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 = ;
    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:
       : aload_0
       : invokespecial #                  // Method java/lang/Object."<init>":
()V
       : aload_0
       : iconst_1
       : invokestatic  #                  // Method java/lang/Integer.valueOf:
(I)Ljava/lang/Integer;
       : putfield      #                  // Field aa:Ljava/lang/Integer;
      : aload_0
      : ldc           #                  // String sss
      : putfield      #                  // Field ss:Ljava/lang/String;
      : return

  public static void main(java.lang.String[]);
    Code:
       : getstatic     #                  // Field java/lang/System.out:Ljava/
io/PrintStream;
       : ldc           #                  // String Hello Java
       : invokevirtual #                  // Method java/io/PrintStream.printl
n:(Ljava/lang/String;)V
       : return
}

输出附加信息

javap -v Hello

Classfile /D:/WWW//javap/Hello.class
  Last modified --; size  bytes
  MD5 checksum 9eb7401f16043460fa36db8275c0f7c4
  Compiled from "Hello.java"
public class Hello
  minor version:
  major version:
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   # = Methodref          #.#        // java/lang/Object."<init>":()V
   # = Methodref          #.#        // java/lang/Integer.valueOf:(I)Ljava/
lang/Integer;
   # = Fieldref           #.#         // Hello.aa:Ljava/lang/Integer;
   # = String             #            // sss
   # = Fieldref           #.#         // Hello.ss:Ljava/lang/String;
   # = Fieldref           #.#        // java/lang/System.out:Ljava/io/Print
Stream;
   # = String             #            // Hello Java
   # = Methodref          #.#        // java/io/PrintStream.println:(Ljava/
lang/String;)V
   # = Class              #            // Hello
  # = Class              #            // java/lang/Object
  # = Utf8               aa
  # = Utf8               Ljava/lang/Integer;
  # = Utf8               ss
  # = Utf8               Ljava/lang/String;
  # = Utf8               <init>
  # = Utf8               ()V
  # = Utf8               Code
  # = Utf8               LineNumberTable
  # = Utf8               main
  # = Utf8               ([Ljava/lang/String;)V
  # = Utf8               SourceFile
  # = Utf8               Hello.java
  # = NameAndType        #:#        // "<init>":()V
  # = Class              #            // java/lang/Integer
  # = NameAndType        #:#        // valueOf:(I)Ljava/lang/Integer;
  # = NameAndType        #:#        // aa:Ljava/lang/Integer;
  # = Utf8               sss
  # = NameAndType        #:#        // ss:Ljava/lang/String;
  # = Class              #            // java/lang/System
  # = NameAndType        #:#        // out:Ljava/io/PrintStream;
  # = Utf8               Hello Java
  # = Class              #            // java/io/PrintStream
  # = NameAndType        #:#        // println:(Ljava/lang/String;)V
  # = Utf8               Hello
  # = Utf8               java/lang/Object
  # = Utf8               java/lang/Integer
  # = Utf8               valueOf
  # = Utf8               (I)Ljava/lang/Integer;
  # = Utf8               java/lang/System
  # = Utf8               out
  # = Utf8               Ljava/io/PrintStream;
  # = Utf8               java/io/PrintStream
  # = Utf8               println
  # = 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=, locals=, args_size=
         : aload_0
         : invokespecial #                  // Method java/lang/Object."<init>
":()V
         : aload_0
         : iconst_1
         : invokestatic  #                  // Method java/lang/Integer.valueO
f:(I)Ljava/lang/Integer;
         : putfield      #                  // Field aa:Ljava/lang/Integer;
        : aload_0
        : ldc           #                  // String sss
        : putfield      #                  // Field ss:Ljava/lang/String;
        : return
      LineNumberTable:
        line :
        line :
        line : 

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=, locals=, args_size=
         : getstatic     #                  // Field java/lang/System.out:Ljav
a/io/PrintStream;
         : ldc           #                  // String Hello Java
         : invokevirtual #                  // Method java/io/PrintStream.prin
tln:(Ljava/lang/String;)V
         : return
      LineNumberTable:
        line :
        line :
}
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 :
      line :
      line : 

  public static void main(java.lang.String[]);
    LineNumberTable:
      line :
      line :
}

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//javap/Hello.class
  Last modified --; size  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[]);
}