Class文件实例

以一段非常简单的java代码为例来分析
java源码：

public class Test16 {
    public static void main(String[] args){
        System.out.println("hello world");
    }
}

调用 javap -verbose Test16.class 命令
class文件：

  Last modified 2015-11-9; size 549 bytes
  MD5 checksum 1e43842e300dc909fc52e655aa582ded
  Compiled from "Test16.java"
public class com.othertest.Test16
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#20         // java/lang/Object."<init>":()V
   #2 = Fieldref           #21.#22        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = String             #23            // hello world
   #4 = Methodref          #24.#25        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #26            // com/othertest/Test16
   #6 = Class              #27            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               LocalVariableTable
  #12 = Utf8               this
  #13 = Utf8               Lcom/othertest/Test16;
  #14 = Utf8               main
  #15 = Utf8               ([Ljava/lang/String;)V
  #16 = Utf8               args
  #17 = Utf8               [Ljava/lang/String;
  #18 = Utf8               SourceFile
  #19 = Utf8               Test16.java
  #20 = NameAndType        #7:#8          // "<init>":()V
  #21 = Class              #28            // java/lang/System
  #22 = NameAndType        #29:#30        // out:Ljava/io/PrintStream;
  #23 = Utf8               hello world
  #24 = Class              #31            // java/io/PrintStream
  #25 = NameAndType        #32:#33        // println:(Ljava/lang/String;)V
  #26 = Utf8               com/othertest/Test16
  #27 = Utf8               java/lang/Object
  #28 = Utf8               java/lang/System
  #29 = Utf8               out
  #30 = Utf8               Ljava/io/PrintStream;
  #31 = Utf8               java/io/PrintStream
  #32 = Utf8               println
  #33 = Utf8               (Ljava/lang/String;)V
{
  public com.othertest.Test16();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 6: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/othertest/Test16;

  public static void main(java.lang.String...);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC, ACC_VARARGS
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String hello world
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 9: 0
        line 10: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  args   [Ljava/lang/String;
}
SourceFile: "Test16.java"

1. 大体结构
1) 魔数(CAFEBABY)
2) class文件版本
3) 常量池（Constant Pool）
4) 实例构造函数
5) 方法：
访问标志（flags）
类索引、父类索引与接口索引集合(this_class 、super_class)
字段表集合（field_info）
方法表集合
属性表结合（包含有Code属性、ConstantValue属性等等）
2. 每个方法都有一个Code属性，其中
stack: 操作数栈需要的深度
local: 局部变量表需要的容量
单位是slot（一个slot等于32bit，在各种类型的数据中，除了long和double占2个slot之外，其余的所有类型都只占1个slot）
3. Java虚拟机的指令是由一个字节长度的、代表着某种操作含义的数字（操作码）以及跟随其后的零至多个代表此操作所需的参数（操作数）而构成。各种字节码指令，e.g.加载指令（iload）、存储指令（istore）、运算指令（iadd）等等。