Eclipse JDT与jdk7u10生成的switch代码对比

public static void main(String[] a) {
	String target="abc";
	String r;
	switch(target){
	case "abc":
		r="cba";
		break;
	default:
	case "def":
		r="fed";
	}
	System.out.println(r);
}

Eclipse JDT

Stack: 2, Locals: 4
 0  ldc <String "abc"> [16]
 2  astore_1 [target]
 3  aload_1 [target]
 4  dup
 5  astore_3
 6  invokevirtual java.lang.String.hashCode() : int [18]
 9  lookupswitch default: 66
	  case 96354: 36
	  case 99333: 48
36  aload_3
37  ldc <String "abc"> [16]
39  invokevirtual java.lang.String.equals(java.lang.Object) : boolean [24]
42  ifne 60
45  goto 66
48  aload_3
49  ldc <String "def"> [28]
51  invokevirtual java.lang.String.equals(java.lang.Object) : boolean [24]
54  ifne 66
57  goto 66
60  ldc <String "cba"> [30]
62  astore_2 [r]
63  goto 69
66  ldc <String "fed"> [32]
68  astore_2 [r]
69  getstatic java.lang.System.out : java.io.PrintStream [34]
72  aload_2 [r]
73  invokevirtual java.io.PrintStream.println(java.lang.String) : void [40]
76  return

经过：
1.switch通过hashcode找出大概位置（因为有hashcode冲撞的可能）
2.调用String.equals方法找到正确目标字节码位置，直接执行操作

JDK7u10

stack=2, locals=5, args_size=1
 0: ldc           #2                  // String abc
 2: astore_1      
 3: aload_1       
 4: astore_3      
 5: iconst_m1     
 6: istore        4
 8: aload_3       
 9: invokevirtual #3                  // Method java/lang/String.hashCode:()I
12: lookupswitch  { // 2
		   96354: 40
		   99333: 55
		 default: 67
	}
40: aload_3       
41: ldc           #2                  // String abc
43: invokevirtual #4                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
46: ifeq          67
49: iconst_0      
50: istore        4
52: goto          67
55: aload_3       
56: ldc           #5                  // String def
58: invokevirtual #4                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
61: ifeq          67
64: iconst_2      
65: istore        4
67: iload         4
69: lookupswitch  { // 2
			   0: 96
			   2: 102
		 default: 102
	}
96: ldc           #6                  // String cba
98: astore_2      
99: goto          105
102: ldc           #7                  // String fed
104: astore_2      
105: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
108: aload_2       
109: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
112: return        

经过：
1.switch通过hashcode找出大概位置（因为有hashcode冲撞的可能）
2.调用String.equals方法找到正确目标，此目标已经在内部生成字节码时用int值代替
3.通过switch代替的int值，找到正确目标字节码位置，直接执行操作


总结：
Eclipse JDT对switch字符串生成的字节码更高效，更紧凑

作者：翁志艺