根据字节码探讨java自增运算符的原理
public class Test { static int x, y; public static void main(String args[]) { x++; myMethod(); System.out.println(x + y + ++x); } public static void myMethod() { y = x++ + ++x; } }
如果以上代码的结果你很自信能做对,那么本文或许对你帮助不大,但仍然可以看下java底层的实现.在最后将给出以上代码的结果以及解析.
本文中的例子主要针对以下情况:
①x=y++
②x=++y
③x=x++
④x=++x
a:x,y为形参
b:x,y为成员变量
废话不多说,直接上代码
代码1(①+b):
public class Test { static int x,y; public static void main(String args[]){ test(); } public static void test(){ x = y++; System.out.println(x+""+y); } }
结果:01
test()字节码:
0: getstatic #3 // Field y:I 3: dup 4: iconst_1 5: iadd 6: putstatic #3 // Field y:I 9: putstatic #4 // Field x:I 12: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 15: getstatic #4 // Field x:I 18: invokevirtual #6 // Method java/io/PrintStream.println:(I)V 21: return
解释:
0:y(值)入操作栈
3:得到y(值)的一个快照y'(值)
(个人认为相当于是将栈顶元素也就是y(值)复制了一份,然后将复制得到的y'(值)入操作栈,现在操作栈中有y(值)和y'(值))
4:常量1入操作栈
5:常量1和y'(值)弹出栈,进行加操作,并将结果s入栈
6:将结果s弹出栈,赋给y(变量)(此时y==1)
9:将y(值)弹出栈,赋给x(变量)(此时x==0)
因为y(值)入操作栈之后没有修改,所以x依旧是0,而y变成了1
代码2(②+b):
public class Test { static int x,y; public static void main(String args[]){ test(); } public static void test(){ x = ++y; System.out.println(x+""+y); } }
结果11
test()字节码:
0: getstatic #3 // Field y:I 3: iconst_1 4: iadd 5: dup 6: putstatic #3 // Field y:I 9: putstatic #4 // Field x:I 12: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 15: getstatic #4 // Field x:I 18: invokevirtual #6 // Method java/io/PrintStream.println:(I)V 21: return
解释:
0:y(值)入操作栈
3:常量1入操作栈
4:常量1和y(值)弹出栈,进行加操作,并将结果s入栈
5:得到栈顶元素也就是s的快照s',并入操作栈
6:将s'弹出栈,并赋给y(变量)(此时y==1)
9:将s弹出栈,并赋给x(变量)(此时x==1)
代码3(③+b):
public class Test { static int x; public static void main(String args[]){ test(); } public static void test(){ x = x++; System.out.println(x); } }
结果:0
test()字节码:
0: getstatic #3 // Field x:I 3: dup 4: iconst_1 5: iadd 6: putstatic #3 // Field x:I 9: putstatic #3 // Field x:I 12: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream; 15: getstatic #3 // Field x:I 18: invokevirtual #5 // Method java/io/PrintStream.println:(I)V 21: return
解释:
0:x(值)入操作栈
3:得到x(值)的快照x',入操作栈
4:常量1入操作栈
5:常量1和x'弹出操作栈,进行加操作,将结果s入操作栈
6:将s弹出栈,并赋给x(变量)(此时x==1)
9:将x(值)弹出栈,并赋给x(变量)(此时x的值被覆盖,x==0)
代码4(④+b):
public class Test { static int x; public static void main(String args[]){ test(); } public static void test(){ x = ++x; System.out.println(x); } }
结果:1
test()字节码:
0: getstatic #3 // Field x:I 3: iconst_1 4: iadd 5: dup 6: putstatic #3 // Field x:I 9: putstatic #3 // Field x:I 12: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream; 15: getstatic #3 // Field x:I 18: invokevirtual #5 // Method java/io/PrintStream.println:(I)V 21: return
解释:
0:x(值)入操作栈
3:常量1如入操作栈
4:常量1和x(值)弹出操作栈,进行加操作,并将结果s入操作栈
5:得到栈顶元素s的快照s',入操作栈
6.将s'弹出操作栈,并赋给x(变量)(此时x==1)
9:将s弹出操作栈,并赋给x(变量)(此时x==1)
代码5(①+a):
public class Test { public static void main(String args[]){ test(0,0); } public static void test(int x,int y){ x = y++; System.out.println(x+""+y); } }
结果:01
test()字节码:
