java-i++和++i从jvm层面解释
2020-12-07 本文已影响0人
倔强青铜弟中弟
代码如下：
public static void main(String arg[]) {
int a = 0;
a++;
System.out.println(a);
int b = 0;
++b;
System.out.println(b);
int c = 0;
System.out.println(b++);
int d = 0;
System.out.println(++d);
}
查看编译后的class文件
查看方式：可以直接使用命令：javap -v main.class。或者使用idea扩展插件jclasslib。使用方法可以自己百度吧。我不经常查看class文件，是使用的是命令。下面是完整的输出结果
Classfile /E:/MyProject/kite-school/kite-school-security/target/classes/com/school/mainTest.class
Last modified 2020年12月7日; size 854 bytes
MD5 checksum 6687beca749c37871dca7cd1caba837c
Compiled from "mainTest.java"
public class com.school.mainTest
minor version: 0
major version: 52
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #4 // com/school/mainTest
super_class: #5 // java/lang/Object
interfaces: 0, fields: 0, methods: 2, attributes: 2
Constant pool:
#1 = Methodref #5.#31 // java/lang/Object."<init>":()V
#2 = Fieldref #32.#33 // java/lang/System.out:Ljava/io/PrintStream;
#3 = Methodref #34.#35 // java/io/PrintStream.println:(I)V
#4 = Class #36 // com/school/mainTest
#5 = Class #37 // java/lang/Object
#6 = Class #38 // com/school/mainTest$1
#7 = Utf8 InnerClasses
#8 = Class #39 // com/school/mainTest$ObjectClass
#9 = Utf8 ObjectClass
#10 = Class #40 // com/school/mainTest$ClassTest
#11 = Utf8 ClassTest
#12 = Utf8 <init>
#13 = Utf8 ()V
#14 = Utf8 Code
#15 = Utf8 LineNumberTable
#16 = Utf8 LocalVariableTable
#17 = Utf8 this
#18 = Utf8 Lcom/school/mainTest;
#19 = Utf8 main
#20 = Utf8 ([Ljava/lang/String;)V
#21 = Utf8 arg
#22 = Utf8 [Ljava/lang/String;
#23 = Utf8 a
#24 = Utf8 I
#25 = Utf8 b
#26 = Utf8 c
#27 = Utf8 d
#28 = Utf8 MethodParameters
#29 = Utf8 SourceFile
#30 = Utf8 mainTest.java
#31 = NameAndType #12:#13 // "<init>":()V
#32 = Class #41 // java/lang/System
#33 = NameAndType #42:#43 // out:Ljava/io/PrintStream;
#34 = Class #44 // java/io/PrintStream
#35 = NameAndType #45:#46 // println:(I)V
#36 = Utf8 com/school/mainTest
#37 = Utf8 java/lang/Object
#38 = Utf8 com/school/mainTest$1
#39 = Utf8 com/school/mainTest$ObjectClass
#40 = Utf8 com/school/mainTest$ClassTest
#41 = Utf8 java/lang/System
#42 = Utf8 out
#43 = Utf8 Ljava/io/PrintStream;
#44 = Utf8 java/io/PrintStream
#45 = Utf8 println
#46 = Utf8 (I)V
{
public com.school.mainTest();
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 17: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/school/mainTest;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=5, args_size=1
0: iconst_0
1: istore_1
2: iinc 1, 1
5: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
8: iload_1
9: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
12: iconst_0
13: istore_2
14: iinc 2, 1
17: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
20: iload_2
21: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
24: iconst_0
25: istore_3
26: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
29: iload_2
30: iinc 2, 1
33: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
36: iconst_0
37: istore 4
39: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
42: iinc 4, 1
45: iload 4
47: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
50: return
LineNumberTable:
line 33: 0
line 34: 2
line 35: 5
line 37: 12
line 38: 14
line 39: 17
line 41: 24
line 42: 26
line 44: 36
line 45: 39
line 46: 50
LocalVariableTable:
Start Length Slot Name Signature
0 51 0 arg [Ljava/lang/String;
2 49 1 a I
14 37 2 b I
26 25 3 c I
39 12 4 d I
MethodParameters:
Name Flags
arg
}
SourceFile: "mainTest.java"
InnerClasses:
static #6; // class com/school/mainTest$1
static #11= #10 of #4; // ClassTest=class com/school/mainTest$ClassTest of class com/school/mainTest
重点：
%DHAY97J{)VT54QE4GA2N5W.png
可以看到i++和++i的不同主要在于对操作数栈和局部变量表的操作顺序不同，导致当操作i的时候，发现i的值有所不同。这里大家可以参考下面的命令解释，然后再仔细看看我的截图，就会有很好的了解。更多的 指令集见：java class源文件指令集解释
使用命令解释
D5982X30(X7H2$Q{2(2~%BT.jpg
i++和++i线程安全问题
i++和++i的线程安全分为两种情况：
1、如果i是局部变量（在方法里定义的），那么是线程安全的。因为局部变量是线程私有的，别的线程访问不到，其实也可以说没有线程安不安全之说，因为别的线程对他造不成影响。
2、如果i是全局变量（类的成员变量），那么是线程不安全的。因为如果是全局变量的话，同一进程中的不同线程都有可能访问到。
可以使用Synchronized锁或者Lock接口的实现类锁，都可以
如果有大量线程同时执行i++操作，i变量的副本拷贝到每个线程的线程栈，当同时有两个线程栈以上的线程读取线程变量，假如此时是1的话，那么同时执行i++操作，再写入到全局变量，最后两个线程执行完，i会等于3而不会是2，所以，出现不安全性。
