Switch关键字理解
2020-06-14 本文已影响0人
ericsonyc
问题诞生
在一次代码review中，被老板喷了下为啥用switch而不用if；对此有点疑惑，当时的理由是：写switch会遗漏break，导致代码出现异常逻辑；当使用switch时，若break漏写的确会导致代码逻辑错误；但这理由并不充分，那么switch到底有啥优点呢？
下面我们探寻下switch的原理！
Switch用法
二话不说，先上代码：以java为例
int type = 1;
switch (type) {
case 1:
System.out.println("1");
break;
case 2:
System.out.println("2");
break;
case 3:
System.out.println("3");
break;
case 4:
System.out.println("4");
break;
case 5:
System.out.println("5");
break;
default:
System.out.println("default");
break;
}
switch用于条件选择，分支逻辑；switch接受基本类型：int，char，enum，short等，此外还支持String类型。
先查看下上述代码的字节码：
Code:
stack=2, locals=2, args_size=1
0: iconst_4
1: istore_1
2: iload_1
3: tableswitch { // 1 to 5
1: 36
2: 47
3: 58
4: 69
5: 80
default: 91
}
36: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
39: ldc #3 // String 1
41: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
44: goto 99
47: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
50: ldc #5 // String 2
52: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
55: goto 99
58: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
61: ldc #6 // String 3
63: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
66: goto 99
69: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
72: ldc #7 // String 4
74: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
77: goto 99
80: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
83: ldc #8 // String 5
85: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
88: goto 99
91: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
94: ldc #9 // String default
96: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
99: return
LineNumberTable:
line 9: 0
line 10: 2
line 12: 36
line 13: 44
line 15: 47
line 16: 55
line 18: 58
line 19: 66
line 21: 69
line 22: 77
line 24: 80
line 25: 88
line 27: 91
line 30: 99
从上述字节码可以看出，int类型的switch变成了tableswitch的一张表；运行时，直接从表中查找跳转地址，查询时间O(1)。
下面再看看String类型的switch代码：
String type = "love";
switch (type) {
case "the":
System.out.println("1");
break;
case "auto":
System.out.println("2");
break;
case "hello":
System.out.println("3");
break;
case "world":
System.out.println("4");
break;
case "love":
System.out.println("5");
break;
default:
System.out.println("default");
break;
}
字符串类型的switch的字节码又是咋样的呢？如下：
Code:
stack=2, locals=4, args_size=1
0: ldc #2 // String love
2: astore_1
3: aload_1
4: astore_2
5: iconst_m1
6: istore_3
7: aload_2
8: invokevirtual #3 // Method java/lang/String.hashCode:()I
11: lookupswitch { // 5
114801: 60
3005871: 74
3327858: 116
99162322: 88
113318802: 102
default: 127
}
60: aload_2
61: ldc #4 // String the
63: invokevirtual #5 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
66: ifeq 127
69: iconst_0
70: istore_3
71: goto 127
74: aload_2
75: ldc #6 // String auto
77: invokevirtual #5 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
80: ifeq 127
83: iconst_1
84: istore_3
85: goto 127
88: aload_2
89: ldc #7 // String hello
91: invokevirtual #5 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
94: ifeq 127
97: iconst_2
98: istore_3
99: goto 127
102: aload_2
103: ldc #8 // String world
105: invokevirtual #5 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
108: ifeq 127
111: iconst_3
112: istore_3
113: goto 127
116: aload_2
117: ldc #2 // String love
119: invokevirtual #5 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
122: ifeq 127
125: iconst_4
126: istore_3
127: iload_3
128: tableswitch { // 0 to 4
0: 164
1: 175
2: 186
3: 197
4: 208
default: 219
}
164: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
167: ldc #10 // String 1
169: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
172: goto 227
175: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
178: ldc #12 // String 2
180: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
183: goto 227
186: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
189: ldc #13 // String 3
191: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
194: goto 227
