Development Tools: IEDA, JDK1.8, WinHex

1. Byte code file structure

source code

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;
    }
}

Javap-verbose parses a byte code file and outputs the magic number, version number, constant pool, class information, class construction method, method information in class, class variables and member variables of the byte code file

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"

WinHex: Open the class file

2. Magic Number

1. Magic number: The first four bytes of all class byte code files are magic numbers. Magic number is a fixed value of 0XCAFEBABE. When JVM loads classes, it verifies that the data conforms to the specifications.

2. Version number

2. Version number:

The four bytes after magic number are version information, the first two bytes (00 00 00) are minor version: 0, the last two bytes (00 34) are major version: 52, the last two bytes are hexadecimal conversion 52, and the corresponding JDK1.8; so the class file version number = 1.8.0; can be verified with java-version.Since JVM is downward compatible, how your byte code file version number is less than or equal to the current version of JVM can run in the current JVM.

Next section: Constant Pool for JAVA Byte Code Files