java字节码文件 helloworld

Java代码

\A.java
public class A{}

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javac A.java \得到 A.class 
javap -v A.class

下面是javap工具帮我们生成的文本的字节码文件

Classfile /home/yao/test/javap/A.class
  Last modified Aug 30, 2016; size 176 bytes
  MD5 checksum f35fe1f44d02cd491e6561a30327ceb5
  Compiled from "A.java"
public class A
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #3.#10         // java/lang/Object."<init>":()V
   #2 = Class              #11            // A
   #3 = Class              #12            // java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               SourceFile
   #9 = Utf8               A.java
  #10 = NameAndType        #4:#5          // "<init>":()V
  #11 = Utf8               A
  #12 = Utf8               java/lang/Object
{
  public A();
    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 1: 0
}

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不过瘾？自己按照java规范翻译一遍～

\通过linux hexdump -C A.class 可以以16进制方式查看文件
00000000  ca fe ba be 00 00 00 34  00 0d 0a 00 03 00 0a 07  |.......4........|
00000010  00 0b 07 00 0c 01 00 06  3c 69 6e 69 74 3e 01 00  |........<init>..|
00000020  03 28 29 56 01 00 04 43  6f 64 65 01 00 0f 4c 69  |.()V...Code...Li|
00000030  6e 65 4e 75 6d 62 65 72  54 61 62 6c 65 01 00 0a  |neNumberTable...|
00000040  53 6f 75 72 63 65 46 69  6c 65 01 00 06 41 2e 6a  |SourceFile...A.j|
00000050  61 76 61 0c 00 04 00 05  01 00 01 41 01 00 10 6a  |ava........A...j|
00000060  61 76 61 2f 6c 61 6e 67  2f 4f 62 6a 65 63 74 00  |ava/lang/Object.|
00000070  21 00 02 00 03 00 00 00  00 00 01 00 01 00 04 00  |!...............|
00000080  05 00 01 00 06 00 00 00  1d 00 01 00 01 00 00 00  |................|
00000090  05 2a b7 00 01 b1 00 00  00 01 00 07 00 00 00 06  |.*..............|
000000a0  00 01 00 00 00 01 00 01  00 08 00 00 00 02 00 09  |................|
000000b0

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下面就按java规范来翻译一下， 
java官方字节码规范 https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.7.24

->ca fe ba be
    magic u4
->00 00 00 34   
    minor_verion u2;major_version u2,jkd8
->00 0d ->13
    constant_pool_count
->#1 0a 00 03 00 0a    
    [CONSTANT_METHOD] u1 tag; u2 class_index; u2 name_and_type_index
->#2 07 00 0b 
    [CONSTANT_CLASS]  u1 tag; u2 name_index
->#3 07 00 0c
    [CONSTANT_CLASS]  u1 tag; u2 name_index
->#4 01 00 06 3c 69 6e 69 74 3e
    [CONSTANT_Utf8]   u1 tag; u2 length; u1[length] literal  <init>
->#5 01 00 03 28 29 56
    [CONSTANT_Utf8]   u1 tag; u2 length; u1[length] literal  ()V
->#6 01 00 04 43 6f 64 65
    [CONSTANT_Utf8]   u1 tag; u2 length; u1[length] literal  Code
->#7 01 00 0f 4c 69 6e 65 4e 75 6d 62 65 72  54 61 62 6c 65
    [CONSTANT_Utf8]   u1 tag; u2 length; u1[length] literal  LineNumberTable
->#8 01 00 0a 53 6f 75 72 63 65 46 69  6c 65
    [CONSTANT_Utf8]   u1 tag; u2 length; u1[length] literal  SourceFile
->#9 01 00 06 41 2e 6a 61 76 61
    [CONSTANT_Utf8]   u1 tag; u2 length; u1[length] literal  A.java
->#10 0c 00 04 00 05
    [CONSTANT_NAEMANDTYPE] u1 tag;u2 name_index;u2 descriptor_index;  00 04-><init> 00 05>()V
->#11 01 00 01 41
    [CONSTANT_Utf8]   u1 tag; u2 length; u1[length] literal  A
->#12 01 00 10 6a 61 76 61 2f 6c 61 6e 67  2f 4f 62 6a 65 63 74
    [CONSTANT_Utf8]   u1 tag; u2 length; u1[length] literal  java/lange/Object
->00 21 
    access flag u2, ACC_PUBLIC,ACC_SUPPER
->00 02
    u2 this_class
->00 03 
    u2 super_class
->00 00 
    u2 interfaces count
->00 00 
    u2 fields count 
->00 01 
    u2 methods count
{ method info #1
->00 01
    u2 access flag  public
->00 04
    u2 name index <init>
->00 05
    u2 descriptor index ()V
->00 01
    u2 attributes_count
->00 06
    u2 attribute_name_index  Code
->00 00 00 1d
    u4 attribute_length
->00 01
    u2 max_stack
->00 01
    u2 max_locals
->00 00 00 05
    u4 code_length
->2a b7 00 01 b1  
    u1 code[length]
->00 00
    u2 exception_table_length; 
->00 01 
    u2 attributes_count
->00 07 
    u2 attributes_name ->  LineNumberTable
->00 00 00 06 
    u4 attributes len
->00 01 
    u2 line_number_table_length
->00 00
    u2 start pc
->00 01 
    u2 linue number
}   
->00 01
    u2 attributes count
->00 08
    u2 attribute_name_index  > SourceFIle
->00 00 00 02
    u4 attribute_length
->00 09
    u1 info[attribute_length] 00 09 -> #9 A.java