字节码文件的分析

Posted 2021-03-25 yangxiaohui227

//环境的搭建:

1.创建一个类并编译它

 2.使用java命令:javap -c -verbose build/classes/java/main/com/yang/jvm/Test.class 将16进制的字节码文件转成可视化的文件内容如下:

 

 具体内容如下:

 

Classfile /F:/jvmdemo/build/classes/java/main/com/yang/jvm/Test.class
  Last modified 2019-10-17; size 573 bytes
  MD5 checksum f85b42e59f656b2cae810d27e91ccd7d
  Compiled from "Test.java"
public class com.yang.jvm.Test
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#25         // java/lang/Object."<init>":()V
   #2 = Fieldref           #5.#26         // com/yang/jvm/Test.i:I
   #3 = String             #27            // hello
   #4 = Fieldref           #5.#28         // com/yang/jvm/Test.str:Ljava/lang/String;
   #5 = Class              #29            // com/yang/jvm/Test
   #6 = Class              #30            // java/lang/Object
   #7 = Utf8               i
   #8 = Utf8               I
   #9 = Utf8               str
  #10 = Utf8               Ljava/lang/String;
  #11 = Utf8               <init>
  #12 = Utf8               ()V
  #13 = Utf8               Code
  #14 = Utf8               LineNumberTable
  #15 = Utf8               LocalVariableTable
  #16 = Utf8               this
  #17 = Utf8               Lcom/yang/jvm/Test;
  #18 = Utf8               getI
  #19 = Utf8               ()I
  #20 = Utf8               setI
  #21 = Utf8               (I)V
  #22 = Utf8               <clinit>
  #23 = Utf8               SourceFile
  #24 = Utf8               Test.java
  #25 = NameAndType        #11:#12        // "<init>":()V
  #26 = NameAndType        #7:#8          // i:I
  #27 = Utf8               hello
  #28 = NameAndType        #9:#10         // str:Ljava/lang/String;
  #29 = Utf8               com/yang/jvm/Test
  #30 = Utf8               java/lang/Object
{
  public com.yang.jvm.Test();
    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_5
         6: putfield      #2                  // Field i:I
         9: return
      LineNumberTable:
        line 3: 0
        line 4: 4
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      10     0  this   Lcom/yang/jvm/Test;

  public int getI();
    descriptor: ()I
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #2                  // Field i:I
         4: ireturn
      LineNumberTable:
        line 9: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/yang/jvm/Test;

  public void setI(int);
    descriptor: (I)V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: iload_1
         2: putfield      #2                  // Field i:I
         5: return
      LineNumberTable:
        line 13: 0
        line 14: 5
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       6     0  this   Lcom/yang/jvm/Test;
            0       6     1     i   I

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: ldc           #3                  // String hello
         2: putstatic     #4                  // Field str:Ljava/lang/String;
         5: return
      LineNumberTable:
        line 6: 0
}
SourceFile: "Test.java"

此外:idea也有插件可以得到上面的字节码内容,安装插件jclasslib:使用该插件的好处是,对应的指令如aload_0可以点击,链接到oracle官网进行解释

 

 安装重启后,鼠标点击在对应的java文件上,不是class文件,如Test.java文件,然后点击view菜单如图:

 

 在右边可以看到: