一、javap是什么？

javap是将字节码进行反编译(与javac对应)，可以查看java编译器为我们生成的字节码。通过它，我们可以对照源代码和字节码，从而了解很多编译器内部的工作。

二、javap的使用

➜ ~ javap -help

用法: javap

其中, 可能的选项包括:

-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 指定查找用户类文件的位置

-cp 指定查找用户类文件的位置

-bootclasspath 覆盖引导类文件的位置

➜ ~ javap -version //输出jdk版本

1.8.0_131

示例代码如下：

1 package com.meituan.mtlearner.cheetah.service.test;

2

3 public class CommonTest {

4 private String name;

5

6 public static void main(String[] args) throws Exception{

7 String a = "222";

8 System.out.println(a);

9 throw new Exception();

10 }

11 }

➜ test javap -v CommonTest //和javap -verbose一样，输出额外信息

警告: 二进制文件CommonTest包含com.meituan.mtlearner.cheetah.service.test.CommonTest

Classfile /Users/shichangmin/workspace/cheetah/target/classes/com/meituan/mtlearner/cheetah/service/test/CommonTest.class

Last modified May 21, 2018; size 727 bytes

MD5 checksum 7110663fa12a58f7d74917d709a67c66

Compiled from "CommonTest.java"

public class com.meituan.mtlearner.cheetah.service.test.CommonTest

minor version: 0

major version: 52

flags: ACC_PUBLIC, ACC_SUPER

Constant pool:

#1 = Methodref #8.#26 // java/lang/Object."":()V

#2 = String #27 // 222

#3 = Fieldref #28.#29 // java/lang/System.out:Ljava/io/PrintStream;

#4 = Methodref #30.#31 // java/io/PrintStream.println:(Ljava/lang/String;)V

#5 = Class #32 // java/lang/Exception

#6 = Methodref #5.#26 // java/lang/Exception."":()V

#7 = Class #33 // com/meituan/mtlearner/cheetah/service/test/CommonTest

#8 = Class #34 // java/lang/Object

#9 = Utf8 name

#10 = Utf8 Ljava/lang/String;

#11 = Utf8

#12 = Utf8 ()V

#13 = Utf8 Code

#14 = Utf8 LineNumberTable

#15 = Utf8 LocalVariableTable

#16 = Utf8 this

#17 = Utf8 Lcom/meituan/mtlearner/cheetah/service/test/CommonTest;

#18 = Utf8 main

#19 = Utf8 ([Ljava/lang/String;)V

#20 = Utf8 args

#21 = Utf8 [Ljava/lang/String;

#22 = Utf8 a

#23 = Utf8 Exceptions

#24 = Utf8 SourceFile

#25 = Utf8 CommonTest.java

#26 = NameAndType #11:#12 // "":()V

#27 = Utf8 222

#28 = Class #35 // java/lang/System

#29 = NameAndType #36:#37 // out:Ljava/io/PrintStream;

#30 = Class #38 // java/io/PrintStream

#31 = NameAndType #39:#40 // println:(Ljava/lang/String;)V

#32 = Utf8 java/lang/Exception

#33 = Utf8 com/meituan/mtlearner/cheetah/service/test/CommonTest

#34 = Utf8 java/lang/Object

#35 = Utf8 java/lang/System

#36 = Utf8 out

#37 = Utf8 Ljava/io/PrintStream;

#38 = Utf8 java/io/PrintStream

#39 = Utf8 println

#40 = Utf8 (Ljava/lang/String;)V

{

public com.meituan.mtlearner.cheetah.service.test.CommonTest();

descriptor: ()V

flags: ACC_PUBLIC

Code:

stack=1, locals=1, args_size=1

0: aload_0

1: invokespecial #1 // Method java/lang/Object."":()V

4: return

LineNumberTable:

line 3: 0

LocalVariableTable:

Start Length Slot Name Signature

0 5 0 this Lcom/meituan/mtlearner/cheetah/service/test/CommonTest;

public static void main(java.lang.String[]) throws java.lang.Exception;

descriptor: ([Ljava/lang/String;)V

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=2, args_size=1

0: ldc #2 // String 222

2: astore_1

3: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;

6: aload_1

7: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V

10: new #5 // class java/lang/Exception

13: dup

14: invokespecial #6 // Method java/lang/Exception."":()V

17: athrow

LineNumberTable: //代码中的行数对应code中的序号，例如示例代码的第7行(String a = "222";)对应Code块中的0号(idc #2 //String 222)

line 7: 0

line 8: 3

line 9: 10

LocalVariableTable: //局部变量表，列出了所有栈帧中的局部变量

Start Length Slot Name Signature

0 18 0 args [Ljava/lang/String;

3 15 1 a Ljava/lang/String;

Exceptions:

throws java.lang.Exception

}

SourceFile: "CommonTest.java"

➜ test javap -l CommonTest

警告: 二进制文件CommonTest包含com.meituan.mtlearner.cheetah.service.test.CommonTest

Compiled from "CommonTest.java"

public class com.meituan.mtlearner.cheetah.service.test.CommonTest {

public com.meituan.mtlearner.cheetah.service.test.CommonTest();

LineNumberTable:

line 3: 0

LocalVariableTable:

Start Length Slot Name Signature

0 5 0 this Lcom/meituan/mtlearner/cheetah/service/test/CommonTest;

public static void main(java.lang.String[]) throws java.lang.Exception;

LineNumberTable:

line 7: 0

line 8: 3

line 9: 10

LocalVariableTable:

Start Length Slot Name Signature

0 18 0 args [Ljava/lang/String;

3 15 1 a Ljava/lang/String;

}

➜ test javap -c CommonTest

警告: 二进制文件CommonTest包含com.meituan.mtlearner.cheetah.service.test.CommonTest

Compiled from "CommonTest.java"

public class com.meituan.mtlearner.cheetah.service.test.CommonTest {

public com.meituan.mtlearner.cheetah.service.test.CommonTest();

Code:

0: aload_0

1: invokespecial #1 // Method java/lang/Object."":()V

4: return

public static void main(java.lang.String[]) throws java.lang.Exception;

Code:

0: ldc #2 // String 222

2: astore_1

3: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;

6: aload_1

7: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V

10: new #5 // class java/lang/Exception

13: dup

14: invokespecial #6 // Method java/lang/Exception."":()V

17: athrow

}

基本上javap -v就包含了所有的反编译信息，如果觉得不够，就增加个help里面的标签就好了。

其实通过javap -v展示的内容不难看出字节码文件的组成：最后修改时间，文件大小、MD5签名、全限定名、版本、访问权限标签、常量池、成员变量信息、方法信息、属性信息等。(http://www.blogjava.net/DLevin/archive/2011/09/05/358033.html)

而每个方法都包含至少三部分：签名、访问权限符、Code，Code中又包含指令集、对应信息、局部变量表。

对照书本上或者一些文档中的字节码文件的组成，通过javap这个小工具来了解字节码文件(.class)更直观一些。