从JVM指令看i++

介绍

最近在B站看面试题，碰到了这么一道题目¹(参考提供了链接)，虽然感觉出题人是憨憨，题目出得没啥意思，转念一想，既然看到了并且花了好些时间才弄明白，那就顺便水一篇博客。

题目如下：

public class Main {
    public static void main(String[] args) {
        int i = 1;
        i = i++;
        int j = i++;
        int k = i + ++i * i++;
        System.out.println("i=" + i);
        System.out.println("j=" + j);
        System.out.println("k=" + k);
    }
}

给出程序运行结果。参考所提供的链接视频中给出了答题思路，但是我没听清楚，本文展示我的分析过程。

首先，对生成的class文件进行反汇编，同时保留额外的编译信息：

javap -v Main.class
Classfile /D:/Private/Code/Java/Hello/target/classes/top/hellooooo/interview/Main.class
  Last modified 26 Oct 2020; size 1170 bytes
  MD5 checksum c05c1e0da276f3927cba38f6d89d57f7
  Compiled from "Main.java"
public class top.hellooooo.interview.Main
  minor version: 0
  major version: 55
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #7                          // top/hellooooo/interview/Main
  super_class: #8                         // java/lang/Object
  interfaces: 0, fields: 0, methods: 2, attributes: 3
Constant pool:
   #1 = Methodref          #8.#27         // java/lang/Object."<init>":()V
   #2 = Fieldref           #28.#29        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = InvokeDynamic      #0:#33         // #0:makeConcatWithConstants:(I)Ljava/lang/String;
   #4 = Methodref          #34.#35        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = InvokeDynamic      #1:#33         // #1:makeConcatWithConstants:(I)Ljava/lang/String;
   #6 = InvokeDynamic      #2:#33         // #2:makeConcatWithConstants:(I)Ljava/lang/String;
   #7 = Class              #38            // top/hellooooo/interview/Main
   #8 = Class              #39            // java/lang/Object
   #9 = Utf8               <init>
  #10 = Utf8               ()V
  #11 = Utf8               Code
  #12 = Utf8               LineNumberTable
  #13 = Utf8               LocalVariableTable
  #14 = Utf8               this
  #15 = Utf8               Ltop/hellooooo/interview/Main;
  #16 = Utf8               main
  #17 = Utf8               ([Ljava/lang/String;)V
  #18 = Utf8               args
  #19 = Utf8               [Ljava/lang/String;
  #20 = Utf8               i
  #21 = Utf8               I
  #22 = Utf8               j
  #23 = Utf8               k
  #24 = Utf8               MethodParameters
  #25 = Utf8               SourceFile
  #26 = Utf8               Main.java
  #27 = NameAndType        #9:#10         // "<init>":()V
  #28 = Class              #40            // java/lang/System
  #29 = NameAndType        #41:#42        // out:Ljava/io/PrintStream;
  #30 = Utf8               BootstrapMethods
  #31 = MethodHandle       6:#43          // REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/in
voke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite
;
  #32 = String             #44            // i=\u0001
  #33 = NameAndType        #45:#46        // makeConcatWithConstants:(I)Ljava/lang/String;
  #34 = Class              #47            // java/io/PrintStream
  #35 = NameAndType        #48:#49        // println:(Ljava/lang/String;)V
  #36 = String             #50            // j=\u0001
  #37 = String             #51            // k=\u0001
  #38 = Utf8               top/hellooooo/interview/Main
  #39 = Utf8               java/lang/Object
  #40 = Utf8               java/lang/System
  #41 = Utf8               out
  #42 = Utf8               Ljava/io/PrintStream;
  #43 = Methodref          #52.#53        // java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandle
s$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
  #44 = Utf8               i=\u0001
  #45 = Utf8               makeConcatWithConstants
  #46 = Utf8               (I)Ljava/lang/String;
  #47 = Utf8               java/io/PrintStream
  #48 = Utf8               println
  #49 = Utf8               (Ljava/lang/String;)V
  #50 = Utf8               j=\u0001
  #51 = Utf8               k=\u0001
  #52 = Class              #54            // java/lang/invoke/StringConcatFactory
  #53 = NameAndType        #45:#58        // makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang
/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
  #54 = Utf8               java/lang/invoke/StringConcatFactory
  #55 = Class              #60            // java/lang/invoke/MethodHandles$Lookup
  #56 = Utf8               Lookup
  #57 = Utf8               InnerClasses
  #58 = Utf8               (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljav
a/lang/Object;)Ljava/lang/invoke/CallSite;
  #59 = Class              #61            // java/lang/invoke/MethodHandles
  #60 = Utf8               java/lang/invoke/MethodHandles$Lookup
  #61 = Utf8               java/lang/invoke/MethodHandles
{
  public top.hellooooo.interview.Main();
    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 8: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Ltop/hellooooo/interview/Main;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=4, args_size=1
         0: iconst_1
         1: istore_1
         2: iload_1
         3: iinc          1, 1
         6: istore_1
         7: iload_1
         8: iinc          1, 1
        11: istore_2
        12: iload_1
        13: iinc          1, 1
        16: iload_1
        17: iload_1
        18: iinc          1, 1
        21: imul
        22: iadd
        23: istore_3
        24: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        27: iload_1
        28: invokedynamic #3,  0              // InvokeDynamic #0:makeConcatWithConstants:(I)Ljava/lang/String;
        33: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        36: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        39: iload_2
        40: invokedynamic #5,  0              // InvokeDynamic #1:makeConcatWithConstants:(I)Ljava/lang/String;
        45: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        48: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        51: iload_3
        52: invokedynamic #6,  0              // InvokeDynamic #2:makeConcatWithConstants:(I)Ljava/lang/String;
        57: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        60: return
      LineNumberTable:
        line 10: 0
        line 11: 2
        line 12: 7
        line 13: 12
        line 14: 24
        line 15: 36
        line 16: 48
        line 17: 60
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      61     0  args   [Ljava/lang/String;
            2      59     1     i   I
           12      49     2     j   I
           24      37     3     k   I
    MethodParameters:
      Name                           Flags
      args
}
SourceFile: "Main.java"
InnerClasses:
  public static final #56= #55 of #59;    // Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
  0: #31 REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang
/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
    Method arguments:
      #32 i=\u0001
  1: #31 REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang
/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
    Method arguments:
      #36 j=\u0001
  2: #31 REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang
/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
    Method arguments:
      #37 k=\u0001

