Java学习：详解字节码class文件

背景：通过上一篇查看字节码文件知道如何查看java字节码文件后，可以继续仔细研究下字节码文件的内容，看看里面究竟承载了哪些内容

借用jclasslib插件的一张截图，可以看到字节码文件主要分为6个部分：

一、General Information

Major version： 52 代表 jdk 1.8，51代表 jdk 1.7

Minor version：代表jdk的次版本号

Constant pool count/Interfaces count/Methods count/Fields count/Attributes count ：分别代表常量池数量、继承接口数、方法数量、类字段数量、类属性数量

Access flags：标识该类或接口的访问属性，各种访问属性如下图

二、Constant Pool 常量池

常量池，里面定义了类、方法、接口中涉及到的常量，包括类、方法、字段的名称、类型等等

Interfaces
类（接口）继承的接口
Fields
从常量池中解析出来的类接口字段，常量池中类型为Fieldref的即类的字段
字段举例：

字段的各个属性如下：

cp_info：constantPool_info，表示常量池信息

Name: 字段名称，cp_info_#18对应常量池中的#18，如下图

Descriptor：字段类型，cp_info_#19如下图：

Access flags：字段的访问类型

对应字节码内容

其中Fieldref表明是类的字段，分别看#16和#61是什么
1.#16如下：

引用#76如下：

2.#61如下：

引用#18，#19如下：

三、Methods 方法

从方法表中解析出来的类（接口）中定义的方法
举例：

一个方法主要包括：

Name：方法名称
Descriptor：方法描述符
Access flags：方法访问属性
Code：代码
其中Code包括以下几个属性：
1.LineNumberTable：行号表
start_pc; // 字节码的行号，即偏移量
line_number; // 源码的行号

2.LocalVariableTable：局部变量表，方法中用到的局部变量这里都能看到

start_pc; // 局部变量起始偏移量
length; //局部变量作用范围长度
name_index; //局部变量名称，指向常量池索引
descriptor_index; //局部变量描述符，指向常量池索引
index; //所在局部变量数组的索引位置, 如果为long或double在index和index+1

Attributes
类（接口）的属性

通过javap查看的字节码内容，分为3个部分，jclasslib是对这些信息进行了一些解析整合：
通用信息：

Classfile /Users/kisrosen/05_PrivateProject/MailContent.class
  Last modified 2017-2-3; size 2196 bytes
  MD5 checksum d58da5b7eb14cacb8456ec74b829c9ae
  Compiled from "MailContent.java"
public class com.wow.mailutils.MailContent
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER

