目的拆解分析反编译字节码 解析成人能够理解的结构 ，然后再对字节码文件进一步分析
public class test {
private static int classV =2;
public static void main(String[] args) {
classV =200;
int localV =4;
localV =400;
}
}
看不懂 那就使用人能看的懂的汇编语言查看类文件结构和代码指令。
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 {};
}
-help
–help
-?
-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
}
-public
-protected
-private
-p
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."<init>":()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
}
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."<init>":()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 <init>
#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 <clinit>
#20 = Utf8 SourceFile
#21 = Utf8 test.java
#22 = NameAndType #7:#8 // "<init>":()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."<init>":()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
}
1.1从45开始因为正式发布之前可能 其他版本号可能已经被用了
1.5改为5.0 也是差不多原因 为了表示重要性更名
Corresponding major version 指定版本 和"Supported major versions"兼容范围
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];
}
常量池字节码结构和常量池结构部分对应关系
