从面试题i++和++i来看java字节码

我们先来看一道经典面试题

public class Demo05 {
    public static void main(String[] args) {
        int a = 10;
        int b = a++ + ++a + a--;
        System.out.println(a);
        System.out.println(b);
    }
}

使用

-v Demo05.class```进行反编译. -v表示输出类文件的详细信息

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Classfile /E:/workplace/abc/jvmStudy/out/production/jvmStudy/demo1/Demo05.class
Last modified 2020-2-23; size 567 bytes
MD5 checksum e378885aa1eef9f5fd48fda536b4467b
Compiled from “Demo05.java”
public class demo1.Demo05
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:

#1 = Methodref #5.#22 // java/lang/Object.”“:()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 // demo1/Demo05

#5 = Class #28 // java/lang/Object

#6 = Utf8

#7 = Utf8 ()V

#8 = Utf8 Code

#9 = Utf8 LineNumberTable

#10 = Utf8 LocalVariableTable

#11 = Utf8 this

#12 = Utf8 Ldemo1/Demo05;

#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 Demo05.java

#22 = NameAndType #6:#7 // ““:()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 demo1/Demo05

#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 demo1.Demo05();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object.”“:()V
4: return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Ldemo1/Demo05;

public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: 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/PrintStream.println:(I)V
25: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
28: iload_2
29: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
32: return
LineNumberTable:
line 5: 0
line 6: 3
line 7: 18
line 8: 25
line 9: 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: “Demo05.java”




现在我们逐行来分析反编译后的字节码

Classfile /E:/workplace/jvmStudy/out/production/jvmStudy/demo1/Demo05.class //字节码原文件所在位置
Last modified 2020-2-23; size 567 bytes //最后修改时间,以及文件大小
MD5 checksum e378885aa1eef9f5fd48fda536b4467b //MD5签名
Compiled from “Demo05.java” //字节码文件是由哪个文件编译得来的
public class demo1.Demo05 //类名
minor version: 0 // 小版本号
major version: 52 //大版本号 52 对应java8
flags: ACC_PUBLIC, ACC_SUPER //ACC_PUBLIC表示类的访问修饰符为Public ACC_SUPER,具体见表1




## 表1 类访问标记（access flags）

| Flag Name      | Value | Interpretation      |
| -------------- | ----- | ------------------- |
| ACC_PUBLIC     | 1     | 标识是否是 public        |
| ACC_FINAL      | 10    | 标识是否是 final         |
| ACC_SUPER      | 20    | 已经不用了               |
| ACC_INTERFACE  | 200   | 标识是类还是接口            |
| ACC_ABSTRACT   | 400   | 标识是否是 abstract      |
| ACC_SYNTHETIC  | 1000  | 编译器自动生成，不是用户源代码编译生成 |
| ACC_ANNOTATION | 2000  | 标识是否是注解类            |
| ACC_ENUM       | 4000  | 标识是否是枚举类            |

---

Constant pool: