重学 i++ 和++i

public void calc(){
        int i = 1;
        i = i++;
        System.out.println(i);
    }

对应的字节码指令如下：

calc:()V
000:    iconst_1                                {Lsample/HelloWorld;, .} | {}
001:    istore_1                                {Lsample/HelloWorld;, .} | {I}
002:    iload_1                                 {Lsample/HelloWorld;, I} | {}
003:    iinc 1 1                                {Lsample/HelloWorld;, I} | {I}
004:    istore_1                                {Lsample/HelloWorld;, I} | {I}
005:    getstatic System.out                    {Lsample/HelloWorld;, I} | {}
006:    iload_1                                 {Lsample/HelloWorld;, I} | {Ljava/io/PrintStream;}
007:    invokevirtual PrintStream.println       {Lsample/HelloWorld;, I} | {Ljava/io/PrintStream;, I}
008:    return                                  {Lsample/HelloWorld;, I} | {}

public void calc(){
        int i = 1;
        i = ++i;
        System.out.println(i);
    }


对应的字节码指令如下：

calc:()V
000:    iconst_1                                {Lsample/HelloWorld;, .} | {}
001:    istore_1                                {Lsample/HelloWorld;, .} | {I}
002:    iinc 1 1                                {Lsample/HelloWorld;, I} | {}
003:    iload_1                                 {Lsample/HelloWorld;, I} | {}
004:    istore_1                                {Lsample/HelloWorld;, I} | {I}
005:    getstatic System.out                    {Lsample/HelloWorld;, I} | {}
006:    iload_1                                 {Lsample/HelloWorld;, I} | {Ljava/io/PrintStream;}
007:    invokevirtual PrintStream.println       {Lsample/HelloWorld;, I} | {Ljava/io/PrintStream;, I}
008:    return                                  {Lsample/HelloWorld;, I} | {}

iload istore image.png iinc.png

指定包名，统计方法耗时

//代码

public void calc(){
        int i = 1;
        i = i++;
        System.out.println(i);
    }

//对应的字节码指令如下
calc:()V
000:    iconst_1                                {Lsample/HelloWorld;, .} | {I}
001:    istore_1                                {Lsample/HelloWorld;, I} | {}
002:    iload_1                                 {Lsample/HelloWorld;, I} | {I} //把局部变量表索引为1位置的值   放到操作数栈中  （操作数栈顶元素的值为 1）
003:    iinc 1 1                                {Lsample/HelloWorld;, I} | {I} //把局部变量表索引为1位置的值加1  (局部变量表该位置的值 i=2)
//栈顶元素弹栈并把值（栈顶元素的值为1）存到局部变量表索引为1的位置（局部变量表索引为1位置的值由2变为1）
004:    istore_1                                {Lsample/HelloWorld;, I} | {}  
005:    getstatic System.out                    {Lsample/HelloWorld;, I} | {Ljava/io/PrintStream;} 
006:    iload_1                                 {Lsample/HelloWorld;, I} | {Ljava/io/PrintStream;, I}
007:    invokevirtual PrintStream.println       {Lsample/HelloWorld;, I} | {}
008:    return                                  

//代码

public void calc(){
        int i = 1;
        i = ++i;
        System.out.println(i);
    }
//对应的字节码指令如下
calc:()V
000:    iconst_1                                {Lsample/HelloWorld;, .} | {I}
001:    istore_1                                {Lsample/HelloWorld;, I} | {}  
002:    iinc 1 1                                {Lsample/HelloWorld;, I} | {} //把局部变量表索引为1位置的值加1 (i = 2)
003:    iload_1                                 {Lsample/HelloWorld;, I} | {I}//把局部变量表索引为1位置的值放到操作数栈中（栈顶元素值为2）
//栈顶元素弹栈并把值（栈顶元素的值为2）存到局部变量表索引为1的位置（局部变量表索引为1位置的值由1变为2）
004:    istore_1                                {Lsample/HelloWorld;, I} | {}
005:    getstatic System.out                    {Lsample/HelloWorld;, I} | {Ljava/io/PrintStream;}
006:    iload_1                                 {Lsample/HelloWorld;, I} | {Ljava/io/PrintStream;, I}
007:    invokevirtual PrintStream.println       {Lsample/HelloWorld;, I} | {}
008:    return