1、解析字节码的作用
通过反编译生成的字节码文件，我们可以深入的了解java代码的工作机制。但是，自己分析类文件结构太麻烦了！除了使用第三方的jclasslib工具之外，oracle官方也提供了工具：javap
javap是jdk自带的反解析工具。它的作用就是根据class字节码文件，反解析出当前类对应的code区（字节码指令）、局部变量表、异常表和代码行偏移量映射表、常量池等信息。
通过局部变量表，我们可以查看局部变量的作用域范围、所在槽位等信息，甚至可以看到槽位复用等信息。
2、javac -g操作
解析字节码文件得到的信息中，有些信息（如局部变量表、指令和代码行偏移量映射表、常量池中方法的参数名称等等）需要在使用javac编译成class文件时，指定参数才能输出
比如，你直接javac xx.java，就不会在生成对应的局部变量表等信息，如果你使用javac -g xx.java就可以生成所有相关信息了。如果你使用的eclipse或IDEA，则默认情况下，eclipse、IDEA在编译时会帮你生成局部变量表、指令和代码行偏移量映射表等信息的。
3、javap的用法
javap的用法格式：
javap <options> <classes>其中，classes就是你要反编译的class文件
在命令行中直接输入javap或javap -help可以看到javap的options有如下选项：
这里重组一下：
-help --help -? 输出此用法消息
-version 版本信息，其实是当前javap所在jdk的版本信息，不是class在哪个jdk下生成的。
-public 仅显示公共类和成员
-protected 显示受保护的/公共类和成员
-p -private 显示所有类和成员
-package 显示程序包/受保护的/公共类 和成员 (默认)
-sysinfo 显示正在处理的类的系统信息 (路径, 大小, 日期, MD5 散列,源文件名)
-constants 显示静态最终常量
-s 输出内部类型签名
-l 输出行号和本地变量表
-c 对代码进行反汇编
-v -verbose 输出附加信息（包括行号、本地变量表，反汇编等详细信息）
-classpath <path> 指定查找用户类文件的位置
-cp <path> 指定查找用户类文件的位置
-bootclasspath <path> 覆盖引导类文件的位置
一般常用的是-v -l -c三个选项。
javap -l 会输出行号和本地变量表信息。
javap -c 会对当前class字节码进行反编译生成汇编代码。
javap -v classxx 除了包含-c内容外，还会输出行号、局部变量表信息、常量池等信息。
4、使用举例
java
public class JavapTest {
private int num;
boolean flag;
protected char gender;
public String info;
public static final int COUNTS = 1;
static{
String url = "www.atguigu.com";
}
{
info = "java";
}
public JavapTest(){
}
private JavapTest(boolean flag){
this.flag = flag;
}
private void methodPrivate(){
}
int getNum(int i){
return num + i;
}
protected char showGender(){
return gender;
}
public void showInfo(){
int i = 10;
System.out.println(info + i);
}
}希望输出的信息比较完整的话，使用如下操作：javap -v -p JavapTest.class
对应的内容信息为：
Classfile /C:/Users/songhk/Desktop/2/JavapTest.class
Last modified 2020-9-6; size 1358 bytes
MD5 checksum 526b4a845e4d98180438e4c5781b7e88
Compiled from "JavapTest.java"
public class com.atguigu.java1.JavapTest
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #16.#46 // java/lang/Object."<init>":()V
#2 = String #47 // java
#3 = Fieldref #15.#48 // com/atguigu/java1/JavapTest.info:Ljava/lang/String;
#4 = Fieldref #15.#49 // com/atguigu/java1/JavapTest.flag:Z
#5 = Fieldref #15.#50 // com/atguigu/java1/JavapTest.num:I
#6 = Fieldref #15.#51 // com/atguigu/java1/JavapTest.gender:C
#7 = Fieldref #52.#53 // java/lang/System.out:Ljava/io/PrintStream;
#8 = Class #54 // java/lang/StringBuilder
#9 = Methodref #8.#46 // java/lang/StringBuilder."<init>":()V
#10 = Methodref #8.#55 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#11 = Methodref #8.#56 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#12 = Methodref #8.#57 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#13 = Methodref #58.#59 // java/io/PrintStream.println:(Ljava/lang/String;)V
#14 = String #60 // www.atguigu.com
#15 = Class #61 // com/atguigu/java1/JavapTest
#16 = Class #62 // java/lang/Object
#17 = Utf8 num
#18 = Utf8 I
#19 = Utf8 flag
#20 = Utf8 Z
#21 = Utf8 gender
#22 = Utf8 C
#23 = Utf8 info
#24 = Utf8 Ljava/lang/String;
#25 = Utf8 COUNTS
#26 = Utf8 ConstantValue
#27 = Integer 1
#28 = Utf8 <init>
#29 = Utf8 ()V
#30 = Utf8 Code
#31 = Utf8 LineNumberTable
#32 = Utf8 LocalVariableTable
#33 = Utf8 this
#34 = Utf8 Lcom/atguigu/java1/JavapTest;
#35 = Utf8 (Z)V
#36 = Utf8 methodPrivate
#37 = Utf8 getNum
#38 = Utf8 (I)I
#39 = Utf8 i
#40 = Utf8 showGender
#41 = Utf8 ()C
#42 = Utf8 showInfo
#43 = Utf8 <clinit>
#44 = Utf8 SourceFile
#45 = Utf8 JavapTest.java
#46 = NameAndType #28:#29 // "<init>":()V
#47 = Utf8 java
#48 = NameAndType #23:#24 // info:Ljava/lang/String;
#49 = NameAndType #19:#20 // flag:Z
#50 = NameAndType #17:#18 // num:I
#51 = NameAndType #21:#22 // gender:C
