FTD JVM Internals
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The document discusses the Java Virtual Machine (JVM) and its key components. It explains that the JVM is a virtual computing environment that executes Java bytecode. The bytecode is executed by the JVM using an operand stack. The document also discusses how objects and classes are represented in memory, including details on object headers, class file formats, and the classloading process. It concludes by mentioning the jdb debugging tool for debugging Java programs without an IDE.
![Java Code Java Bytecode Expression
double d[][][]; [[[D
Object mymethod(int I, double d, Thread t) (IDLjava/lang/Thread;)Ljava/lang/Object;](https://image.slidesharecdn.com/ftd-jvm-internals-141017122753-conversion-gate02/85/FTD-JVM-Internals-18-320.jpg)
![String explanation
Field Type Size (bytes)
HEADER 16
value “Java” char[] (16 + 8) + (4 * 2) = 32
offset int 4
count int 4
hash int 4
PADDING = 60 + 4
TOTAL 64 bytes](https://image.slidesharecdn.com/ftd-jvm-internals-141017122753-conversion-gate02/85/FTD-JVM-Internals-29-320.jpg)
![Class File Format
ClassFile {
u4 magic;
u2 minor_version;
u2 major_version;
u2 constant_pool_count;
cp_info contant_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];
}](https://image.slidesharecdn.com/ftd-jvm-internals-141017122753-conversion-gate02/85/FTD-JVM-Internals-33-320.jpg)
![JVM initialization order
1. Create a initial class using Bootstrap
classloader;
2. links the initial class;
3. initializes it;
4. and invokes the public class method void
main(String[])](https://image.slidesharecdn.com/ftd-jvm-internals-141017122753-conversion-gate02/85/FTD-JVM-Internals-36-320.jpg)
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FTD JVM Internals
- 1. JVM Internals FTD Group
- 2. acknowledgment ● God ● iMasters ● FTD Group
- 3. who we are? FTD Group http://friendstechday.wordpress.com/about/
- 5. Java Virtual Machine ● The Java Virtual Machine Specification ● Written in C/C++ ● Stack based VM ● Different implementations ● Symbolic reference ● Garbage Collected ● Java Bytecode ● JIT (Just-In-Time)
- 8. 1 + 2 = ?
- 9. (+ 1 2) Lisp? RPN?
- 10. operand stack PUSH 1 1 1 PUSH 2 2 ADD 3
- 11. operand stack - bytecode iconst_1 1 1 iconst_2 2 iadd 3
- 12. Java bytecode is the instruction set of the Java virtual machine opcode = 1 or 2 bytes 256 possible opcodes 198 are currently in use 51 are reserved for future use 3 are set aside as permanently unimplemented list of all opcodes: http://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html
- 13. variable declaration int i; 0: iconst_0 // push 0 to top of the operand stack 1: istore_1 // Pop value from top of operand stack and store as local variable 1
- 14. object instantiation Object ftd = new Object(); 0: new #2 // Class java/lang/Object 1: dup // duplicate the value on top of the stack 2: invokespecial #3 // Method java/lang/Object “<init>()V”
- 15. #1, #4, #100? #Wat?
- 16. Runtime Constant Pool The constant pool contains the following types: ● Integer, Long, Float, Double ● String, Uft8 ● Class ● NameAndType ● Fieldref, Methodref, InterfaceMethodref
- 17. Java Bytecode Type Description B byte signed byte C char Unicode character D double double-precision floating-point value F float single-precision floating-point value I int integer J long long integer L<classname> reference an instance of class <classname> S short signed short Z boolean true or false [ reference one array dimension
- 18. Java Code Java Bytecode Expression double d[][][]; [[[D Object mymethod(int I, double d, Thread t) (IDLjava/lang/Thread;)Ljava/lang/Object;
- 20. Not only Java Stack lives, but also Heap.
- 21. Java Heap memory
- 22. Code Cache used for compilation and storage of methods that have been compiled to native code by the JIT compiler
- 23. if >= java1.8 no PermGen yes Metaspace
- 24. everything is wonderful in Java?.... it depends bro!
- 25. which the size of a Java String with "Java" content? String java = “Java”;
- 26. answer: 64 bytes Ahh… how.. WATTT???
- 27. Java memory consumption 8 bytes Object Header 8 bytes Object reference (x86 may use XX:+UseCompressedOops) 8 bytes alignment (WAT?)
- 28. data alignment 5 bytes used 3 bytes paddind 8 bytes alignment
- 29. String explanation Field Type Size (bytes) HEADER 16 value “Java” char[] (16 + 8) + (4 * 2) = 32 offset int 4 count int 4 hash int 4 PADDING = 60 + 4 TOTAL 64 bytes
- 30. which the size of a java. lang.Boolean? Boolean b = false;
- 31. answer: 24 bytes Ahh… how.. WATTT???
- 32. Ok .. But what about classes? OOP?
- 33. Class File Format ClassFile { u4 magic; u2 minor_version; u2 major_version; u2 constant_pool_count; cp_info contant_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]; }
- 35. And what about the Classloader?
- 36. JVM initialization order 1. Create a initial class using Bootstrap classloader; 2. links the initial class; 3. initializes it; 4. and invokes the public class method void main(String[])
- 37. find, check headers class file, super loading, binary created 3 steps, verifying and preparing the type and its direct supers structurally correct, and obeys the semantic requirements allocation of memory foe data structures (optional) checking corrected symbolic references executing initialization method <clinit>
- 39. Java debug without ide? Is it possible?
- 40. jdb (included in the JDK 1.2 package)
- 41. THANK YOU! FTD Group JOIN US! “And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what {is} that good, and acceptable, and perfect, will of God.” (Romans 12:2)