首先來看一下ClassFile，類註釋如下：

   A JVM class file.
 
   Generic Java classfiles have one additional attribute for classes,
   methods and fields:

    "Signature" (u4 attr-length, u2 signature-index)

 
   A signature gives the full Java type of a method or field. When
   used as a class attribute, it indicates type parameters, followed
 

   by supertype, followed by all interfaces.
   
      methodOrFieldSignature ::= type
      classSignature         ::= [ typeparams ] supertype { interfacetype }
  
   The type syntax in signatures is extended as follows:
   
      type       ::= ... | classtype | methodtype | typevar
      classtype  ::= classsig { ‘.‘ classsig }
 

      classig    ::= ‘L‘ name [typeargs] ‘;‘
      methodtype ::= [ typeparams ] ‘(‘ { type } ‘)‘ type
      typevar    ::= ‘T‘ name ‘;‘
      typeargs   ::= ‘<‘ type { type } ‘>‘
      typeparams ::= ‘<‘ typeparam { typeparam } ‘>‘
      typeparam  ::= name ‘:‘ type
   
   This class defines constants used in class files as well
 

   as routines to convert between internal ``.‘‘ and external ``/‘‘
   separators in class names.

一些常量定義（由虛擬機嚴格規定）如下：

public final static int JAVA_MAGIC = 0xCAFEBABE;  // 魔數

    // 常量池中14種常量項
    public final static int CONSTANT_Utf8 = 1;
    public final static int CONSTANT_Unicode = 2;
    public final static int CONSTANT_Integer = 3;
    public final static int CONSTANT_Float = 4;
    public final static int CONSTANT_Long = 5;
    public final static int CONSTANT_Double = 6;
    public final static int CONSTANT_Class = 7;
    public final static int CONSTANT_String = 8;
    public final static int CONSTANT_Fieldref = 9;
    public final static int CONSTANT_Methodref = 10;
    public final static int CONSTANT_InterfaceMethodref = 11;
    public final static int CONSTANT_NameandType = 12;
    public final static int CONSTANT_MethodHandle = 15;
    public final static int CONSTANT_MethodType = 16;
    public final static int CONSTANT_InvokeDynamic = 18;

    public final static int MAX_PARAMETERS = 0xff; // max_parameters
    public final static int MAX_DIMENSIONS = 0xff; // max_dimensions
    public final static int MAX_CODE = 0xffff; // max_code
    public final static int MAX_LOCALS = 0xffff; // max_locals
    public final static int MAX_STACK = 0xffff; // max_stack

ClassReader類的註釋如下：

 This class provides operations to read a classfile into an internal
   representation. The internal representation is anchored in (立足於) a
   ClassSymbol which contains in its scope symbol representations
   for all other definitions in the classfile.
 
   Top-level Classes themselves appear as members of the scopes of PackageSymbols.
   
  Error Diagnoses
  Buffer Access
  Constant Pool Access
  Reading Types
  Reading Attributes
  Reading Java-language annotations
  Reading Symbols
  Adjusting flags
  Loading Classes
  Loading Packages

參考文章：

http://blog.csdn.net/column/details/jvm-principle.html

Java類文件結構及javac的ClassReader類解讀