java6虚拟机_Java 虚拟机之六：javap工具
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(jié)碼。通過它，我們可以對照源代碼和字節(jié)碼，從而了解很多編譯器內部的工作。
javap命令的常用參數(shù)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(shù),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(tǒng)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(jié)碼的指令
-verbose? 輸出棧大小，變量(locals)和方法參數(shù)的個數(shù)
-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(huán)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(shù)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(jié)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(jié)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(qū)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(qū)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(qū)域。
1).代碼區(qū)域一開始會聲明該方法中的操作數(shù)棧(stack=3) 和局部變量數(shù)目(locals=1)的最大值，以及該方法接收參數(shù)的個數(shù)(args_size=1)這里局部變量指的是字節(jié)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(jié)碼，每條字節(jié)碼均標注了對應的偏移量，這是用來定位字節(jié)碼的。
3).接下來的行數(shù)表(LineNumberTable)則是Java源程序到字節(jié)碼偏移量的映射。行數(shù)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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