0: iload_1 1: iinc 1, 1 4: istore_0 5: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream; 8: new #4 // class java/lang/StringBuilder 11: dup 12: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V 15: iload_0 16: invokevirtual #6 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder; 19: ldc #7 // String 21: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 24: iload_1 25: invokevirtual #6 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder; 28: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 31: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 34: return
解释:
0:将本地变量区的y(值)入操作栈
1:将本地变量y加1(y==1)
4:将0中的y(值)弹出栈,并赋给本地变量区的x(x==0)
代码6(②+a):
public class Test { public static void main(String args[]){ test(0,0); } public static void test(int x,int y){ x = ++y; System.out.println(x+""+y); } }
结果:11
test()字节码:
0: iinc 1, 1
3: iload_1
4: istore_0
5: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
8: new #4 // class java/lang/StringBuilder
11: dup
12: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
15: iload_0
16: invokevirtual #6 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
19: ldc #7 // String
21: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: iload_1
25: invokevirtual #6 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
28: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
31: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
34: return
解释:
0:将本地变量y加1(此时y==1)
3:将本地变量y(值)入操作栈
4:将y(值)弹出操作栈,并赋给x(此时x==1)
代码7(③+a):
public class Test { public static void main(String args[]){ test(0); } public static void test(int x){ x = x++; System.out.println(x); } }
结果:0
test()字节码:
0: iload_0 1: iinc 0, 1 4: istore_0 5: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream; 8: iload_0 9: invokevirtual #4 // Method java/io/PrintStream.println:(I)V 12: return
解释:
0:本地变量x(值)入操作栈
1:本地变量x加1(此时x==1)
4:将x(值)弹出栈,并赋给本地变量x(此时x==0)
代码8(④+a):
public class Test { public static void main(String args[]){ test(0); } public static void test(int x){ x = ++x; System.out.println(x); } }
结果:1
test()字节码:
0: iinc 0, 1
3: iload_0
4: istore_0
5: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
8: iload_0
9: invokevirtual #4 // Method java/io/PrintStream.println:(I)V
12: return
解释:
0:本地变量x加1(此时x==1)
3:本地变量x(值)入操作栈
4:将x(值)弹出操作栈,并赋给x(此时x==1)
事实上i++和++i在底层的实现都是先自增,区别在于返回值.i++返回自增前的值,++i返回自增后的值
现在来看看一开始那段代码的结果和解析:
结果:11
myMethod()字节码:
0: getstatic #2 // Field x:I 3: dup 4: iconst_1 5: iadd 6: putstatic #2 // Field x:I 9: getstatic #2 // Field x:I 12: iconst_1 13: iadd 14: dup 15: putstatic #2 // Field x:I 18: iadd 19: putstatic #5 // Field y:I 22: return
解释:
0:变量x(值)入操作栈(栈状态:x0)
3:得到栈顶元素x(值)的快照x'(值),并入操作栈(栈状态:x0->x0')
4:常量1入操作栈(栈状态:x0->x0'->1)
5:常量1和x'(值)弹出操作栈,进行加操作,将结果s0入操作栈(栈状态:x0->s0)
6:弹出s0,并赋给x(变量)(栈状态:x0,此时x(变量)==2)
9:将修改后的x(值)入操作栈(栈状态:x0->x1)
12:常量1入操作栈(栈状态:x0->x1->1)
13:常量1和X1(值)弹出操作栈,进行加操作,将结果s1入操作栈(栈状态:x0->s1)
14:得到栈顶元素s1(值)的快照s1'(值),并入操作栈(栈状态:x0->s1->s1')
15:弹出s1'并赋给x(变量)(栈状态:x0->s1)
18:s1和x0弹出栈,进行加操作,将结果s2入栈(栈状态:s2)
19:弹出s2,并赋给y(变量)
所以在myMethod之后x的值为经过两次自增后的值,为x+2==3,而y的值为x0+s1,其中x0为最初传进来的值==1,s1是x经过两次自增后的值==3,所以y==4
x==3,y==4
最后输出的结果就是3+4+4==11
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