Overview of sun.misc.Unsafe class. Fun, dirty and maybe some practical examples.

1. Unsafe Java
Misha Kozik
@mishadoff

2. Average C developer

3. Java is Safe!
● No pointers
● No direct memory allocation
● Compile-time checking
● Platform-independent types
● Checked Exceptions
● Garbage collection
● etc.

4. Not really...

5. Not really...
sun.misc.Unsafe

6. “API” Overview
Info Synchronization
● addressSize ● monitorEnter
● pageSize ● tryMonitorEnter
● monitorExit
Objects ● compareAndSwapInt
● allocateInstance
● putOrderedInt
● objectFieldOffset
● park
● unpark
Classes
● staticFieldOffset Memory
● defineClass ● allocateMemory
● defineAnonymousClass ● copyMemory
● ensureClassInitialized ● freeMemory
● getAddress
Arrays ● getInt
● arrayBaseOffset ● putInt
● arrayIndexScale
TOTAL: 105

7. Instantiation

8. “Trusted” code with bootloader
java -Xbootclasspath:/usr/jdk1.7.0/jre/lib/rt.jar:.
com.mishadoff.magic.UnsafeClient

9. Reflection

10. Usage

11. Avoid Initialization

12. Memory Corruption

13. #define TRUE FALSE

14. Very Unchecked Cast

15. Dynamic Classes

16. sizeOf

17. sizeOf

18. Shallow Copy

19. Hide Password

20. Throw Exception

21. Fast Serialization
● Serializable (so slooooow)
● Externalizable (requires schema)
● protobuf (dependency)
● kryo (dependency)
● Unsafe

22. Fast Serialization: Write

23. Fast Serialization: Read

24. Fast Serialization: Performance
Benchmarks: https://github.com/eishay/jvm-serializers/wiki

25. Super Array

26. Off-heap Collections
● Not limited to Heap capacity
● Not limited to Integer.MAX_VALUE size
● No boundary checks
● Not under GC management
● Partially available at java.nio.*
● Useful for math computations
● Realtime programming
“The price of greatness is responsibility.”
– Winston Churchill

27. Concurrency

28. Concurrency: Client

29. Concurrency: Stupid Counter

30. Concurrency: Stupid Counter
Counter result: 99542945
Time passed in ms: 679

31. Concurrency: Sync Counter

32. Concurrency: Sync Counter
Counter result: 100000000
Time passed in ms: 10136

33. Concurrency: Lock Counter

34. Concurrency: Lock Counter
Counter result: 100000000
Time passed in ms: 8065

35. Concurrency: Atomic Counter

36. Concurrency: Atomic Counter
Counter result: 100000000
Time passed in ms: 6552

37. Concurrency: CAS Counter
CAS = Compare And Swap

38. Concurrency: CAS Counter
CAS = Compare And Swap
Counter result: 100000000
Time passed in ms: 6454

39. Lock-free Data Structures
Intuition
● Create a copy of current state
● Modify it
● CAS
● Repeat if it fails
Problems
● Hard to implement.
● Problem ABA (DCAS, LL/SC)
● Reordering (Memory Barriers)

40. And finally...

41. Unsafe.park()
“Block current thread, returning when a balancing
unpark occurs, or a balancing unpark has already
occurred, or the thread is interrupted, or, if not
absolute and time is not zero, the given time
nanoseconds have elapsed, or if absolute, the given
deadline in milliseconds since Epoch has passed, or
spuriously (i.e., returning for no "reason"). Note:
This operation is in the Unsafe class only because
unpark is, so it would be strange to place it
elsewhere.”
– Javadoc

42. Disclaimer
● May be removed/changed
● Not portable
● Depends on architecture
● Depends on JVM implementation
● Depends on major/minor JVM
● Never use it in production
Gentle reminder:
One error may cause
JVM crash

43. Projects
Project Lambda
Project Jigsaw

44. Links
● Javadoc http://www.docjar.com/docs/api/sun/misc/Unsafe.html
● My Blog http://mishadoff.github.com/blog/java-magic-part-4-sun-dot-misc-dot-unsafe/
● Tricks with Direct Memory Access
http://highlyscalable.wordpress.com/2012/02/02/direct-memory-access-in-java/
● StackOverflow http://stackoverflow.com/questions/5574241/interesting-uses-of-sun-misc-unsafe
● Native C/C++ Like Performance For Java Object Serialisation
http://mechanical-sympathy.blogspot.de/2012/07/native-cc-like-performance-for-java.html
● Big Arrays in Java http://www.omsn.de/blog/big-arrays-in-java
● Lock-Free Wait-Free Hash Table
http://www.azulsystems.com/about_us/presentations/lock-free-hash
● Java theory and practice: Going atomic
www.ibm.com/developerworks/java/library/j-jtp11234/
● Fast multipurpose serialization (RU) http://www.javaspecialist.ru/2012/07/blog-post.html

45. Thanks!
Q?