Jvm internals 2015 - CorkJUG
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This document provides an introduction to virtual machines and the Java Virtual Machine (JVM). It discusses how the JVM mimics a real machine by loading and executing Java bytecode. It also describes classloaders, garbage collection algorithms, and how code is executed via interpretation or compilation. Examples are provided of how memory is managed dynamically and how the JVM performs just-in-time compilation to native code.
Jvm internals 2015 - CorkJUG
- 1. JVM INTERNALS Introduction toVirtual Machines and the JVM 1 LuizTeston www.fracta.cc www.corkjug.com
- 4. Mimics a Real Machine 4
- 5. Loads and execute code 5
- 7. Java classes are loaded on demand 7
- 8. How they are loaded is up to the classloader 8
- 9. 9
- 10. 10
- 12. 12
- 13. 13
- 15. boot jars LibraryA.jar LibraryB.jar Load class “Main” 15
- 16. boot jars LibraryA.jar LibraryB.jar Load class “Main” Not Here 16
- 17. boot jars LibraryA.jar LibraryB.jar Load class “Main” Not Here Not Here 17
- 18. boot jars LibraryA.jar LibraryB.jar Load class “Main” Not Here Not Here Wait… 18
- 19. boot jars LibraryA.jar LibraryB.jar Load class “Main” Not Here Not Here Not Here run time generated code 19
- 20. boot jars LibraryA.jar LibraryB.jar Load class “Main” Not Here Not Here Not Here run time generated code Here! 20
- 22. INTERPRETING 22
- 23. Line of text AST 23
- 24. Simple example: sum of two numbers 24
- 25. 25
- 27. int variable: jint variable: i 27
- 28. int variable: jint variable: i sum 28
- 29. int variable: jint variable: i sum return 29
- 30. Errors usually are caught at runtime 30
- 31. COMPILING 31
- 32. Code compiled to binary prior to the execution. 32
- 33. Some errors can be caught at compile time 33
- 34. Binary can be: native code,VM bytecode and so on… 34
- 35. Bytecode: Byte sized OPCODE 35
- 37. Works like your processor 37
- 38. Simple example: summing two numbers 38
- 39. R1 R2 RN… 39
- 40. R1 R2 RN… 1 40
- 42. R1 R2 RN… 3 42
- 43. STACK BASEDVM 43
- 44. Works like you HP48G calculator 44
- 45. 45
- 46. 1 46
- 47. 1 2 47
- 48. 1 2 + 48
- 49. 3 49
- 50. JVM IS STACK BASED 50
- 51. Eventually byte code is compiled to native code on the fly 51
- 53. C/C++ Approach: memory as a big array 53
- 55. index size variable 1 1 i 55
- 56. index size variable 1 1 i 2 2 l 56
- 57. index size variable 1 1 i 2 2 l 3 4 c 57
- 58. index size variable 1 1 i 3 4 c 58
- 59. Few caveats for this approach 59
- 61. Possible memory fragmentation 4 sized var doesn’t fit 61
- 63. Possible memory leak used vars unused vars 63
- 65. Possible invalid pointer variable pointing here 65
- 66. JVM based approach: Garbage Collector 66
- 69. 69 VISIBLE REF
- 70. 70
- 71. 71
- 72. 72
- 73. COPY 73
- 74. 74 VISIBLE REF
- 75. 75
- 76. 76
- 77. 77
- 78. 78
- 79. 79
- 80. IN PRACTICE 80
- 84. Eden Survivor 1 Survivor 2 off you goGC 84
- 86. Eden Survivor 1 Survivor 2 GC t ? 86
- 87. Eden Survivor 1 Survivor 2 GC t ? Wait… 87
- 88. Eden Survivor 1 Survivor 2 GC ? t Wait… 88
- 89. Eden Survivor 1 Survivor 2 GC ? t off you go 89
- 90. Eden Survivor 1 Survivor 2 GC ? t t2 90
- 91. Eden Survivor 1 Survivor 2 GC ? t t2 t is still used within t2 91
- 92. Eden Survivor 1 Survivor 2 GC ? t t2 ? is not used… 92
- 93. Eden Survivor 1 Survivor 2 GC ? t t2 Whatever, I have plenty of memory 93
- 94. Few caveats for this approach 94
- 95. GC Wait… GC needs to be properly configured 95
- 96. GC … GC can be unpredictable 96
- 97. GC No control over memory layout 97
- 98. GC No control over memory layout Let me do this job! 98
- 100. GC Having a GC doing the hard work is good. 100
- 101. WHAT ABOUT PERM GEN? 101
- 102. 102 @Deprecated
- 103. HANDS ON 103
- 104. • Create a .java file • Compile it into a .class file • Analyse its binary content
- 105. 105
- 106. 106
- 107. 107
- 108. 108 magic number
- 109. 109 Source code info (can be removed by compilation args)
- 110. 110 Optimisations?
- 111. 111 Yes, we appended to a string, but strings are immutable in Java.
- 113. Time to take a look on the JVM Assembly code 113
- 114. 114
- 115. 115
- 116. 116 main method
- 117. 117 arg to int into a variable
- 120. 120
- 121. 121 Constructor
- 122. 122 can you see the loop?
- 123. 123 can you see the loop?
- 124. 124 add method changing a field
- 125. Going further, let’s look the native assembly code 125
- 126. • Google for java dissablembler plugin. Install it (hdis-i386 or hdis-amd64) • See the native assembly output • Enjoy analysing it
- 128. large iteration count, so it can be JIT compiled
- 130. native code for java.lang.Object constructor
- 132. native code for addTimes
- 133. related to the loop within addTimes
- 134. native assembly
- 135. related bytecode
- 137. native code for add method
- 138. bytecode for add method
- 139. QUESTIONS?
- 140. WHO AM I Luiz Teston • 15 years on the field mostly as a consultant, working on non trivial projects using Java, C++ and functional programming • lteston@fracta.cc • http://fracta.cc • http://linkedin.com/in/teston • http://twitter.com/FeuTeston
- 141. KEEP INTOUCH Feedback about the presentation appreciated. lteston@fracta.cc www.fracta.cc
- 142. REFERENCES • Garbage Collection:Algorithms for Automatic Dynamic Memory Management, by Richard Jones and Rafael Lins. • Virtual Machines:Versatile Platforms for Systems and Processes, by Jim Smith and Ravi Nair. • https://docs.oracle.com/javase/7/docs/api/java/lang/ClassLoader.html • http://www.ibm.com/developerworks/ibm/library/it-haggar_bytecode/ • http://docs.oracle.com/javase/specs/jvms/se8/html/index.html • http://mechanical-sympathy.blogspot.ie/2013/06/printing-generated-assembly-code- from.html • http://www.slideshare.net/CharlesNutter/redev-2011-jvm-jit-for-dummies-what-the-jvm- does-with-your-bytecode-when-youre-not-looking 142