解析字节码的作用
- 通过反编译生成字节码文件,可以深入了解Java工作机制,但自己分析类文件结构太麻烦,除了第三方的jclasslib工具外,官方提供了javap
- javap是jdk自带的反解析工具,它的作用是根据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、代码行偏移量映射表、常量池等信息
- 通过局部变量表,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信息
java -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)需要在使用javac编译成class文件时,指定参数才能输出
- 直接使用javac xx.java,不会生成对应局部变量表等信息,使用javac -g xx.java可以生成相关信息,如使用eclipse或IDEA,默认情况下,在编译时会帮你生成局部变量表,指令和代码偏移量映射表等信息
javap
- classes是要反编译的class文件
- 在命令行直接输入javap或javap -help可以看到javap的options有如下选项
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -version JavapTest.class
1.8.0_131
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {boolean flag;protected char gender;public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();int getNum(int);protected char showGender();public void showInfo();static {};
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -public JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();public void showInfo();
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -protected JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {protected char gender;public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();protected char showGender();public void showInfo();
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -private JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {private int num;boolean flag;protected char gender;public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();private com.chapter09.JavapTest(boolean);private void methodPrivate();int getNum(int);protected char showGender();public void showInfo();static {};
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -package JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {boolean flag;protected char gender;public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();int getNum(int);protected char showGender();public void showInfo();static {};
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -sysinfo JavapTest.class
Classfile /C:/Users/Administrator/IdeaProjects/jvm/target/classes/com/chapter09/JavapTest.class Last modified 2024-5-27; size 1348 bytes MD5 checksum 85dc41e7e2e7128d8899b5d131d7975f Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {boolean flag;protected char gender;public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();int getNum(int);protected char showGender();public void showInfo();static {};
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -constants JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {boolean flag;protected char gender;public java.lang.String info;public static final int COUNTS = 1;public com.chapter09.JavapTest();int getNum(int);protected char showGender();public void showInfo();static {};
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -s JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {boolean flag;descriptor: Zprotected char gender;descriptor: Cpublic java.lang.String info;descriptor: Ljava/lang/String;public static final int COUNTS;descriptor: Ipublic com.chapter09.JavapTest();descriptor: ()Vint getNum(int);descriptor: (I)Iprotected char showGender();descriptor: ()Cpublic void showInfo();descriptor: ()Vstatic {};descriptor: ()V
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -l JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {boolean flag;protected char gender;public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();LineNumberTable:line 20: 0line 18: 4line 20: 10LocalVariableTable:Start Length Slot Name Signature0 11 0 this Lcom/chapter09/JavapTest;int getNum(int);LineNumberTable:line 26: 0LocalVariableTable:Start Length Slot Name Signature0 7 0 this Lcom/chapter09/JavapTest;0 7 1 i Iprotected char showGender();LineNumberTable:line 29: 0LocalVariableTable:Start Length Slot Name Signature0 5 0 this Lcom/chapter09/JavapTest;public void showInfo();LineNumberTable:line 32: 0line 33: 3line 34: 30LocalVariableTable:Start Length Slot Name Signature0 31 0 this Lcom/chapter09/JavapTest;3 28 1 i Istatic {};LineNumberTable:line 15: 0line 16: 3LocalVariableTable:Start Length Slot Name Signature
