javap的使用总结

符号阅读：497 2021-06-16 22:34:55 评论：0

今天听的分享里在介绍String时，提到了javap，学习了好久的java，但是好像从来没有了解过这个工具。

javap是JDK提供的一个命令行工具,javap能对给定的class文件提供的字节代码进行反编译。

通过它，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,对更深入地理解如何提高程序执行的效率等问题有极大的帮助。

使用格式

javap <options> <classes>

常用: javap -c -v 类名

  -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 <path>        指定查找用户类文件的位置 
  -cp <path>               指定查找用户类文件的位置 
  -bootclasspath <path>    覆盖引导类文件的位置

新建Hello.java

public class Hello { 
    private Integer aa = 1; 
    public String ss = "sss"; 
    public static void main(String[] args) {    
    System.out.println("Hello Java"); 
    } 
}

不带参数

javap Hello

Compiled from "Hello.java" 
public class Hello { 
  public java.lang.String ss; 
  public Hello(); 
  public static void main(java.lang.String[]); 
}

c 对代码进行反汇编

javap -c Hello

Compiled from "Hello.java" 
public class Hello { 
  public java.lang.String ss; 
 
  public Hello(); 
    Code: 
       0: aload_0 
       1: invokespecial #1                  // Method java/lang/Object."<init>": 
()V 
       4: aload_0 
       5: iconst_1 
       6: invokestatic  #2                  // Method java/lang/Integer.valueOf: 
(I)Ljava/lang/Integer; 
       9: putfield      #3                  // Field aa:Ljava/lang/Integer; 
      12: aload_0 
      13: ldc           #4                  // String sss 
      15: putfield      #5                  // Field ss:Ljava/lang/String; 
      18: return 
 
  public static void main(java.lang.String[]); 
    Code: 
       0: getstatic     #6                  // Field java/lang/System.out:Ljava/ 
io/PrintStream; 
       3: ldc           #7                  // String Hello Java 
       5: invokevirtual #8                  // Method java/io/PrintStream.printl 
n:(Ljava/lang/String;)V 
       8: return 
}

输出附加信息

javap -v Hello

Classfile /D:/WWW/11/javap/Hello.class 
  Last modified 2019-4-23; size 602 bytes 
  MD5 checksum 9eb7401f16043460fa36db8275c0f7c4 
  Compiled from "Hello.java" 
public class Hello 
  minor version: 0 
  major version: 52 
  flags: ACC_PUBLIC, ACC_SUPER 
Constant pool: 
   #1 = Methodref          #10.#23        // java/lang/Object."<init>":()V 
   #2 = Methodref          #24.#25        // java/lang/Integer.valueOf:(I)Ljava/ 
lang/Integer; 
   #3 = Fieldref           #9.#26         // Hello.aa:Ljava/lang/Integer; 
   #4 = String             #27            // sss 
   #5 = Fieldref           #9.#28         // Hello.ss:Ljava/lang/String; 
   #6 = Fieldref           #29.#30        // java/lang/System.out:Ljava/io/Print 
Stream; 
   #7 = String             #31            // Hello Java 
   #8 = Methodref          #32.#33        // java/io/PrintStream.println:(Ljava/ 
lang/String;)V 
   #9 = Class              #34            // Hello 
  #10 = Class              #35            // java/lang/Object 
  #11 = Utf8               aa 
  #12 = Utf8               Ljava/lang/Integer; 
  #13 = Utf8               ss 
  #14 = Utf8               Ljava/lang/String; 
  #15 = Utf8               <init> 
  #16 = Utf8               ()V 
  #17 = Utf8               Code 
  #18 = Utf8               LineNumberTable 
  #19 = Utf8               main 
  #20 = Utf8               ([Ljava/lang/String;)V 
  #21 = Utf8               SourceFile 
  #22 = Utf8               Hello.java 
  #23 = NameAndType        #15:#16        // "<init>":()V 
  #24 = Class              #36            // java/lang/Integer 
  #25 = NameAndType        #37:#38        // valueOf:(I)Ljava/lang/Integer; 
  #26 = NameAndType        #11:#12        // aa:Ljava/lang/Integer; 
  #27 = Utf8               sss 
  #28 = NameAndType        #13:#14        // ss:Ljava/lang/String; 
  #29 = Class              #39            // java/lang/System 
  #30 = NameAndType        #40:#41        // out:Ljava/io/PrintStream; 
  #31 = Utf8               Hello Java 
  #32 = Class              #42            // java/io/PrintStream 
  #33 = NameAndType        #43:#44        // println:(Ljava/lang/String;)V 
  #34 = Utf8               Hello 
  #35 = Utf8               java/lang/Object 
  #36 = Utf8               java/lang/Integer 
  #37 = Utf8               valueOf 
  #38 = Utf8               (I)Ljava/lang/Integer; 
  #39 = Utf8               java/lang/System 
  #40 = Utf8               out 
  #41 = Utf8               Ljava/io/PrintStream; 
  #42 = Utf8               java/io/PrintStream 
  #43 = Utf8               println 
  #44 = Utf8               (Ljava/lang/String;)V 
{ 
  public java.lang.String ss; 
    descriptor: Ljava/lang/String; 
    flags: ACC_PUBLIC 
 
