一：簡介

javap是JDK自帶的反匯編器，可以查看java編譯器為我們生成的字節碼。通過它，我們可以對照源代碼和字節碼，從而了解很多編譯器內部的工作。

javap命令的常用參數有：

-l 打印行和本地變量表

-public 只顯示公共類和成員變量

-protected? 只顯示公共的、受保護的類和變量

-private? 顯示所有的類和成員變量

-p? 顯示所有的類和成員變量

-Joption? ?給jvm指定參數,例如:

javap -J-version

javap -J-Djava.security.manager -J-Djava.security.policy=MyPolicy

-s 打印內部類型簽名(用于泛型)

-sysinfo 打印被處理類的系統信息(路徑,大小,日志,MD5哈希)

-constants 顯示static final常量

-c 輸出分解后的代碼。例如，類中每一個方法內，包含java字節碼的指令

-verbose? 輸出棧大小，變量(locals)和方法參數的個數

-classpath path 指定查詢類的路徑.如果設置該值,則會覆蓋默認值和CLASSPATH環境變量。

-bootclasspath path 指定啟動類的路徑,默認值是jre/lib/rt.jar 和 其他少數的jar.

-extdir dirs? 覆蓋 java.ext.dirs路徑

二：實例

1.Java源代碼

public?class?Demo?{

private?String?name?=?"張三";

public?static?void?main(String?args[])?{

System.out.println("Hello?World.");

}

private?void?sayHello()?{

System.out.println("Hello?"?+?name);