常量池：

Constant pool:
   #1 = Methodref          #17.#60        // java/lang/Object."<init>":()V
   #2 = Fieldref           #16.#61        // com/wow/mailutils/MailContent.title:Ljava/lang/String;
   #3 = Fieldref           #16.#62        // com/wow/mailutils/MailContent.toNames:Ljava/util/List;
   #4 = Fieldref           #16.#63        // com/wow/mailutils/MailContent.ccNames:Ljava/util/List;
   #5 = Fieldref           #16.#64        // com/wow/mailutils/MailContent.mailFrom:Ljava/lang/String;
   #6 = Fieldref           #16.#65        // com/wow/mailutils/MailContent.mailBody:Ljava/lang/String;
   #7 = Class              #66            // java/lang/StringBuilder
   #8 = Methodref          #7.#60         // java/lang/StringBuilder."<init>":()V
   #9 = InterfaceMethodref #67.#68        // java/util/List.iterator:()Ljava/util/Iterator;
  #10 = InterfaceMethodref #69.#70        // java/util/Iterator.hasNext:()Z
  #11 = InterfaceMethodref #69.#71        // java/util/Iterator.next:()Ljava/lang/Object;
  #12 = Class              #72            // java/lang/String
  #13 = Methodref          #7.#73         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #14 = String             #74            // ,
  #15 = Methodref          #7.#75         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #16 = Class              #76            // com/wow/mailutils/MailContent
  #17 = Class              #77            // java/lang/Object
  #18 = Utf8               title
  #19 = Utf8               Ljava/lang/String;
  #20 = Utf8               toNames
  #21 = Utf8               Ljava/util/List;
  #22 = Utf8               Signature
  #23 = Utf8               Ljava/util/List<Ljava/lang/String;>;
  #24 = Utf8               ccNames
  #25 = Utf8               mailFrom
  #26 = Utf8               mailBody
  #27 = Utf8               <init>
  #28 = Utf8               ()V
  #29 = Utf8               Code
  #30 = Utf8               LineNumberTable
  #31 = Utf8               LocalVariableTable
  #32 = Utf8               this
  #33 = Utf8               Lcom/wow/mailutils/MailContent;
  #34 = Utf8               getTitle
  #35 = Utf8               ()Ljava/lang/String;
  #36 = Utf8               setTitle
  #37 = Utf8               (Ljava/lang/String;)V
  #38 = Utf8               getToNames
  #39 = Utf8               ()Ljava/util/List;
  #40 = Utf8               ()Ljava/util/List<Ljava/lang/String;>;
  #41 = Utf8               setToNames
  #42 = Utf8               (Ljava/util/List;)V
  #43 = Utf8               LocalVariableTypeTable
  #44 = Utf8               (Ljava/util/List<Ljava/lang/String;>;)V
  #45 = Utf8               getCcNames
  #46 = Utf8               setCcNames
  #47 = Utf8               getMailFrom
  #48 = Utf8               setMailFrom
  #49 = Utf8               getMailBody
  #50 = Utf8               setMailBody
  #51 = Utf8               getToNamesString
  #52 = Utf8               toName
  #53 = Utf8               toNameString
  #54 = Utf8               Ljava/lang/StringBuilder;
  #55 = Utf8               StackMapTable
  #56 = Class              #66            // java/lang/StringBuilder
  #57 = Class              #78            // java/util/Iterator
  #58 = Utf8               SourceFile
  #59 = Utf8               MailContent.java
  #60 = NameAndType        #27:#28        // "<init>":()V
  #61 = NameAndType        #18:#19        // title:Ljava/lang/String;
  #62 = NameAndType        #20:#21        // toNames:Ljava/util/List;
  #63 = NameAndType        #24:#21        // ccNames:Ljava/util/List;
  #64 = NameAndType        #25:#19        // mailFrom:Ljava/lang/String;
  #65 = NameAndType        #26:#19        // mailBody:Ljava/lang/String;
  #66 = Utf8               java/lang/StringBuilder
  #67 = Class              #79            // java/util/List
  #68 = NameAndType        #80:#81        // iterator:()Ljava/util/Iterator;
  #69 = Class              #78            // java/util/Iterator
  #70 = NameAndType        #82:#83        // hasNext:()Z
  #71 = NameAndType        #84:#85        // next:()Ljava/lang/Object;
  #72 = Utf8               java/lang/String
  #73 = NameAndType        #86:#87        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #74 = Utf8               ,
  #75 = NameAndType        #88:#35        // toString:()Ljava/lang/String;
  #76 = Utf8               com/wow/mailutils/MailContent
  #77 = Utf8               java/lang/Object
  #78 = Utf8               java/util/Iterator
  #79 = Utf8               java/util/List
  #80 = Utf8               iterator
  #81 = Utf8               ()Ljava/util/Iterator;
  #82 = Utf8               hasNext
  #83 = Utf8               ()Z
  #84 = Utf8               next
  #85 = Utf8               ()Ljava/lang/Object;
  #86 = Utf8               append
  #87 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #88 = Utf8               toString

方法表：

{
  public com.wow.mailutils.MailContent();
    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 8: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/wow/mailutils/MailContent;

  public java.lang.String getTitle();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #2                  // Field title:Ljava/lang/String;
         4: areturn
      LineNumberTable:
        line 35: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/wow/mailutils/MailContent;

  public void setTitle(java.lang.String);
    descriptor: (Ljava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         2: putfield      #2                  // Field title:Ljava/lang/String;
         5: return
      LineNumberTable:
        line 39: 0
        line 40: 5
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       6     0  this   Lcom/wow/mailutils/MailContent;
            0       6     1 title   Ljava/lang/String;

  public java.util.List<java.lang.String> getToNames();
    descriptor: ()Ljava/util/List;
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #3                  // Field toNames:Ljava/util/List;
         4: areturn
      LineNumberTable:
        line 43: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/wow/mailutils/MailContent;
    Signature: #40                          // ()Ljava/util/List<Ljava/lang/String;>;

  public void setToNames(java.util.List<java.lang.String>);
    descriptor: (Ljava/util/List;)V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         2: putfield      #3                  // Field toNames:Ljava/util/List;
         5: return
      LineNumberTable:
        line 47: 0
        line 48: 5
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       6     0  this   Lcom/wow/mailutils/MailContent;
            0       6     1 toNames   Ljava/util/List;
      LocalVariableTypeTable:
        Start  Length  Slot  Name   Signature
            0       6     1 toNames   Ljava/util/List<Ljava/lang/String;>;
    Signature: #44                          // (Ljava/util/List<Ljava/lang/String;>;)V

