JVM学习-javap解析Class文件

丢爸2024-06-01 4:03
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
  • 格式 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
java 复制代码 
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 {};
}

// -public 显示公共信息
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();
}
// -protected 
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();
}
// -private || -p  大于或等于private权限
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 {};
}
// -package     显示非私有的信息
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 {};
}
// -sysinfo
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     //字节码文件路径
  Last modified 2024-5-27; size 1348 bytes             //日期
  MD5 checksum 85dc41e7e2e7128d8899b5d131d7975f       //MD5散列
  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 {};
}
// -constants
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 {};
}
// -s 输出内部类型签名
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: Z
  protected char gender;
    descriptor: C
  public java.lang.String info;
    descriptor: Ljava/lang/String;
  public static final int COUNTS;
    descriptor: I
  public com.chapter09.JavapTest();
    descriptor: ()V

  int getNum(int);
    descriptor: (I)I

  protected char showGender();
    descriptor: ()C

  public void showInfo();
    descriptor: ()V

  static {};
    descriptor: ()V
}
// -l 输出行号和本地变量表
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: 0
      line 18: 4
      line 20: 10
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      11     0  this   Lcom/chapter09/JavapTest;

  int getNum(int);
    LineNumberTable:
      line 26: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       7     0  this   Lcom/chapter09/JavapTest;
          0       7     1     i   I

  protected char showGender();
    LineNumberTable:
      line 29: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       5     0  this   Lcom/chapter09/JavapTest;

  public void showInfo();
    LineNumberTable:
      line 32: 0
      line 33: 3
      line 34: 30
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      31     0  this   Lcom/chapter09/JavapTest;
          3      28     1     i   I

  static {};
    LineNumberTable:
      line 15: 0
      line 16: 3
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
}
// -c 对代码进行反编译
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: 0
      line 18: 4
      line 20: 10
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      11     0  this   Lcom/chapter09/JavapTest;

  int getNum(int);
    LineNumberTable:
      line 26: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       7     0  this   Lcom/chapter09/JavapTest;
          0       7     1     i   I

  protected char showGender();
    LineNumberTable:
      line 29: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       5     0  this   Lcom/chapter09/JavapTest;

  public void showInfo();
    LineNumberTable:
      line 32: 0
      line 33: 3
      line 34: 30
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      31     0  this   Lcom/chapter09/JavapTest;
          3      28     1     i   I

  static {};
    LineNumberTable:
      line 15: 0
      line 16: 3
    LocalVariableTable:
      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_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: ldc           #2                  // String java
       7: putfield      #3                  // Field info:Ljava/lang/String;
      10: return

  int getNum(int);
    Code:
       0: aload_0
       1: getfield      #5                  // Field num:I
       4: iload_1
       5: iadd
       6: ireturn

  protected char showGender();
    Code:
       0: aload_0
       1: getfield      #6                  // Field gender:C
       4: ireturn

  public void showInfo();
    Code:
       0: bipush        10
       2: istore_1
       3: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
       6: new           #8                  // class java/lang/StringBuilder
       9: dup
      10: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V
      13: aload_0
      14: getfield      #3                  // Field info:Ljava/lang/String;
      17: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      20: iload_1
      21: invokevirtual #11                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      24: invokevirtual #12                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      27: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      30: return