197: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
200: ldc #14 // String 4
202: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
205: goto 227
208: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
211: ldc #15 // String 5
213: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
216: goto 227
219: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
222: ldc #16 // String default
224: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
227: return
LineNumberTable:
line 9: 0
line 10: 3
line 12: 164
line 13: 172
line 15: 175
line 16: 183
line 18: 186
line 19: 194
line 21: 197
line 22: 205
line 24: 208
line 25: 216
line 27: 219
line 30: 227
从上面看出来，String的switch使用了tableswitch和lookupswitch两种表；我们来分析下这段字节码，其实做了下面几件事：
- JVM先创建tableswitch，用索引来表示真正执行的代码；
- 然后根据String的hashCode生成lookupswitch，key为hashCode，value为偏移量，主要用于跳转到tableswitch；
当一个String进入时，会在lookup中进行比较，时间复杂度为O(n)；然后tableswitch的时间复杂度为O(1)；整体的复杂度为O(n)。
那什么时候用tableswitch，什么时候用lookupswitch呢？？？
首先看下官方的介绍：
Where the cases of the switch are sparse, the table representation of the tableswitch instruction becomes inefficient in terms of space. The lookupswitch instruction may be used instead. The lookupswitch instruction pairs int keys (the values of the case labels) with target offsets in a table. When a lookupswitch instruction is executed, the value of the expression of the switch is compared against the keys in the table. If one of the keys matches the value of the expression, execution continues at the associated target offset. If no key matches, execution continues at the default target.
用中文翻译下就是：
当switch中case的值可以被表示为一个表中的索引值时，则使用tableswitch，比如：1，2，3，4这样的；当case的值非常稀疏，那用tableswitch做索引值时，会耗费大量的空间；所以lookupswitch的做法是：把case的int值和偏移量作为一对放在一个表里。
switch的问题解决了，还有一个问题，它比if有没有啥优点？？？
先上if的代码：
int type = 5;
if (type == 1) {
System.out.println("1");
} else if (type == 2) {
System.out.println("2");
} else if (type == 3) {
System.out.println("3");
} else if (type == 4) {
System.out.println("4");
} else if (type == 5) {
System.out.println("5");
} else {
System.out.println("default");
}</pre>
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="java" cid="n36" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-size: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; background-position: inherit inherit; background-repeat: inherit inherit;">Code:
stack=2, locals=2, args_size=1
0: iconst_5
1: istore_1
2: iload_1
3: iconst_1
4: if_icmpne 18
7: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
10: ldc #3 // String 1
12: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
15: goto 90
18: iload_1
19: iconst_2
20: if_icmpne 34
23: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
26: ldc #5 // String 2
28: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
31: goto 90
34: iload_1
35: iconst_3
36: if_icmpne 50
39: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
42: ldc #6 // String 3
44: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
47: goto 90
50: iload_1
51: iconst_4
52: if_icmpne 66
55: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
58: ldc #7 // String 4
60: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
63: goto 90
66: iload_1
67: iconst_5
68: if_icmpne 82
71: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
74: ldc #8 // String 5
76: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
79: goto 90
82: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
85: ldc #9 // String default
87: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
90: return
LineNumberTable:
line 9: 0
line 10: 2
line 11: 7
line 12: 18
line 13: 23
line 14: 34
line 15: 39
line 16: 50
line 17: 55
line 18: 66
line 19: 71
line 21: 82
line 23: 90
相比于相应的switch代码，if代码每次都会加载下条件，而switch只是第一次加载条件；因此，理论上switch比if快。
后记
本文还有几个待解决的问题：
（1）tableswitch和lookupswitch内部如何实现的？跳表，需要深入研究下；
（2）当代码中条件特别多的时候，代码会出现大量的分支，应该如何减少if或者switch代码？
参考
[1] Java字节码：http://gityuan.com/2015/10/24/jvm-bytecode-grammar/
