al. (VEE’10)
2 Makewave AB Corp. (2014)
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15. Functional Evaluation
Micro benchmarks
§ 10 scenarios of stale references, divided by:
– Visibility from the OSGi framework
– Scope of stale references
– Synchronization on stale references
– Finalization of objects holding stale references
§ Stale references detection results
Scenario 1 2 3 4 5 6 7 8 9 10
J3/Hotspot û û û û û û û û û û
Service Coroner1 ü û û û û û û û û û
Incinerator ü ü ü ü ü ü ü ü ü ü
§ Incinerator also eliminates all detected stale references
1 Gama and Donsez (SEAA’08)
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16. Performance Evaluation
DaCapo1 benchmark suite
Low-end computer High-end computer
Average cost in execution time of DaCapo test suites on J3 and on Incinerator
Execution cost < 4%
1 Blackburn et al. (OOPSLA’06)
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17. Conclusion
Incinerator
§ Incinerator detects and eliminates stale references, avoiding:
– Memory leaks
– Data corruption
– Physical hazards
§ And makes stale references visible to Java/OSGi developers
§ Execution cost < 4% è Tolerable in:
– Constrained production environments
– Test environments
§ Implementation is mostly independent of:
– The OSGi framework implementation
– The garbage collection algorithm
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Thank you!