  public java.util.List<java.lang.String> getCcNames();
    descriptor: ()Ljava/util/List;
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #4                  // Field ccNames:Ljava/util/List;
         4: areturn
      LineNumberTable:
        line 51: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/wow/mailutils/MailContent;
    Signature: #40                          // ()Ljava/util/List<Ljava/lang/String;>;

  public void setCcNames(java.util.List<java.lang.String>);
    descriptor: (Ljava/util/List;)V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         2: putfield      #4                  // Field ccNames:Ljava/util/List;
         5: return
      LineNumberTable:
        line 55: 0
        line 56: 5
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       6     0  this   Lcom/wow/mailutils/MailContent;
            0       6     1 ccNames   Ljava/util/List;
      LocalVariableTypeTable:
        Start  Length  Slot  Name   Signature
            0       6     1 ccNames   Ljava/util/List<Ljava/lang/String;>;
    Signature: #44                          // (Ljava/util/List<Ljava/lang/String;>;)V

  public java.lang.String getMailFrom();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #5                  // Field mailFrom:Ljava/lang/String;
         4: areturn
      LineNumberTable:
        line 59: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/wow/mailutils/MailContent;

  public void setMailFrom(java.lang.String);
    descriptor: (Ljava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         2: putfield      #5                  // Field mailFrom:Ljava/lang/String;
         5: return
      LineNumberTable:
        line 63: 0
        line 64: 5
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       6     0  this   Lcom/wow/mailutils/MailContent;
            0       6     1 mailFrom   Ljava/lang/String;

  public java.lang.String getMailBody();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #6                  // Field mailBody:Ljava/lang/String;
         4: areturn
      LineNumberTable:
        line 67: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/wow/mailutils/MailContent;

  public void setMailBody(java.lang.String);
    descriptor: (Ljava/lang/String;)V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         2: putfield      #6                  // Field mailBody:Ljava/lang/String;
         5: return
      LineNumberTable:
        line 71: 0
        line 72: 5
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       6     0  this   Lcom/wow/mailutils/MailContent;
            0       6     1 mailBody   Ljava/lang/String;

  public java.lang.String getToNamesString();
    descriptor: ()Ljava/lang/String;
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=4, args_size=1
         0: new           #7                  // class java/lang/StringBuilder
         3: dup
         4: invokespecial #8                  // Method java/lang/StringBuilder."<init>":()V
         7: astore_1
         8: aload_0
         9: getfield      #3                  // Field toNames:Ljava/util/List;
        12: invokeinterface #9,  1            // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
        17: astore_2
        18: aload_2
        19: invokeinterface #10,  1           // InterfaceMethod java/util/Iterator.hasNext:()Z
        24: ifeq          53
        27: aload_2
        28: invokeinterface #11,  1           // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
        33: checkcast     #12                 // class java/lang/String
        36: astore_3
        37: aload_1
        38: aload_3
        39: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        42: pop
        43: aload_1
        44: ldc           #14                 // String ,
        46: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        49: pop
        50: goto          18
        53: aload_1
        54: invokevirtual #15                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        57: areturn
      LineNumberTable:
        line 76: 0
        line 78: 8
        line 80: 37
        line 81: 43
        line 82: 50
        line 84: 53
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           37      13     3 toName   Ljava/lang/String;
            0      58     0  this   Lcom/wow/mailutils/MailContent;
            8      50     1 toNameString   Ljava/lang/StringBuilder;
      StackMapTable: number_of_entries = 2
        frame_type = 253 /* append */
          offset_delta = 18
          locals = [ class java/lang/StringBuilder, class java/util/Iterator ]
        frame_type = 250 /* chop */
          offset_delta = 34
}
SourceFile: "MailContent.java"

四、Fields 字段

字段主要是指类的字段定义，通常会在类的clinit和init方法里初始化

字段主要包含以下类型：

比如常用的LOG，字节码中描述了字段的名字、描述符（引用了常量池），以及访问标志

最后：jclasslib的一个小技巧，code里面点击可以跳转查看说明

最后编辑于：2021.05.11 17:25:23

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Java学习：详解字节码class文件

一、General Information

二、Constant Pool 常量池

三、Methods 方法

四、Fields 字段

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