JVMJVM05(从字节码角度分析i++和++i的执行流程)

Posted 2022-02-17 温文艾尔

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• 🍅文章发布日期：2022.02.06
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  文章目录

  • 1.i++
  • 执行流程

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1.i++

原始代码

public class Demo03 
    public static void main(String[] args) 
        int a = 10;
        int b = a++ + ++a + a--;
        System.out.println(a);
        System.out.println(b);
    

javap反编译后的.class字节码文件

Classfile /F:/2021年12月之后的Idea项目/jvmProject/out/production/jvmProj
ect/com/wql/jvm/ClassInfo/Demo03.class
  Last modified 2022年1月22日; size 599 bytes
  SHA-256 checksum 2fb1f080d8858a312c0f3c65356a84fea66a987ae74607ef26ac5
aa49fbedfee
  Compiled from "Demo03.java"
public class com.wql.jvm.ClassInfo.Demo03
  minor version: 0
  major version: 52
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #4                          // com/wql/jvm/ClassInfo/Demo0
3
  super_class: #5                         // java/lang/Object
  interfaces: 0, fields: 0, methods: 2, attributes: 1
Constant pool:
   #1 = Methodref          #5.#22         // java/lang/Object."<init>":(
)V
   #2 = Fieldref           #23.#24        // java/lang/System.out:Ljava/
io/PrintStream;
   #3 = Methodref          #25.#26        // java/io/PrintStream.println
:(I)V
   #4 = Class              #27            // com/wql/jvm/ClassInfo/Demo0
3
   #5 = Class              #28            // java/lang/Object
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               LocalVariableTable
  #11 = Utf8               this
  #12 = Utf8               Lcom/wql/jvm/ClassInfo/Demo03;
  #13 = Utf8               main
  #14 = Utf8               ([Ljava/lang/String;)V
  #15 = Utf8               args
  #16 = Utf8               [Ljava/lang/String;
  #17 = Utf8               a
  #18 = Utf8               I
  #19 = Utf8               b
  #20 = Utf8               SourceFile
  #21 = Utf8               Demo03.java
  #22 = NameAndType        #6:#7          // "<init>":()V
  #23 = Class              #29            // java/lang/System
  #24 = NameAndType        #30:#31        // out:Ljava/io/PrintStream;
  #25 = Class              #32            // java/io/PrintStream
  #26 = NameAndType        #33:#34        // println:(I)V
  #27 = Utf8               com/wql/jvm/ClassInfo/Demo03
  #28 = Utf8               java/lang/Object
  #29 = Utf8               java/lang/System
  #30 = Utf8               out
  #31 = Utf8               Ljava/io/PrintStream;
  #32 = Utf8               java/io/PrintStream
  #33 = Utf8               println
  #34 = Utf8               (I)V

  public com.wql.jvm.ClassInfo.Demo03();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object
."<init>":()V
         4: return
      LineNumberTable:
        line 9: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/wql/jvm/ClassInfo/Demo03;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: bipush        10
         2: istore_1
         3: iload_1
         4: iinc          1, 1
         7: iinc          1, 1
        10: iload_1
        11: iadd
        12: iload_1
        13: iinc          1, -1
        16: iadd
        17: istore_2
        18: getstatic     #2                  // Field java/lang/System.
out:Ljava/io/PrintStream;
        21: iload_1
        22: invokevirtual #3                  // Method java/io/PrintStr
eam.println:(I)V
        25: getstatic     #2                  // Field java/lang/System.
out:Ljava/io/PrintStream;
        28: iload_2
        29: invokevirtual #3                  // Method java/io/PrintStr
eam.println:(I)V
        32: return
      LineNumberTable:
        line 11: 0
        line 12: 3
        line 13: 18
        line 14: 25
        line 15: 32
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      33     0  args   [Ljava/lang/String;
            3      30     1     a   I
           18      15     2     b   I

SourceFile: "Demo03.java"

本次我们主要分析这部分的操作

         0: bipush        10
         2: istore_1
         3: iload_1
         4: iinc          1, 1
         7: iinc          1, 1
        10: iload_1
        11: iadd
        12: iload_1
        13: iinc          1, -1
        16: iadd
        17: istore_2

分析：

• 注意iinc指令是直接在局部变量slot上进行运算
• a++和++a的区别是先执行iload还是先执行iinc

执行流程

1.bipush 10

含义：将10放入操作数栈中

2.istore_1

含义：弹出栈顶元素，放入局部变量槽的1号位置，对应a=10操作

然后开始执行a++的操作，a++顺序为先iload再自增

3.iload_1

将局部变量a=10压入操作栈栈顶

4.iinc 1 1

第一个参数是对哪个槽位做自增，第二个参数是自增多少
例如iinc 1 1意思是是局部变量表的1号槽位中的变量+1

下面开始++i的操作，顺序是先自增再iload

5.iinc 1 1

局部变量表的1号槽位中的变量+1，此时i=12

6.iload_1

将局部变量12压入操作数栈

7.iadd
弹出操作数栈栈顶的两个数字进行相加10+12 = 22，并将结果压入操作数栈

下面进行i–的操作
8.iload_1
将局部变量表1号槽位的变量压入栈顶

9.iinc 1 -1
对局部变量表1号槽位的变量进行-1操作

10.iadd
弹出栈顶的两个元素进行相加

istore 2
将栈顶元素弹出到局部变量表的2号槽位

故b=34，a=11
方法执行结束