Brief explanation about JVM, it's bytecode mechanism, its GC and so on. The presentation has lot's of references from the web.

1. JVM Internals
How does your preferred platform works
Luiz Fernando Teston
Summa
feu.teston@gmail.com / @FeuTeston

2. Introduction to (J)VMs
Mimics a Machine or OS
Abstraction
Security
Isolation
...

3. Machine/OS mimics
Loads code
Executes code
Takes care of memory
Manage code for security

4. Machine/OS mimics
Loads code :: Classloader
Executes code
Takes care of memory :: GC
Manage code for security

5. Execute code. How?
Register based
Stack based

6. Register based
Less (byte) code
Works like your processor
More difficult to write a compiler
Easier to execute

7. Register based
getvar R1, "a"
getvar R2, "b"
c = a + b
getvar R3, "c"
add R3, R1, R2

8. Stack based (JVM way)
More (byte) code
Works like your HP Calculator ;-)
Easier to write a compiler
Difficult to execute

9. Stack based
push 'a'
getvar
push ‘b'
c = a + b getvar
add
push 'c'
storevar

10. Java Binary Class Layout

11. Java Binary Class Layout
ClassFile {
u4 magic;
u2 minor_version;
u2 major_version;
u2 constant_pool_count;
cp_info constant_pool[constant_pool_count-1];
u2 access_flags;
u2 this_class;
u2 super_class;
u2 interfaces_count;
u2 interfaces[interfaces_count];
u2 fields_count;
field_info fields[fields_count];
u2 methods_count;
method_info methods[methods_count];
u2 attributes_count;
attribute_info attributes[attributes_count];
}

12. Java Binary Class Layout
method_info {
u2 access_flags;
u2 name_index;
u2 descriptor_index;
u2 attributes_count;
attribute_info attributes[attributes_count];
}

13. Java Binary Class Layout
Code_attribute {
u2 attribute_name_index;
u4 attribute_length;
u2 max_stack;
u2 max_locals;
u4 code_length;
u1 code[code_length];
u2 exception_table_length; {
u2 start_pc;
u2 end_pc;
u2 handler_pc;
u2 catch_type;
}
exception_table[exception_table_length];
u2 attributes_count;
attribute_info attributes[attributes_count];
}

14. Sample source
public class Sample {
public int sum(int a, int b){
return a + b;
}
}

15. Sample bytecode
Compiled from "Sample.java"
public class Sample extends java.lang.Object{
public Sample();
public class Sample { Code:
0: aload_0
1: invokespecial 1; //Method java/lang/
#
public int sum(int a, intObject."<init>":()V
b){
4: return
return a + -c Sample
javap
b;
} public int sum(int, int);
Code:
0: iload_1
} 1: iload_2
2: iadd
3: ireturn
}

16. Assembly,
baby

17. Assembly,
baby
Yes, we have low
level stuff too!

18. Manipulating JVM Assembly

19. Stack bytecode execution?
Here we are talking about the method’s code section
Usually it is a switch statement on the opcode
The stack grow and sink as necessary
The opcode instruction are executed consuming and
producing stack entries

20. Creating JVM bytecode
Java Jasmin
Source ASM
Java
Jython
Source
JRuby Clojure
Scala Groovy
and many others...

21. Creating JVM bytecode
Java Jasmin
Source ASM
Java
Jython
Source
compiles 0xCAFEBABE
JRuby Clojure ...
(JVM bytecode)
Scala Groovy
and many others...

22. Creating JVM bytecode
Java Jasmin
Source ASM
Java
Jython
Source
runtime gen 0xCAFEBABE
JRuby Clojure ...
(JVM bytecode)
Scala Groovy
and many others...

23. Many easy ways of
extending bytecodes
Without ‘brushing bytecodes’:
CGLib
Java Proxies
JavaAssist

24. Many easy ways of
extending bytecodes
Without ‘brushing bytecodes’:
CGLib
Java Proxies They’re easy!
JavaAssist

25. Bytecodes can be...
Generated when compiling a language
Generated at runtime during class loading
Modified at runtime during class loading
Extended at runtime dynamically

26. Bytecodes can be...
Generated when compiling a language
}
Generated at runtime during class loading
Modified at runtime during class loading
JVM
Extended at runtime dynamically
Feature

27. Loading code: Classloaders

28. Classloaders
Loads code in a lazy manner
Can do anything while loading code
Create new code
Manipulate existing code
Hierarchal
Can be isolated

29. Javadoc: classloaders are...
A class loader is an object that is responsible for
loading classes.
Given the binary name of a class, a class loader should
attempt to locate or generate data that constitutes a
definition for the class.
A typical strategy is to transform the name into a file
name and then read a "class file" of that name from a
file system.

