A brief explanation on how the JVM loads and execute its code is done here. Register and stack based execution are explained, and different garbage collection algorithms are shown using graphs to make it easy to understand what happens under the hood. This talk uses the Java Virtual Machine as a main example, but most of the concepts extends to any modern virtual machine available today.

1. JVM INTERNALS
Introduction toVirtual Machines and the JVM
1
LuizTeston
www.fracta.cc

2. This presentation is available online at
fracta.cc/presentations/jvm
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3. VIRTUAL MACHINES
3

4. Mimics a Real Machine
4

5. Loads and execute code
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6. CLASSLOADERS
6

7. Java classes are loaded on demand
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8. How they are loaded is up to the classloader
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9. 9

10. 10

11. Classloaders are hierarchical
11

12. 12

13. 13

14. boor jars
LibraryA.jar
LibraryB.jar
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15. boor jars
LibraryA.jar
LibraryB.jar
Load class “Main”
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16. boor jars
LibraryA.jar
LibraryB.jar
Load class “Main”
Not Here
16

17. boor jars
LibraryA.jar
LibraryB.jar
Load class “Main”
Not Here
Not Here
17

18. boor jars
LibraryA.jar
LibraryB.jar
Load class “Main”
Not Here
Not Here
Wait…
18

19. boor jars
LibraryA.jar
LibraryB.jar
Load class “Main”
Not Here
Not Here
Not Here
run time
generated code
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20. boor jars
LibraryA.jar
LibraryB.jar
Load class “Main”
Not Here
Not Here
Not Here
run time
generated code
Here!
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21. HOWTO EXECUTE CODE?
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22. INTERPRETING
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23. Line of text AST
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24. Simple example: sum of two numbers
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25. 25

26. int variable: j
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27. int variable: jint variable: i
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28. int variable: jint variable: i
sum
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29. int variable: jint variable: i
sum
return
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30. Errors usually are caught at runtime
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31. COMPILING
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32. Code compiled to binary prior to the execution.
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33. Some errors can be caught at compile time
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34. Binary can be: native code,VM bytecode and so on…
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35. Bytecode: Byte sized OPCODE
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36. REGISTER BASEDVM
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37. Works like your processor
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38. Simple example: summing two numbers
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39. R1 R2 RN…
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40. R1 R2 RN…
1
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41. R1 R2 RN…
1
+2
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42. R1 R2 RN…
3
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43. STACK BASEDVM
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44. Works like you HP48G calculator
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45. 45

46. 1
46

47. 1
2
47

48. 1
2
+
48

49. 3
49

50. JVM IS STACK BASED
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51. Eventually byte code is compiled
to native code on the fly
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52. DYNAMIC MEMORY
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53. C/C++ Approach: memory as a big array
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54. index size variable
54

55. index size variable
1 1 i
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56. index size variable
1 1 i
2 2 l
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57. index size variable
1 1 i
2 2 l
3 4 c
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58. index size variable
1 1 i
3 4 c
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59. Few caveats for this approach
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60. Possible memory fragmentation
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61. Possible memory fragmentation
4 sized var doesn’t fit
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62. Possible memory leak
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63. Possible memory leak
used vars unused vars
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64. Possible invalid pointer
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65. Possible invalid pointer
variable pointing here
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66. JVM based approach: Garbage Collector
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67. Eden
Survivor 1
Survivor 2
GC
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68. Eden
Survivor 1
Survivor 2
GC
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69. Eden
Survivor 1
Survivor 2
Wait…GC
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70. Eden
Survivor 1
Survivor 2
off you goGC
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71. Eden
Survivor 1
Survivor 2
GC
t
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72. Eden
Survivor 1
Survivor 2
GC
t ?
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73. Eden
Survivor 1
Survivor 2
GC
t ?
Wait…
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74. Eden
Survivor 1
Survivor 2
GC
? t
Wait…
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75. Eden
Survivor 1
Survivor 2
GC
? t
off you go
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76. Eden
Survivor 1
Survivor 2
GC
? t
t2
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77. Eden
Survivor 1
Survivor 2
GC
? t
t2
t is still used within t2
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78. Eden
Survivor 1
Survivor 2
GC
? t
t2
? is not used…
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79. Eden
Survivor 1
Survivor 2
GC
? t
t2
Whatever, I have plenty of memory
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80. Few caveats for this approach
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81. GC Wait…
GC needs to be properly configured
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82. GC …
GC can be unpredictable
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83. GC
No control over memory layout
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84. GC
No control over memory layout
Let me do this job!
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85. GC
Managing memory
is hard!
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86. GC
Having a GC doing the
hard work is good.
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87. HANDS ON
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88. • Create a .java file
• Compile it into a .class file
• Analyse its binary content

89. 89

90. 90

91. 91

92. 92
magic number

93. 93
Source code info
(can be removed by
compilation args)

94. 94
Optimisations?

95. 95
Yes, we appended to a string,
but strings are immutable in Java.

96. 96
the addTimes method

97. Time to take a look on the JVM Assembly code
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98. 98

99. 99

100. 100
main method

101. 101
arg to int into a variable

102. 102
new SumArg1Arg2Times

103. 103
StringBuilder optimisation

104. 104

105. 105
Constructor

106. 106
can you see the loop?

107. 107
can you see the loop?

108. 108
add method
changing a field

109. Going further, let’s look the native assembly code
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110. • Google for java dissablembler plugin. Install
it (hdis-i386 or hdis-amd64)
• See the native assembly output
• Enjoy analysing it

112. large iteration count, so
it can be JIT compiled

114. native code for java.lang.Object
constructor

116. native code for addTimes

117. related to the loop within addTimes

118. native assembly

119. related bytecode

121. native code for add method

122. bytecode for add method

123. QUESTIONS?

124. WHO AM I
Luiz Teston
Principle Engineer, CTO at FRACTA
• 15 years on the field, 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

125. KEEP INTOUCH
Feedback about the presentation appreciated.
lteston@fracta.cc
www.fracta.cc

126. 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
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