The document discusses the structure and contents of Java class files. A Java class file contains the compiled bytecode of a Java class and has a specific internal format. It includes sections for the magic number, class file version, constant pool, access flags, superclass and implemented interfaces, fields, methods, and attributes. The constant pool stores strings, numbers, class references and other constants used in the class. Methods are defined by their access flags, code, and other attributes. This internal structure allows class files to be executed by the Java Virtual Machine.

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》