}

}

2.javac編譯后的字節碼

CA?FE?BA?BE?00?00?00?34??00?36?0A?00?0D?00?1E?08

00?1F?09?00?0C?00?20?09??00?21?00?22?08?00?23?0A

00?24?00?25?07?00?26?0A??00?07?00?1E?08?00?27?0A

00?07?00?28?0A?00?07?00??29?07?00?2A?07?00?2B?01

00?04?6E?61?6D?65?01?00??12?4C?6A?61?76?61?2F?6C

61?6E?67?2F?53?74?72?69??6E?67?3B?01?00?06?3C?69

6E?69?74?3E?01?00?03?28??29?56?01?00?04?43?6F?64

65?01?00?0F?4C?69?6E?65??4E?75?6D?62?65?72?54?61

62?6C?65?01?00?12?4C?6F??63?61?6C?56?61?72?69?61

62?6C?65?54?61?62?6C?65??01?00?04?74?68?69?73?01

00?28?4C?63?6F?6D?2F?62??72?69?6C?6C?69?61?6E?74

73?74?61?72?2F?63?69?6D??63?2F?69?6D?2F?64?65?73

6B?74?6F?70?2F?44?65?6D??6F?3B?01?00?04?6D?61?69

6E?01?00?16?28?5B?4C?6A??61?76?61?2F?6C?61?6E?67

2F?53?74?72?69?6E?67?3B??29?56?01?00?04?61?72?67

73?01?00?13?5B?4C?6A?61??76?61?2F?6C?61?6E?67?2F

53?74?72?69?6E?67?3B?01??00?08?73?61?79?48?65?6C

6C?6F?01?00?0A?53?6F?75??72?63?65?46?69?6C?65?01

00?09?44?65?6D?6F?2E?6A??61?76?61?0C?00?10?00?11

01?00?06?E5?BC?A0?E4?B8??89?0C?00?0E?00?0F?07?00

2C?0C?00?2D?00?2E?01?00??0C?48?65?6C?6C?6F?20?57

6F?72?6C?64?2E?07?00?2F??0C?00?30?00?31?01?00?17

6A?61?76?61?2F?6C?61?6E??67?2F?53?74?72?69?6E?67

42?75?69?6C?64?65?72?01??00?06?48?65?6C?6C?6F?20

0C?00?32?00?33?0C?00?34??00?35?01?00?26?63?6F?6D

2F?62?72?69?6C?6C?69?61??6E?74?73?74?61?72?2F?63

69?6D?63?2F?69?6D?2F?64??65?73?6B?74?6F?70?2F?44

65?6D?6F?01?00?10?6A?61??76?61?2F?6C?61?6E?67?2F

4F?62?6A?65?63?74?01?00??10?6A?61?76?61?2F?6C?61

6E?67?2F?53?79?73?74?65??6D?01?00?03?6F?75?74?01

00?15?4C?6A?61?76?61?2F??69?6F?2F?50?72?69?6E?74

53?74?72?65?61?6D?3B?01??00?13?6A?61?76?61?2F?69

6F?2F?50?72?69?6E?74?53??74?72?65?61?6D?01?00?07

70?72?69?6E?74?6C?6E?01??00?15?28?4C?6A?61?76?61

2F?6C?61?6E?67?2F?53?74??72?69?6E?67?3B?29?56?01

00?06?61?70?70?65?6E?64??01?00?2D?28?4C?6A?61?76

61?2F?6C?61?6E?67?2F?53??74?72?69?6E?67?3B?29?4C

6A?61?76?61?2F?6C?61?6E??67?2F?53?74?72?69?6E?67

42?75?69?6C?64?65?72?3B??01?00?08?74?6F?53?74?72

69?6E?67?01?00?14?28?29??4C?6A?61?76?61?2F?6C?61

6E?67?2F?53?74?72?69?6E??67?3B?00?21?00?0C?00?0D

00?00?00?01?00?02?00?0E??00?0F?00?00?00?03?00?01

00?10?00?11?00?01?00?12??00?00?00?39?00?02?00?01

00?00?00?0B?2A?B7?00?01??2A?12?02?B5?00?03?B1?00

00?00?02?00?13?00?00?00??0A?00?02?00?00?00?13?00

04?00?14?00?14?00?00?00??0C?00?01?00?00?00?0B?00

15?00?16?00?00?00?09?00??17?00?18?00?01?00?12?00

00?00?37?00?02?00?01?00??00?00?09?B2?00?04?12?05

B6?00?06?B1?00?00?00?02??00?13?00?00?00?0A?00?02

00?00?00?17?00?08?00?18??00?14?00?00?00?0C?00?01

00?00?00?09?00?19?00?1A??00?00?00?02?00?1B?00?11

00?01?00?12?00?00?00?4B??00?03?00?01?00?00?00?1D

B2?00?04?BB?00?07?59?B7??00?08?12?09?B6?00?0A?2A

B4?00?03?B6?00?0A?B6?00??0B?B6?00?06?B1?00?00?00

02?00?13?00?00?00?0A?00??02?00?00?00?1B?00?1C?00

1C?00?14?00?00?00?0C?00??01?00?00?00?1D?00?15?00

16?00?00?00?01?00?1C?00??00?00?02?00?1D

3.javap返匯編后的內容

javap -p -v Demo

默認情況下javap會打印所有非私有的字段和方法，當加了-p選項后，它還將打印私有的字段和方法；-v 它盡可能地打印所有信息。如果只需要查詢方法對應的字節碼，那么可以用-c代替-v.

=================基本信息(涵蓋了原class文件的相關信息)=======================

Classfile?/D:/workspace/donkey/cimc-im-desktop/target/classes/com/brilliantstar/cimc/im/desktop/Demo.class

Last?modified?2018-9-6;?size?909?bytes

MD5?checksum?0f9cd841c4a2ab1f6a00163e55ca6e0f

Compiled?from?"Demo.java"

public?class?com.brilliantstar.cimc.im.desktop.Demo

minor?version:?0

major?version:?52

flags:?ACC_PUBLIC,?ACC_SUPER

=================常量池(用來存放各種常量及符號引用)=======================

Constant?pool:

#1?=?Methodref??????????#13.#30????????//?java/lang/Object."":()V

#2?=?String?????????????#31????????????//?張三

#3?=?Fieldref???????????#12.#32????????//?com/brilliantstar/cimc/im/desktop/Demo.name:Ljava/lang/String;

#4?=?Fieldref???????????#33.#34????????//?java/lang/System.out:Ljava/io/PrintStream;

#5?=?String?????????????#35????????????//?Hello?World.

#6?=?Methodref??????????#36.#37????????//?java/io/PrintStream.println:(Ljava/lang/String;)V

#7?=?Class??????????????#38????????????//?java/lang/StringBuilder

#8?=?Methodref??????????#7.#30?????????//?java/lang/StringBuilder."":()V

#9?=?String?????????????#39????????????//?Hello

#10?=?Methodref??????????#7.#40?????????//?java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

#11?=?Methodref??????????#7.#41?????????//?java/lang/StringBuilder.toString:()Ljava/lang/String;

#12?=?Class??????????????#42????????????//?com/brilliantstar/cimc/im/desktop/Demo

#13?=?Class??????????????#43????????????//?java/lang/Object

#14?=?Utf8???????????????name

#15?=?Utf8???????????????Ljava/lang/String;

#16?=?Utf8???????????????

#17?=?Utf8???????????????()V

#18?=?Utf8???????????????Code

#19?=?Utf8???????????????LineNumberTable

#20?=?Utf8???????????????LocalVariableTable

#21?=?Utf8???????????????this

#22?=?Utf8???????????????Lcom/brilliantstar/cimc/im/desktop/Demo;

#23?=?Utf8???????????????main

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

#25?=?Utf8???????????????args

#26?=?Utf8???????????????[Ljava/lang/String;

#27?=?Utf8???????????????sayHello

#28?=?Utf8???????????????SourceFile

#29?=?Utf8???????????????Demo.java

#30?=?NameAndType????????#16:#17????????//?"":()V

#31?=?Utf8???????????????張三

#32?=?NameAndType????????#14:#15????????//?name:Ljava/lang/String;

#33?=?Class??????????????#44????????????//?java/lang/System

#34?=?NameAndType????????#45:#46????????//?out:Ljava/io/PrintStream;

#35?=?Utf8???????????????Hello?World.

