javap 是 Java 开发工具包（JDK）中的一个工具，用于反编译 Java 字节码文件，以便查看类的结构、方法、字段以及其他相关信息。通过 javap 命令，开发人员可以快速了解 Java 类的内部结构，包括类的成员、访问修饰符、方法签名等信息。
javap <options> <classes>
-help --help -? 输出此用法消息
-version 版本信息
-v -verbose 输出附加信息
-l 输出行号和本地变量表
-public 仅显示公共类和成员
-protected 显示受保护的/公共类和成员
-package 显示程序包/受保护的/公共类和成员 (默认)
-p -private 显示所有类和成员
-c 对代码进行反汇编
-s 输出内部类型签名
-sysinfo 显示正在处理的类的系统信息 (路径, 大小, 日期, MD5 散列)
-constants 显示最终常量
-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
（1）使用 javap classname 命令显示指定类的反编译信息。例如：
C:\Program Files\Java\jdk1.8.0_171\bin> javap Demo20231207193016.class
Compiled from "Demo20231207193016.java"
public class com.huangxin.demo202312.Demo20231207193016 {
public com.huangxin.demo202312.Demo20231207193016();
public static void main(java.lang.String[]) throws java.lang.Exception;
}注意：我将 Demo20231207193016.class 字节码文件拷贝到了 javap 命令所在的目录了。
（2）使用 -c 选项显示反编译后的字节码指令。例如：
C:\Program Files\Java\jdk1.8.0_171\bin> javap -c Demo20231207193016.class
Compiled from "Demo20231207193016.java"
public class com.huangxin.demo202312.Demo20231207193016 {
public com.huangxin.demo202312.Demo20231207193016();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]) throws java.lang.Exception;
Code:
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: invokestatic #3 // Method java/lang/System.currentTimeMillis:()J
6: invokevirtual #4 // Method java/io/PrintStream.println:(J)V
9: ldc2_w #5 // long 1000l
12: invokestatic #7 // Method java/lang/Thread.sleep:(J)V
15: goto 0
}（3）使用 -s 选项显示内部类型签名。例如：
C:\Program Files\Java\jdk1.8.0_171\bin> javap -s Demo20231207193016.class
Compiled from "Demo20231207193016.java"
public class com.huangxin.demo202312.Demo20231207193016 {
public com.huangxin.demo202312.Demo20231207193016();
descriptor: ()V
public static void main(java.lang.String[]) throws java.lang.Exception;
descriptor: ([Ljava/lang/String;)V
}（4）使用 -verbose 选项显示详细的信息，包括类的构造器、方法、字段等。例如：
C:\Program Files\Java\jdk1.8.0_171\bin> javap -verbose Demo20231207193016.class
Classfile /C:/Program Files/Java/jdk1.8.0_171/bin/Demo20231207193016.class
Last modified 2023-12-7; size 738 bytes
MD5 checksum 3ed0670d75604ce43ff4e500139a89ac
Compiled from "Demo20231207193016.java"
public class com.huangxin.demo202312.Demo20231207193016
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #9.#26 // java/lang/Object."<init>":()V
#2 = Fieldref #27.#28 // java/lang/System.out:Ljava/io/PrintStream;
#3 = Methodref #27.#29 // java/lang/System.currentTimeMillis:()J
#4 = Methodref #30.#31 // java/io/PrintStream.println:(J)V
#5 = Long 1000l
#7 = Methodref #32.#33 // java/lang/Thread.sleep:(J)V
#8 = Class #34 // com/huangxin/demo202312/Demo20231207193016
#9 = Class #35 // java/lang/Object
#10 = Utf8 <init>
#11 = Utf8 ()V
#12 = Utf8 Code
#13 = Utf8 LineNumberTable
#14 = Utf8 LocalVariableTable
#15 = Utf8 this
#16 = Utf8 Lcom/huangxin/demo202312/Demo20231207193016;
#17 = Utf8 main
#18 = Utf8 ([Ljava/lang/String;)V
#19 = Utf8 args
#20 = Utf8 [Ljava/lang/String;
#21 = Utf8 StackMapTable
#22 = Utf8 Exceptions
#23 = Class #36 // java/lang/Exception
#24 = Utf8 SourceFile
#25 = Utf8 Demo20231207193016.java
#26 = NameAndType #10:#11 // "<init>":()V
#27 = Class #37 // java/lang/System
#28 = NameAndType #38:#39 // out:Ljava/io/PrintStream;
#29 = NameAndType #40:#41 // currentTimeMillis:()J
#30 = Class #42 // java/io/PrintStream
#31 = NameAndType #43:#44 // println:(J)V
#32 = Class #45 // java/lang/Thread
#33 = NameAndType #46:#44 // sleep:(J)V
#34 = Utf8 com/huangxin/demo202312/Demo20231207193016
#35 = Utf8 java/lang/Object
#36 = Utf8 java/lang/Exception
#37 = Utf8 java/lang/System
#38 = Utf8 out
#39 = Utf8 Ljava/io/PrintStream;
#40 = Utf8 currentTimeMillis
#41 = Utf8 ()J
#42 = Utf8 java/io/PrintStream
#43 = Utf8 println
#44 = Utf8 (J)V
#45 = Utf8 java/lang/Thread
#46 = Utf8 sleep
{
public com.huangxin.demo202312.Demo20231207193016();
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 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/huangxin/demo202312/Demo20231207193016;
public static void main(java.lang.String[]) throws java.lang.Exception;
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: invokestatic #3 // Method java/lang/System.currentTimeMillis:()J
6: invokevirtual #4 // Method java/io/PrintStream.println:(J)V
9: ldc2_w #5 // long 1000l
12: invokestatic #7 // Method java/lang/Thread.sleep:(J)V
15: goto 0
LineNumberTable:
line 11: 0
line 12: 9
LocalVariableTable:
Start Length Slot Name Signature
0 18 0 args [Ljava/lang/String;
StackMapTable: number_of_entries = 1
frame_type = 0 /* same */
Exceptions:
throws java.lang.Exception
}
SourceFile: "Demo20231207193016.java"（5）使用 -classpath 选项指定 javap 工具查找类文件的路径。例如：
C:\Program Files\Java\jdk1.8.0_171\bin>javap -classpath E:\~hx_demo\target\classes com.huangxin.demo202312.Demo20231207193016
Compiled from "Demo20231207193016.java"
public class com.huangxin.demo202312.Demo20231207193016 {
public com.huangxin.demo202312.Demo20231207193016();
public static void main(java.lang.String[]) throws java.lang.Exception;
}
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