  public Hello(); 
    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: iconst_1 
         6: invokestatic  #2                  // Method java/lang/Integer.valueO 
f:(I)Ljava/lang/Integer; 
         9: putfield      #3                  // Field aa:Ljava/lang/Integer; 
        12: aload_0 
        13: ldc           #4                  // String sss 
        15: putfield      #5                  // Field ss:Ljava/lang/String; 
        18: return 
      LineNumberTable: 
        line 1: 0 
        line 2: 4 
        line 3: 12 
 
  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     #6                  // Field java/lang/System.out:Ljav 
a/io/PrintStream; 
         3: ldc           #7                  // String Hello Java 
         5: invokevirtual #8                  // Method java/io/PrintStream.prin 
tln:(Ljava/lang/String;)V 
         8: return 
      LineNumberTable: 
        line 5: 0 
        line 6: 8 
} 
SourceFile: "Hello.java"

输出行号和本地变量表
javap -l Hello

Compiled from "Hello.java" 
public class Hello { 
  public java.lang.String ss; 
 
  public Hello(); 
    LineNumberTable: 
      line 1: 0 
      line 2: 4 
      line 3: 12 
 
  public static void main(java.lang.String[]); 
    LineNumberTable: 
      line 5: 0 
      line 6: 8 
}

javap -p Hello

Compiled from "Hello.java" 
public class Hello { 
  private java.lang.Integer aa; 
  public java.lang.String ss; 
  public Hello(); 
  public static void main(java.lang.String[]); 
}

javap -public Hello

Compiled from "Hello.java" 
public class Hello { 
  public java.lang.String ss; 
  public Hello(); 
  public static void main(java.lang.String[]); 
}

javap -private Hello

Compiled from "Hello.java" 
public class Hello { 
  private java.lang.Integer aa; 
  public java.lang.String ss; 
  public Hello(); 
  public static void main(java.lang.String[]); 
}

输出内部类型签名
javap -s Hello

Compiled from "Hello.java" 
public class Hello { 
  public java.lang.String ss; 
    descriptor: Ljava/lang/String; 
  public Hello(); 
    descriptor: ()V 
 
  public static void main(java.lang.String[]); 
    descriptor: ([Ljava/lang/String;)V 
}

显示正在处理的类的
javap -sysinfo  Hello

Classfile /D:/WWW/11/javap/Hello.class 
  Last modified 2019-4-23; size 602 bytes 
  MD5 checksum 9eb7401f16043460fa36db8275c0f7c4 
  Compiled from "Hello.java" 
public class Hello { 
  public java.lang.String ss; 
  public Hello(); 
  public static void main(java.lang.String[]); 
}