Some research on java class file.

1. Let’s Talk about
Java Class File
Yongqiang Li

2. Outlines
 Brief
Introduction to Java Class File
 Java Class Format and Layout
 References

3. Brief Introduction to Java Class
File
 Has .class extension
 Compiled from java source files (.java)
 Platform-independent
 Machine-readable
 Stores so-called “bytecodes”

4. Java Source File (ClassA.java)
javac ClassA.java
Java Class File (ClassA.class)

5.  Bytecode

6.  Only “javac” can generate class file?
JRuby Clojure
Jython Java Class
Groovy Scala

7. Java Class Format and Layout
 Class File contains different sections.
 Magic Number
 Version of Class File Format
 Constant Pool
 Access Flags
 This Class
 Super Class
 Interfaces
 Fields
 Methods
 Attributes

9. Magic Number
4 bytes
 Always “CAFEBABE” ( 咖啡宝贝 )

10. Version of Class File Format
2 bytes for minor version
 2 bytes for major version
 Java major version starts from 45 (JDK 1.1)
 Java SE 5 is 49, SE 6 is 50 (0x32), SE 7 is 51.

11. Constant Pool
2 bytes used to store constant count
 Index of constant starts from 1 not 0.

12. Tag Byte Constant Type Additional Bytes
0x01 UTF8-String 2+x
0x03 Integer 4 (Big endian)
0x04 Float 4 (IEEE 754 floating-
point)
0x05 Long 8 (Big endian)
0x06 Double 8 (IEEE 754 floating-
point)
0x07 Class Reference 2
0x08 String Reference 2
0x09 Field Reference 2+2 (first 2 for class
Ref)
0x0A Method Reference 2+2 (first 2 for class
Ref)
0x0B Interface Method Ref. 2+2 (first 2 for class
Ref)
0x0C Name and Type Ref. 2+2

13.  Constant Count
0x0021(hex) = 33 (dec)

14.  Double
• 0x06: Double Tag
• 0x4000 1893 74bc 6a7f: 2.012

15. • 0x01: UTF-8 Tag
 UTF-8 • 0x0006: “String” has 6 char.
• 0x5374 7269 6367: “String”

16.  More Strings
 Field/Method
Name
 Field/Method
Type
 Attribute Name

17.  Class Reference
• 0x07: Class Ref Tag
• 0x0002: Constant Index

18. Access Flags
 For Class
 2 Bytes

19. This and Super
 Both are 2 bytes
 Direct to Class Reference in Constant
Pool.
This Ref: 0x0001
Super Ref: 0x0003

20. Interfaces
2 bytes for count of interfaces
 2 * interfaces_count bytes

21. Fields
2 bytes for fields count
 field_info * fields_count

22.  Attribute Structure

23.  Field Access
 2 bytes

24. • 0x0001: Only 1 Field
• 0x0019: public static final
• 0x0005: “obj”
• 0x0006: Descriptor “Ljava/lang/Object;”
• 0x0002: 2 Attributes
• 1st: 0x0007 (Deprecated)
• 2nd: 0x0008 (RuntimeVisibleAnnotations)

25. Methods
2 bytes for methods count
 method_info * methods_count
Method Access Flags

26.  Method Attributes
 CodeAttributes

27. • Max Stack: 2
• Max Locals: 3
• Args Size: 2
• Code:

28.  JVM Opcode
 Based on stack
not register
 About 200
opcode in JVM 6

30.  0: push the integer in
slot 0 to stack
 1: push the integer in
slot 1 to stack
 2: add first 2 integers in
stack and result will be
push into stack
 3: load the integer
from stack top to
integer in slot 2
 4: push the integer in
slot 2 to stack
 5: return an integer

32.  Exception Attributes

33. Who can help me on this?
 Magic Number
 Version of Class File Format
 Constant Pool
 Access Flags
 This Class
 Super Class
 Interfaces
 Fields
 Methods
 Attributes

35. References
 JVM
6 Specification
 《深入浅出 Java 虚拟机》