  static {};
    Code:
       0: ldc           #14                 // String www.lotus.com
       2: astore_0
       3: return
}
// -v 显示最全的字节码反编译数据
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         //位置
  Last modified 2024-5-27; size 1348 bytes             //日期，大小
  MD5 checksum 85dc41e7e2e7128d8899b5d131d7975f        //MD5散列值
  Compiled from "JavapTest.java"
public class com.chapter09.JavapTest
  minor version: 0                     //副版本号
  major version: 52                   //主版本号
  flags: ACC_PUBLIC, ACC_SUPER           //访问标识
Constant pool:
   #1 = Methodref          #16.#46        // java/lang/Object."<init>":()V
   #2 = String             #47            // java
   #3 = Fieldref           #15.#48        // com/chapter09/JavapTest.info:Ljava/lang/String;
   #4 = Fieldref           #15.#49        // com/chapter09/JavapTest.flag:Z
   #5 = Fieldref           #15.#50        // com/chapter09/JavapTest.num:I
   #6 = Fieldref           #15.#51        // com/chapter09/JavapTest.gender:C
   #7 = Fieldref           #52.#53        // java/lang/System.out:Ljava/io/PrintStream;
   #8 = Class              #54            // java/lang/StringBuilder
   #9 = Methodref          #8.#46         // java/lang/StringBuilder."<init>":()V
  #10 = Methodref          #8.#55         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #11 = Methodref          #8.#56         // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  #12 = Methodref          #8.#57         // java/lang/StringBuilder.toString:()Ljava/lang/String;
  #13 = Methodref          #58.#59        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #14 = String             #60            // www.lotus.com
  #15 = Class              #61            // com/chapter09/JavapTest
  #16 = Class              #62            // java/lang/Object
  #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        // "<init>":()V
  #47 = Utf8               java
  #48 = NameAndType        #23:#24        // info:Ljava/lang/String;
  #49 = NameAndType        #19:#20        // flag:Z
  #50 = NameAndType        #17:#18        // num:I
  #51 = NameAndType        #21:#22        // gender:C
  #52 = Class              #63            // java/lang/System
  #53 = NameAndType        #64:#65        // out:Ljava/io/PrintStream;
  #54 = Utf8               java/lang/StringBuilder
  #55 = NameAndType        #66:#67        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #56 = NameAndType        #66:#68        // append:(I)Ljava/lang/StringBuilder;
  #57 = NameAndType        #69:#70        // toString:()Ljava/lang/String;
  #58 = Class              #71            // java/io/PrintStream
  #59 = NameAndType        #72:#73        // println:(Ljava/lang/String;)V
  #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
{ //-----------------------------------------------方法
//字段表集合信息
  boolean flag;          //字段名
    descriptor: Z        //字段描述符(字段类型)
    flags:               //字段访问标识

  protected char gender;
    descriptor: C
    flags: ACC_PROTECTED

  public java.lang.String info;
    descriptor: Ljava/lang/String;
    flags: ACC_PUBLIC

  public static final int COUNTS;
    descriptor: I
    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
    ConstantValue: int 1                             //常量字段的属性：ConstantValue，保存常量的值
//-----------------------------------------------方法
  public com.chapter09.JavapTest();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: aload_0
         5: ldc           #2                  // String java
         7: putfield      #3                  // Field info:Ljava/lang/String;
        10: return
      LineNumberTable:
        line 20: 0
        line 18: 4
        line 20: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  this   Lcom/chapter09/JavapTest;

  int getNum(int);
    descriptor: (I)I
    flags:
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: getfield      #5                  // Field num:I
         4: iload_1
         5: iadd
         6: ireturn
      LineNumberTable:
        line 26: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       7     0  this   Lcom/chapter09/JavapTest;
            0       7     1     i   I

  protected char showGender();
    descriptor: ()C
    flags: ACC_PROTECTED
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #6                  // Field gender:C
         4: ireturn
      LineNumberTable:
        line 29: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/chapter09/JavapTest;

  public void showInfo();
    descriptor: ()V                                   //方法描述符：方法的形参列表、返回值类型
    flags: ACC_PUBLIC                                 //方法的访问标识
    Code:                                             //方法Code属性
      stack=3, locals=2, args_size=1                  //stack：操作数栈的最大深度，locals：局部变量表的长度，args_size：方法接收参数的个数
   //偏移量  操作码        操作数
         0: bipush        10
         2: istore_1
         3: getstatic     #7 //#7常量表索引                  // Field java/lang/System.out:Ljava/io/PrintStream;
         6: new           #8                  // class java/lang/StringBuilder
         9: dup
        10: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V
        13: aload_0
        14: getfield      #3                  // Field info:Ljava/lang/String;
        17: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        20: iload_1
        21: invokevirtual #11                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        24: invokevirtual #12                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        27: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        30: return
      LineNumberTable:             //行号表，当前字节码指令偏移量与Java源程序中代码的行号一一对应关系
        line 32: 0
        line 33: 3
        line 34: 30
      LocalVariableTable:          //局部变量表，描述内部局部变量的相关信息
        Start  Length  Slot  Name   Signature
            0      31     0  this   Lcom/chapter09/JavapTest;
            3      28     1     i   I

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=1, args_size=0
         0: ldc           #14                 // String www.lotus.com
         2: astore_0
         3: return
      LineNumberTable:
        line 15: 0
        line 16: 3
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
}
SourceFile: "JavapTest.java"                         //附加属性：当前字节码对应的源文件的文件名
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