javap的使用
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[]); }