#52 = Class #63 // java/lang/System
#53 = NameAndType #64:#65 // out:Ljava/io/PrintStream;
#54 = Utf8 java/lang/StringBuilder
#55 = NameAndType #66:#67 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#56 = NameAndType #66:#68 // append:(I)Ljava/lang/StringBuilder;
#57 = NameAndType #69:#70 // toString:()Ljava/lang/String;
#58 = Class #71 // java/io/PrintStream
#59 = NameAndType #72:#73 // println:(Ljava/lang/String;)V
#60 = Utf8 www.atguigu.com
#61 = Utf8 com/atguigu/java1/JavapTest
#62 = Utf8 java/lang/Object
#63 = Utf8 java/lang/System
#64 = Utf8 out
#65 = Utf8 Ljava/io/PrintStream;
#66 = Utf8 append
#67 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#68 = Utf8 (I)Ljava/lang/StringBuilder;
#69 = Utf8 toString
#70 = Utf8 ()Ljava/lang/String;
#71 = Utf8 java/io/PrintStream
#72 = Utf8 println
#73 = Utf8 (Ljava/lang/String;)V
{
private int num;
descriptor: I
flags: ACC_PRIVATE
boolean flag;
descriptor: Z
flags:
protected char gender;
descriptor: C
flags: ACC_PROTECTED
public java.lang.String info;
descriptor: Ljava/lang/String;
flags: ACC_PUBLIC
public static final int COUNTS;
descriptor: I
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: int 1
public com.atguigu.java1.JavapTest();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String java
7: putfield #3 // Field info:Ljava/lang/String;
10: return
LineNumberTable:
line 20: 0
line 18: 4
line 22: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Lcom/atguigu/java1/JavapTest;
private com.atguigu.java1.JavapTest(boolean);
descriptor: (Z)V
flags: ACC_PRIVATE
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String java
7: putfield #3 // Field info:Ljava/lang/String;
10: aload_0
11: iload_1
12: putfield #4 // Field flag:Z
15: return
LineNumberTable:
line 23: 0
line 18: 4
line 24: 10
line 25: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this Lcom/atguigu/java1/JavapTest;
0 16 1 flag Z
private void methodPrivate();
descriptor: ()V
flags: ACC_PRIVATE
Code:
stack=0, locals=1, args_size=1
0: return
LineNumberTable:
line 28: 0
LocalVariableTable:
Start Length Slot Name Signature
0 1 0 this Lcom/atguigu/java1/JavapTest;
int getNum(int);
descriptor: (I)I
flags:
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: getfield #5 // Field num:I
4: iload_1
5: iadd
6: ireturn
LineNumberTable:
line 30: 0
LocalVariableTable:
Start Length Slot Name Signature
0 7 0 this Lcom/atguigu/java1/JavapTest;
0 7 1 i I
protected char showGender();
descriptor: ()C
flags: ACC_PROTECTED
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: getfield #6 // Field gender:C
4: ireturn
LineNumberTable:
line 33: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/atguigu/java1/JavapTest;
public void showInfo();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=3, locals=2, args_size=1
0: bipush 10
2: istore_1
3: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
6: new #8 // class java/lang/StringBuilder
9: dup
10: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
13: aload_0
14: getfield #3 // Field info:Ljava/lang/String;
17: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: iload_1
21: invokevirtual #11 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
24: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: return
LineNumberTable:
line 36: 0
line 37: 3
line 38: 30
LocalVariableTable:
Start Length Slot Name Signature
0 31 0 this Lcom/atguigu/java1/JavapTest;
3 28 1 i I
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=1, locals=1, args_size=0
0: ldc #14 // String www.atguigu.com
2: astore_0
3: return
LineNumberTable:
line 15: 0
line 16: 3
LocalVariableTable:
Start Length Slot Name Signature
}
SourceFile: "JavapTest.java"