通过JVM指令分析Java中i++与++i的区别

转自
https://blog.youkuaiyun.com/s394500839/article/details/80878485

通过jvm编译指令，来解析为何i++与++i运算过程的区别

i++

public void testFun(){
    int y = 13;
    int x = y++;
}

对应指令结构

public void testFun();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=3, args_size=1
         0: bipush        13
         2: istore_1
         3: iload_1
         4: iinc          1, 1
         7: istore_2
         8: return
      LineNumberTable:
        line 7: 0
        line 8: 3
        line 9: 8

++i

    public void testFun(){
        int y = 13;
        int x = ++y;
    }

 public void testFun();              
   descriptor: ()V                   
   flags: (0x0001) ACC_PUBLIC        
   Code:                             
     stack=1, locals=3, args_size=1  
        0: bipush        13          
        2: istore_1                  
        3: iinc          1, 1        
        6: iload_1                   
        7: istore_2                  
        8: return                    
     LineNumberTable:                
       line 7: 0                     
       line 8: 3                     
       line 9: 8            

分析

i++
3: iload_1 //从局部变量表的slot_1位置加载变量到栈帧中
4: iinc 1, 1 // 对局部变量表的slot_1位置的变量进行+1操作
7: istore_2 // 将栈顶的值保存到slot_2，即对应代码应该是 变量x

++i
3: iinc 1, 1 // 对局部变量表的slot_1的变量y进行+1操作
6: iload_1 // 从局部变量表的slot_1加载变量y到栈帧,此时加载的Y的值已经被+1了
7: istore_2 // 将帧栈的值，保存到局部变量表slot_2即变量x

通过上面的操作即可明显的看到i++与++i在指令的运算顺序的区别