#36?=?Class??????????????#47????????????//?java/io/PrintStream

#37?=?NameAndType????????#48:#49????????//?println:(Ljava/lang/String;)V

#38?=?Utf8???????????????java/lang/StringBuilder

#39?=?Utf8???????????????Hello

#40?=?NameAndType????????#50:#51????????//?append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

#41?=?NameAndType????????#52:#53????????//?toString:()Ljava/lang/String;

#42?=?Utf8???????????????com/brilliantstar/cimc/im/desktop/Demo

#43?=?Utf8???????????????java/lang/Object

#44?=?Utf8???????????????java/lang/System

#45?=?Utf8???????????????out

#46?=?Utf8???????????????Ljava/io/PrintStream;

#47?=?Utf8???????????????java/io/PrintStream

#48?=?Utf8???????????????println

#49?=?Utf8???????????????(Ljava/lang/String;)V

#50?=?Utf8???????????????append

#51?=?Utf8???????????????(Ljava/lang/String;)Ljava/lang/StringBuilder;

#52?=?Utf8???????????????toString

#53?=?Utf8???????????????()Ljava/lang/String;

{

===============字段區域(用來列舉該類中的各個字段)===================

private?java.lang.String?name;

descriptor:?Ljava/lang/String;

flags:?ACC_PRIVATE

public?com.brilliantstar.cimc.im.desktop.Demo();

descriptor:?()V

flags:?ACC_PUBLIC

Code:

stack=2,?locals=1,?args_size=1

0:?aload_0

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

4:?aload_0

5:?ldc???????????#2??????????????????//?String?張三

7:?putfield??????#3??????????????????//?Field?name:Ljava/lang/String;

10:?return

LineNumberTable:

line?19:?0

line?20:?4

LocalVariableTable:

Start??Length??Slot??Name???Signature

0??????11?????0??this???Lcom/brilliantstar/cimc/im/desktop/Demo;

===============方法區域(用來列舉該類中的各個方法)=========================

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

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

flags:?ACC_PUBLIC,?ACC_STATIC

Code:

stack=2,?locals=1,?args_size=1

0:?getstatic?????#4??????????????????//?Field?java/lang/System.out:Ljava/io/PrintStream;

3:?ldc???????????#5??????????????????//?String?Hello?World.

5:?invokevirtual?#6??????????????????//?Method?java/io/PrintStream.println:(Ljava/lang/String;)V

8:?return

LineNumberTable:

line?23:?0

line?24:?8

LocalVariableTable:

Start??Length??Slot??Name???Signature

0???????9?????0??args???[Ljava/lang/String;

private?void?sayHello();

descriptor:?()V

flags:?ACC_PRIVATE

Code:

stack=3,?locals=1,?args_size=1

0:?getstatic?????#4??????????????????//?Field?java/lang/System.out:Ljava/io/PrintStream;

3:?new???????????#7??????????????????//?class?java/lang/StringBuilder

6:?dup

7:?invokespecial?#8??????????????????//?Method?java/lang/StringBuilder."":()V

10:?ldc???????????#9??????????????????//?String?Hello

12:?invokevirtual?#10?????????????????//?Method?java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

15:?aload_0

16:?getfield??????#3??????????????????//?Field?name:Ljava/lang/String;

19:?invokevirtual?#10?????????????????//?Method?java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

22:?invokevirtual?#11?????????????????//?Method?java/lang/StringBuilder.toString:()Ljava/lang/String;

25:?invokevirtual?#6??????????????????//?Method?java/io/PrintStream.println:(Ljava/lang/String;)V

28:?return

LineNumberTable:

line?27:?0

line?28:?28

LocalVariableTable:

Start??Length??Slot??Name???Signature

0??????29?????0??this???Lcom/brilliantstar/cimc/im/desktop/Demo;

}

SourceFile:?"Demo.java"

以其中的sayHello()方法為例，除了方法描述以及訪問權限之外，方法中還包括最為重要的代碼區域。

1).代碼區域一開始會聲明該方法中的操作數棧(stack=3) 和局部變量數目(locals=1)的最大值，以及該方法接收參數的個數(args_size=1)這里局部變量指的是字節碼中的局部變量，而非Java程序中的局部變量。

2).接下來是該方法的字節碼，每條字節碼均標注了對應的偏移量，這是用來定位字節碼的。

3).接下來的行數表(LineNumberTable)則是Java源程序到字節碼偏移量的映射。行數表和局部變量均屬于調試信息，Java虛擬機并不要求class文件必備這些信息。

總結

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