Unsafe Java
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The document discusses the unsafe capabilities provided by the sun.misc.Unsafe class in Java, which allows accessing low-level functionality normally not accessible in the Java language like direct memory access, locking, serialization, and more. While it provides powerful capabilities, sun.misc.Unsafe should be avoided in production code as it depends on specific JVM implementations and could crash the JVM. It presents examples showing how to use Unsafe for fast serialization, lock-free data structures, and off-heap memory, but cautions that it is difficult to use correctly and problems like ABA could occur with lock-free code.
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Unsafe Java
- 1. Unsafe Java Misha Kozik @mishadoff
- 3. Java is Safe! ● No pointers ● No direct memory allocation ● Compile-time checking ● Platform-independent types ● Checked Exceptions ● Garbage collection ● etc.
- 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
- 8. “Trusted” code with bootloader java -Xbootclasspath:/usr/jdk1.7.0/jre/lib/rt.jar:. com.mishadoff.magic.UnsafeClient
- 9. Reflection
- 10. Usage
- 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
- 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
- 30. Concurrency: Stupid Counter Counter result: 99542945 Time passed in ms: 679
- 32. Concurrency: Sync Counter Counter result: 100000000 Time passed in ms: 10136
- 34. Concurrency: Lock Counter Counter result: 100000000 Time passed in ms: 8065
- 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
- 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?