i++与 ++i 的区别

首先看一段代码

        int i=1,j=2;
        int a = i++;
        int b = ++j;

上述代码对应的字节码

 0 iconst_1
 1 istore_1
 2 iconst_2
 3 istore_2
 4 iload_1
 5 iinc 1 by 1
 8 istore_3
 9 iinc 2 by 1
12 iload_2
13 istore 4

从4、5行号的指令可以看出，i++是先将i从局部变量表取出到操作数栈中，然后将局部变量表中的i加1.
++j对应9、12行号的之指令，是先将局部变量表中的j加1，然后取出到操作数栈中。
jdk1.8 jvm规范中对IINC指令的描述：

Description
The index is an unsigned byte that must be an index into the local variable array of the current frame (§2.6). The const is an immediate signed byte. The local variable at index must contain an int. The value const is first sign-extended to an int, and then the local variable at index is incremented by that amount.