java 字节码指令理解

public class Test {
    private int num1 = 1;
    public static int NUM1 = 100;
    public int func(int a,int b){
        return add(a,b);
    }
    public int add(int a,int b) {
        return a+b+num1;
    }
    public int sub(int a, int b) {
        return a-b-NUM1;
    }
}




// 生成class文件
D:\>javac -g Test.java

// 查看class详情
D:\>javap -verbose Test.class
Classfile /D:/Test.class
  Last modified 2022年8月29日; size 652 bytes
  MD5 checksum 6b44a7a347f016d2b30fd5e157922e8b
  Compiled from "Test.java"
public class Test
  minor version: 0
  major version: 55
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #5                          // Test
  super_class: #6                         // java/lang/Object
  interfaces: 0, fields: 2, methods: 5, attributes: 1
Constant pool:
   #1 = Methodref          #6.#26         // java/lang/Object."<init>":()V
   #2 = Fieldref           #5.#27         // Test.num1:I
   #3 = Methodref          #5.#28         // Test.add:(II)I
   #4 = Fieldref           #5.#29         // Test.NUM1:I
   #5 = Class              #30            // Test
   #6 = Class              #31            // java/lang/Object
   #7 = Utf8               num1
   #8 = Utf8               I
   #9 = Utf8               NUM1
  #10 = Utf8               <init>
  #11 = Utf8               ()V
  #12 = Utf8               Code
  #13 = Utf8               LineNumberTable
  #14 = Utf8               LocalVariableTable
  #15 = Utf8               this
  #16 = Utf8               LTest;
  #17 = Utf8               func
  #18 = Utf8               (II)I
  #19 = Utf8               a
  #20 = Utf8               b
  #21 = Utf8               add
  #22 = Utf8               sub
  #23 = Utf8               <clinit>
  #24 = Utf8               SourceFile
  #25 = Utf8               Test.java
  #26 = NameAndType        #10:#11        // "<init>":()V
  #27 = NameAndType        #7:#8          // num1:I
  #28 = NameAndType        #21:#18        // add:(II)I
  #29 = NameAndType        #9:#8          // NUM1:I
  #30 = Utf8               Test
  #31 = Utf8               java/lang/Object
{
  public static int NUM1;
    descriptor: I
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC

  public Test();
    descriptor: ()V
    flags: (0x0001) 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 num1:I
         9: return
      LineNumberTable:
        line 1: 0
        line 2: 4
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      10     0  this   LTest;

  public int func(int, int);
    descriptor: (II)I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=3, args_size=3
         0: aload_0
         1: iload_1
         2: iload_2
         3: invokevirtual #3                  // Method add:(II)I
         6: ireturn
      LineNumberTable:
        line 5: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       7     0  this   LTest;
            0       7     1     a   I
            0       7     2     b   I

  public int add(int, int);
    descriptor: (II)I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=3
         0: iload_1
         1: iload_2
         2: iadd
         3: aload_0
         4: getfield      #2                  // Field num1:I
         7: iadd
         8: ireturn
      LineNumberTable:
        line 8: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  this   LTest;
            0       9     1     a   I
            0       9     2     b   I

  public int sub(int, int);
    descriptor: (II)I
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=3
         0: iload_1
         1: iload_2
         2: isub
         3: getstatic     #4                  // Field NUM1:I
         6: isub
         7: ireturn
      LineNumberTable:
        line 11: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       8     0  this   LTest;
            0       8     1     a   I
            0       8     2     b   I

  static {};
    descriptor: ()V
    flags: (0x0008) ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: bipush        100
         2: putstatic     #4                  // Field NUM1:I
         5: return
      LineNumberTable:
        line 3: 0
}
SourceFile: "Test.java"