30. How classloading
(normally) works
There’s a classloader tree
For each classloader entry:
Search classes inside the classpath directories
Search classes inside jar files in the classpath
Until the class definition is find, searches the child
classloaders

31. Fancy Classloaders
class FancyClassLoader extends ClassLoader {
public Class findClass(String name) {
byte[] b = compileOnTheFly(name + “.fancy”);
return defineClass(name, b, 0, b.length);
}
}

32. Fancy Classloaders
Easy to create,
extend and use!
class FancyClassLoader extends ClassLoader {
public Class findClass(String name) {
byte[] b = compileOnTheFly(name + “.fancy”);
return defineClass(name, b, 0, b.length);
}
}

33. Great Classloader samples
Remote Classloader (RMI)
Think for a minute about the difficulty to build this
without the JVM
Loads classes using network connection
Makes possible to put remote invocation in a next
level

34. Great Classloader samples
AppServer Classloaders per application
Can isolate AppServer classes from the hosted
application
Isolates classes from each application
Makes ease to implement hotdeploy

35. Great Classloader samples
OSGi
Plugin/bundle architecture with lifecycle
Isolates plugins/bundles classes really well
Makes possible to have many versions of the ‘same
class’ on different bundles
Isolate internal classes from plugin/bundle from the
ones who needs to be exported

36. Great Classloader samples
AOP (runtime)
Can implement AOP on the fly (runtime)
Many applications:
Transaction management
Logging
Error tracing

37. Where the garbage
cames from?

38. How to manage memory?
C
malloc/free
C++
new/delete

39. How to manage memory?
}
C
malloc/free
Almost the same...
C++
new/delete

40. Memory fragmentation
Memory as a big byte array...
m[0], m[1] ..... m[MAX]

41. Memory fragmentation
Memory as a big byte array...
m[0], m[1] ..... m[MAX]
x
*x = 1, sizeof(x_type) = 2

42. Memory fragmentation
Memory as a big byte array...
m[0], m[1] ..... m[MAX]
x y
*x = 1, sizeof(x_type) = 2
*y = 1, sizeof(y_type) = 1

43. Memory fragmentation
Memory as a big byte array...
m[0], m[1] ..... m[MAX]
x y z
*x = 1, sizeof(x_type) = 2
*y = 1, sizeof(y_type) = 1
*z = 1, sizeof(z_type) = 6

44. Memory fragmentation
Memory as a big byte array...
m[0], m[1] ..... m[MAX]
x y z
Each item is a index + size.
‘Mallocked’ by C’s runtime.
*x = 1, sizeof(x_type) = 2
*y = 1, sizeof(y_type) = 1
*z = 1, sizeof(z_type) = 6

45. Memory fragmentation
Memory as a big byte array...
WARNING!
m[0], m[1] ..... m[MAX]
Easy to mess with
x y z
Easy to fragment
Each itemEasy to forgot leaks in long runs
is a index + size.
‘Mallocked’ by C’s runtime.
*x = 1, sizeof(x_type) = 2
*y = 1, sizeof(y_type) = 1
*z = 1, sizeof(z_type) = 6

46. Garbage Collection
Possible solution for memory
fragmentation
Memory is no longer managed by
programmer
Runtime takes care of fragmentation
during garbage collecting

47. How GC works?
From time to time it searches from unused
memory
When it find unused memory, it cleans it and it
could defragmentize it
Behavior defined by JVM runtime options + JVM
implementation

48. How GC works?
Parameters includes:
Running in parallel
Intervals per region
Specifying memory sizes for each type of
object
eden
survivor1, survivor 2
PermGen

49. Sample GC-ed environment
eden x y z
survivor 1 a b c
survivor 2 w k l

50. Sample GC-ed environment
eden x y z
survivor 1 a b c
survivor 2 w k l

51. Sample GC-ed environment
eden x z
survivor 1 a b
survivor 2 k l

52. Sample GC-ed environment
eden x z
survivor 1 a b
survivor 2 k l

53. GC in practice
Mark and sweep

54. GC in practice
Mark and sweep

55. GC in practice
Mark and sweep

56. GC in practice
Mark and sweep

57. GC in practice
Copying GC

58. GC in practice
Copying GC

59. GC in practice
Copying GC

60. GC in practice
Copying GC
Copy

61. GC in practice
Copying GC

62. GC in practice
Copying GC

63. Questions? Thanks!
feu.teston@gmail.com / @FeuTeston

64. References on the web
http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/ClassLoader.html
http://java.sun.com/developer/technicalArticles/Networking/classloaders/
http://blog.osgi.org/2011/05/what-you-should-know-about-class.html
http://www.theserverside.com/news/1364680/Understanding-J2EE-Application-Server-ClassLoading-
Architectures
http://www.sidhe.org/~dan/blog/archives/000189.html
http://www.ibm.com/developerworks/ibm/library/it-haggar_bytecode/
http://download.oracle.com/otndocs/jcp/jcfsu-0.1-prc-oth-JSpec/
http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.11
http://asm.ow2.org/doc/tutorial-asm-2.0.html
http://stackoverflow.com/questions/2129044/java-heap-terminology-young-old-and-permanent-generations
http://javarevisited.blogspot.com.br/2011/04/garbage-collection-in-java.html
http://docs.oracle.com/javase/6/docs/technotes/guides/management/jconsole.html
http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html