Switch关键字理解

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/