JAVA byte code file structure of

Development tools: IEDA, JDK1.8, WinHex

A bytecode file structure

Source

package com.jalja.java.bytecode;

/**
 * @Auther: XL
 * @Date: 2020/1/4 12:58
 * @Description:
 */
public class BytecodeTest {
    private int num=1;

    public int getNum() {
        return num;
    }

    public void setNum(int num) {
        this.num = num;
    }
}

Analysis javap -verbose a bytecode file, the magic number information is output bytecode files, version number, constant pool, class information, class constructor, a method of class information, class variables and member variables

F:\workspace\IDEA\study\jalja-base-utils\target\test-classes>javap -verbose com.jalja.java.bytecode.BytecodeTest
Classfile /F:/workspace/IDEA/study/jalja-base-utils/target/test-classes/com/jalja/java/bytecode/BytecodeTest.class
  Last modified 2020-1-4; size 514 bytes
  MD5 checksum b661c2792027e7c9169a0266523412c1
  Compiled from "BytecodeTest.java"
public class com.jalja.java.bytecode.BytecodeTest
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #4.#20         // java/lang/Object."<init>":()V
   #2 = Fieldref           #3.#21         // com/jalja/java/bytecode/BytecodeTest.num:I
   #3 = Class              #22            // com/jalja/java/bytecode/BytecodeTest
   #4 = Class              #23            // java/lang/Object
   #5 = Utf8               num
   #6 = Utf8               I
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               LocalVariableTable
  #12 = Utf8               this
  #13 = Utf8               Lcom/jalja/java/bytecode/BytecodeTest;
  #14 = Utf8               getNum
  #15 = Utf8               ()I
  #16 = Utf8               setNum
  #17 = Utf8               (I)V
  #18 = Utf8               SourceFile
  #19 = Utf8               BytecodeTest.java
  #20 = NameAndType        #7:#8          // "<init>":()V
  #21 = NameAndType        #5:#6          // num:I
  #22 = Utf8               com/jalja/java/bytecode/BytecodeTest
  #23 = Utf8               java/lang/Object
{
  public com.jalja.java.bytecode.BytecodeTest();
    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: putfield      #2                  // Field num:I
         9: return
      LineNumberTable:
        line 8: 0
        line 9: 4
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      10     0  this   Lcom/jalja/java/bytecode/BytecodeTest;

  public int getNum();
    descriptor: ()I
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #2                  // Field num:I
         4: ireturn
      LineNumberTable:
        line 12: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/jalja/java/bytecode/BytecodeTest;

  public void setNum(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 num:I
         5: return
      LineNumberTable:
        line 16: 0
        line 17: 5
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       6     0  this   Lcom/jalja/java/bytecode/BytecodeTest;
            0       6     1   num   I
}
SourceFile: "BytecodeTest.java"

View Code

WinHex: Open class file

Second, the magic number

1, the magic number: the first four bytes of all the class bytecode files are magic number, the magic number is a fixed value 0XCAFEBABE, JVM verifies that the data is compliant class loading.

Second, the version number

2, the version number: 

 After four byte magic number, version information, the first two bytes (00 00) is the minor version number (minor version: 0), the last two bytes (00 34) is the major version number (major version: 52 ), hexadecimal 34 is converted to decimal 52; 52 is the corresponding JDK1.8; therefore the version number of the class file = 1.8.0; java -version used can be verified. Since the JVM is backward compatible, so how do you bytecode file version number is less than equal to the current version of the JVM, you can run in the current JVM.

 

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