java如何拆解字节码文件

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源代码

public class test {    private static int classV =2;    public static void main(String[] args) {        classV =200;        int localV =4;        localV =400;    }}

javap 指令和选项

0:无选项

打印package, protected and public fields, and methods public class com.example.test {  public com.example.test();  public static void main(java.lang.String[]);  static {};}

1：辅助指令

-help
-help
-?

2：代码行号和方法的局部变量表

-l

public class com.example.test {  //默认构造方法  public com.example.test();   //代码行号：命令偏移位置    LineNumberTable:      line 3: 0   //局部变量表    LocalVariableTable:      Start  Length  Slot  Name   Signature          0       5     0  this   Lcom/example/test;  public static void main(java.lang.String[]);  //代码行号：命令偏移位置    LineNumberTable:      line 6: 0      line 7: 6      line 8: 8      line 9: 12    //局部变量表    LocalVariableTable:      Start  Length  Slot  Name   Signature          0      13     0  args   [Ljava/lang/String; //方法参数          8       5     1 localV   I。/局部变量localV        //静态代码块  static {};    LineNumberTable:      line 4: 0}

3 用级别过滤方法 属性 类

-public
-protected
-private
-p

4.反汇编 出汇编指令

javap -c

//纯汇编指令public class com.example.test {  public com.example.test();    Code:       0: aload_0       1: invokespecial #1                  // Method java/lang/Object."":()V       4: return  public static void main(java.lang.String[]);    Code:       0: sipush        200       3: putstatic     #2                  // Field classV:I       6: iconst_4       7: istore_1       8: sipush        400      11: istore_1      12: return  static {};    Code:       0: iconst_2       1: putstatic     #2                  // Field classV:I       4: return}

5 显示verbose详细信息

javap -v

Classfile /Users/zhangshanxue/Downloads/akka-quickstart-java/target/classes/com/example/test.class//javap -sysinfo显示下面3行  Last modified 2021-4-5; size 507 bytes  MD5 checksum 24a0c74751aafd61d0f7f69be9c161db  Compiled from "test.java"public class com.example.test//即1.8 对照表和原因见附录1 //u2类型 即每个占用两个字节  minor version: 0  major version: 52    //类标志 见附录2   //u2类型access_flags 通过位与表示多个权限   flags: ACC_PUBLIC, ACC_SUPER //常量池。class文件结构重点  //u2 2两个字节 表示数量//pool 常量池//constant_pool_count 和constant_pool[]表示为常量池中内容//后面大部分内容依赖此常量池Constant pool:   #1 = Methodref          #4.#22         // java/lang/Object."":()V   #2 = Fieldref           #3.#23         // com/example/test.classV:I   #3 = Class              #24            // com/example/test   #4 = Class              #25            // java/lang/Object   #5 = Utf8               classV   #6 = Utf8               I   #7 = Utf8                  #8 = Utf8               ()V   #9 = Utf8               Code  #10 = Utf8               LineNumberTable  #11 = Utf8               LocalVariableTable  #12 = Utf8               this  #13 = Utf8               Lcom/example/test;  #14 = Utf8               main  #15 = Utf8               ([Ljava/lang/String;)V  #16 = Utf8               args  #17 = Utf8               [Ljava/lang/String;  #18 = Utf8               localV  #19 = Utf8                 #20 = Utf8               SourceFile  #21 = Utf8               test.java  #22 = NameAndType        #7:#8          // "":()V  #23 = NameAndType        #5:#6          // classV:I  #24 = Utf8               com/example/test  #25 = Utf8               java/lang/Object{  public com.example.test();    descriptor: ()V    flags: ACC_PUBLIC    Code:      //代码汇编指令      stack=1, locals=1, args_size=1         0: aload_0         1: invokespecial #1                  // Method java/lang/Object."":()V         4: return         //Javap -l显示下面信息          //行号和上面对应的指令偏移位置            LineNumberTable:        line 3: 0      LocalVariableTable:         //局部变量表 在指令偏移位置start start+length之间有效        Start  Length  Slot  Name   Signature            0       5     0  this   Lcom/example/test;  public static void main(java.lang.String[]);    descriptor: ([Ljava/lang/String;)V    flags: ACC_PUBLIC, ACC_STATIC    Code:     //代码汇编指令           stack=1, locals=2, args_size=1         0: sipush        200         3: putstatic     #2                  // Field classV:I         6: iconst_4         7: istore_1         8: sipush        400        11: istore_1        12: return         //Javap -l显示下面信息         //行号和上面对应的指令偏移位置      LineNumberTable:        line 6: 0        line 7: 6        line 8: 8        line 9: 12      LocalVariableTable:        Start  Length  Slot  Name   Signature            0      13     0  args   [Ljava/lang/String;            8       5     1 localV   I  static {};    descriptor: ()V    flags: ACC_STATIC    Code:      stack=1, locals=0, args_size=0         0: iconst_2         1: putstatic     #2                  // Field classV:I         4: return          //行号和上面对应的指令偏移位置        LineNumberTable:        line 4: 0

直接分析字节码块

1为了方便交流表达class文件的结构

使用u1 u2 u4 u8无符号数表示字节数使用*_info 结尾表示池（数组cp_info、field_info、method_info、attribute_info

ClassFile {    //表示java class的文件格式  固定为cafe baby 4个字节        u4             magic;    //主版本号和次版本号共同决定了类文件格式的版本//u2类型 即每个占用两个字节//56包含56以后support for N.0 and N.65535         u2             minor_version;    //主版本号56（java12）之前 minjor只支持0          u2             major_version;    //常量池数量        u2             constant_pool_count;            cp_info        constant_pool[constant_pool_count-1];            u2             access_flags;    //this_class、super_class、interfaces指向常量池的CONSTANT_Class_info        u2             this_class;            u2             super_class;            u2             interfaces_count;            u2             interfaces[interfaces_count];            u2             fields_count;    //指向常量池structures:CONSTANT_Fieldref_info 结构        field_info     fields[fields_count];            u2             methods_count;    //指向常量池CONSTANT_Methodref_info 结构        method_info    methods[methods_count];            u2             attributes_count;            attribute_info attributes[attributes_count];}

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