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -l JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {boolean flag;protected char gender;public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();LineNumberTable:line 20: 0line 18: 4line 20: 10LocalVariableTable:Start Length Slot Name Signature0 11 0 this Lcom/chapter09/JavapTest;int getNum(int);LineNumberTable:line 26: 0LocalVariableTable:Start Length Slot Name Signature0 7 0 this Lcom/chapter09/JavapTest;0 7 1 i Iprotected char showGender();LineNumberTable:line 29: 0LocalVariableTable:Start Length Slot Name Signature0 5 0 this Lcom/chapter09/JavapTest;public void showInfo();LineNumberTable:line 32: 0line 33: 3line 34: 30LocalVariableTable:Start Length Slot Name Signature0 31 0 this Lcom/chapter09/JavapTest;3 28 1 i Istatic {};LineNumberTable:line 15: 0line 16: 3LocalVariableTable:Start Length Slot Name Signature
}C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -c JavapTest.class
Compiled from "JavapTest.java"
public class com.chapter09.JavapTest {boolean flag;protected char gender;public java.lang.String info;public static final int COUNTS;public com.chapter09.JavapTest();Code:0: aload_01: invokespecial #1 4: aload_05: ldc #2 7: putfield #3 10: returnint getNum(int);Code:0: aload_01: getfield #5 4: iload_15: iadd6: ireturnprotected char showGender();Code:0: aload_01: getfield #6 4: ireturnpublic void showInfo();Code:0: bipush 102: istore_13: getstatic #7 6: new #8 9: dup10: invokespecial #9 13: aload_014: getfield #3 17: invokevirtual #10 20: iload_121: invokevirtual #11 24: invokevirtual #12 27: invokevirtual #13 30: returnstatic {};Code:0: ldc #14 2: astore_03: return
}
C:\Users\Administrator\IdeaProjects\jvm\target\classes\com\chapter09>javap -v JavapTest.class
Classfile /C:/Users/Administrator/IdeaProjects/jvm/target/classes/com/chapter09/JavapTest.class Last modified 2024-5-27; size 1348 bytes MD5 checksum 85dc41e7e2e7128d8899b5d131d7975f Compiled from "JavapTest.java"
public class com.chapter09.JavapTestminor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER
Constant pool:#1 = Methodref #16.#46 #2 = String #47 #3 = Fieldref #15.#48 #4 = Fieldref #15.#49 #5 = Fieldref #15.#50 #6 = Fieldref #15.#51 #7 = Fieldref #52.#53 #8 = Class #54 #9 = Methodref #8.#46 #10 = Methodref #8.#55 #11 = Methodref #8.#56 #12 = Methodref #8.#57 #13 = Methodref #58.#59 #14 = String #60 #15 = Class #61 #16 = Class #62 #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/chapter09/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 #47 = Utf8 java#48 = NameAndType #23:#24 #49 = NameAndType #19:#20 #50 = NameAndType #17:#18 #51 = NameAndType #21:#22 #52 = Class #63 #53 = NameAndType #64:#65 #54 = Utf8 java/lang/StringBuilder#55 = NameAndType #66:#67 #56 = NameAndType #66:#68 #57 = NameAndType #69:#70 #58 = Class #71 #59 = NameAndType #72:#73 #60 = Utf8 www.lotus.com#61 = Utf8 com/chapter09/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
{
boolean flag; descriptor: Z flags: protected char gender;descriptor: Cflags: ACC_PROTECTEDpublic java.lang.String info;descriptor: Ljava/lang/String;flags: ACC_PUBLICpublic static final int COUNTS;descriptor: Iflags: ACC_PUBLIC, ACC_STATIC, ACC_FINALConstantValue: int 1
public com.chapter09.JavapTest();descriptor: ()Vflags: ACC_PUBLICCode:stack=2, locals=1, args_size=10: aload_01: invokespecial #1 4: aload_05: ldc #2 7: putfield #3 10: returnLineNumberTable:line 20: 0line 18: 4line 20: 10LocalVariableTable:Start Length Slot Name Signature0 11 0 this Lcom/chapter09/JavapTest;int getNum(int);descriptor: (I)Iflags:Code:stack=2, locals=2, args_size=20: aload_01: getfield #5 4: iload_15: iadd6: ireturnLineNumberTable:line 26: 0LocalVariableTable:Start Length Slot Name Signature0 7 0 this Lcom/chapter09/JavapTest;0 7 1 i Iprotected char showGender();descriptor: ()Cflags: ACC_PROTECTEDCode:stack=1, locals=1, args_size=10: aload_01: getfield #6 4: ireturnLineNumberTable:line 29: 0LocalVariableTable:Start Length Slot Name Signature0 5 0 this Lcom/chapter09/JavapTest;public void showInfo();descriptor: ()V flags: ACC_PUBLIC Code: stack=3, locals=2, args_size=1 0: bipush 102: istore_13: getstatic #7 6: new #8 9: dup10: invokespecial #9 13: aload_014: getfield #3 17: invokevirtual #10 20: iload_121: invokevirtual #11 24: invokevirtual #12 27: invokevirtual #13 30: returnLineNumberTable: line 32: 0line 33: 3line 34: 30LocalVariableTable: Start Length Slot Name Signature0 31 0 this Lcom/chapter09/JavapTest;3 28 1 i Istatic {};descriptor: ()Vflags: ACC_STATICCode:stack=1, locals=1, args_size=00: ldc #14 2: astore_03: returnLineNumberTable:line 15: 0line 16: 3LocalVariableTable:Start Length Slot Name Signature
}
SourceFile: "JavapTest.java"