取出main方法逻辑：

         0: iconst_1
         1: istore_1
         2: iload_1
         3: iinc          1, 1
         6: istore_1
         7: iload_1
         8: iinc          1, 1
        11: istore_2
        12: iload_1
        13: iinc          1, 1
        16: iload_1
        17: iload_1
        18: iinc          1, 1
        21: imul
        22: iadd
        23: istore_3

JVM指令

instruction	description
iconst_1	将int型1推送至栈顶
istore_1	将栈顶int型数值存入第二个本地变量
iload_1	将第二个int型本地变量推送至栈顶
iinc	把一个常量值加到一个int类型的局部变量上
istore_2	将栈顶int型数值存入第三个本地变量
imul	将栈顶两int型数值相乘并将结果压入栈顶
iadd	将栈顶两int型数值相加并将结果压入栈顶
istore_3	将栈顶int型数值存入第四个本地变量

解析

本题主要涉及JVM Frame的Local Variables以及Operand Stacks（可看The Java® VirtualMachine Specification 2.6.1以及2.6.2）

Operation	LocalVariables（index:value）	OperandStack（top->buttom）
0: iconst_1		1
1: istore_1	2:1(取出栈顶元素存入本地变量表)	null
2: iload_1	2:1	1
3: iinc 1, 1	2:2	1
6: istore_1	2:1	null
7: iload_1	2:1	1
8: iinc 1, 1	2:2	1
11: istore_2	2:2 3:1	null
12: iload_1	2:2 3:1	2
13: iinc 1, 1	2:3 3:1
16: iload_1		3 2
17: iload_1		3 3 2
18: iinc 1, 1	2:4 3:1
21: imul		9 2
22: iadd		11
23: istore_3	2:4 3:1 4:11	null

所以最后输出的结果为：4 1 11

以上仅我一人看法，如发现错误烦请留言帮忙指出。

小结

这个小结是从视频结尾看到的,感觉非常不错,所以转载过来.视频就是参考1

=右边的从左到右加载值一次压入操作数栈
自增,自减操作都是直接修改变量的值,不经过操作数栈
最后的赋值之前,临时结果也是存储在操作数栈中