Class 文件格式

喜欢ヅ旅行 2021-07-24 15:13 415阅读 0赞

# 一 一个简单的 Java 程序 #

package jvm;
    
    public class T0100_ByteCode01 {
    }

# 二 生成的 class 文件 #

用 IDEA 的插件 BinEd 来查看。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

# 三 反编译后文件 #

//
    // Source code recreated from a .class file by IntelliJ IDEA
    // (powered by Fernflower decompiler)
    //
    
    package jvm;
    
    public class T0100_ByteCode01 {
        public T0100_ByteCode01() {
        }
    }

反编译后，自动生成了注释和无参构造器。

# 四 ClassFileFormat 是什么 #

## 1 二进制字节流 ##

## 2 数据类型：u1 u2 u4 u8 和 \_info（表类型） ##

\_info 的来源是 hotspot 源码中的写法。

## 3 查看 16 进制格式的 ClassFile ##

*  sblime notepad
 *  IDEA 插件-BinEd

# 五 class 文件结构 #

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 2][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 3][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 4][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 5][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 6][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 7][]

# 六 观察字节码的方法 #

**1 javap**

E:\javatest1\src\main\java\jvm>javap -v T0100_ByteCode01.class
    Classfile /E:/javatest1/src/main/java/jvm/T0100_ByteCode01.class
      Last modified 2021-6-13; size 210 bytes
      MD5 checksum 398657a721c13378bd97e268bcf522ae
      Compiled from "T0100_ByteCode01.java"
    public class jvm.T0100_ByteCode01
      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            // jvm/T0100_ByteCode01
       #3 = Class              #12            // java/lang/Object
       #4 = Utf8               <init>
       #5 = Utf8               ()V
       #6 = Utf8               Code
       #7 = Utf8               LineNumberTable
       #8 = Utf8               SourceFile
       #9 = Utf8               T0100_ByteCode01.java
      #10 = NameAndType        #4:#5          // "<init>":()V
      #11 = Utf8               jvm/T0100_ByteCode01
      #12 = Utf8               java/lang/Object
    {
      public jvm.T0100_ByteCode01();
        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 3: 0
    }
    SourceFile: "T0100_ByteCode01.java"

**2 JBE-可以直接修改**

**3 JClassLib-IDEA插件之一**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 8][]

## 七 classfile 构成 ##

ClassFile {
        u4 magic;
        u2 minor_version;
        u2 major_version;
        u2 constant_pool_count;
        cp_info constant_pool[constant_pool_count  - 1];
        u2
        ......
    }

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