为什么 array[idx++]+="a" 在 Java 8 中增加了一次 idx 而在 Java 9 和 10 中增加了两次？
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Why does array[idx++]+="a" increase idx once in Java 8 but twice in Java 9 and 10?
提问by Olivier Grégtheitroade
For a challenge, a fellow code golferwrote the following code:
import java.util.*;
public class Main {
public static void main(String[] args) {
int size = 3;
String[] array = new String[size];
Arrays.fill(array, "");
for(int i = 0; i <= 100; ) {
array[i++%size] += i + " ";
}
for(String element: array) {
System.out.println(element);
}
}
}
When running this code in Java 8, we get the following result:
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97 100
2 5 8 11 14 17 20 23 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80 83 86 89 92 95 98 101
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99
When running this code in Java 10, we get the following result:
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100
The numbering is entirely off using Java 10. So what is happening here? Is it a bug in Java 10?
使用 Java 10 完全不进行编号。那么这里发生了什么？这是 Java 10 中的错误吗？
Follow ups from the comments:
来自评论的跟进：
- The issue appears when compiled with Java 9 or later (we found it in Java 10). Compiling this code on Java 8, then running in Java 9 or any later version, including Java 11 early access, gives the expected result.
- This kind of code is non-standard, but is valid according to the spec. It was found by Kevin Cruijssenin a discussion in a golfing challenge, hence the weird use case encountered.
Didier Lfound out that the issue can be reproduced with the much smaller and more understandable code:
class Main { public static void main(String[] args) { String[] array = { "" }; array[test()] += "a"; } static int test() { System.out.println("evaluated"); return 0; } }Result when compiled in Java 8:
evaluatedResult when compiled in Java 9 and 10:
evaluated evaluatedThe issue seems to be limited to the string concatenation and assignment operator (
+=) with an expression with side effect(s) as the left operand, like inarray[test()]+="a",array[ix++]+="a",test()[index]+="a", ortest().field+="a". To enable string concatenation, at least one of the sides must have typeString. Trying to reproduce this on other types or constructs failed.
- 使用 Java 9 或更高版本（我们在 Java 10 中发现）编译时会出现此问题。在 Java 8 上编译此代码，然后在 Java 9 或任何更高版本（包括 Java 11 早期访问）中运行，会给出预期的结果。
- 这种代码是非标准的，但根据规范是有效的。它是由Kevin Cruijssen在一次高尔夫挑战中的讨论中发现的，因此遇到了奇怪的用例。
Didier L发现这个问题可以用更小更易理解的代码重现：
class Main { public static void main(String[] args) { String[] array = { "" }; array[test()] += "a"; } static int test() { System.out.println("evaluated"); return 0; } }evaluatedevaluated evaluated这个问题似乎限于字符串连接和赋值运算符（
+=）与副作用（一个或多个）的表达式作为左操作数，如在array[test()]+="a"，array[ix++]+="a"，test()[index]+="a"，或test().field+="a"。要启用字符串连接，至少一侧必须具有 typeString。尝试在其他类型或构造上重现此失败。
采纳答案by Jorn Vernee
This is a bug in javacstarting from JDK 9 (which made some changes with regard to string concatenation, which I suspect is part of the problem), as confirmed by the javacteam under the bug id JDK-8204322. If you look at the corresponding bytecode for the line:
这是javac从 JDK 9 开始的一个错误（它对字符串连接进行了一些更改，我怀疑这是问题的一部分），正如javac团队在错误 ID JDK-8204322 下确认的那样。如果您查看该行的相应字节码：
array[i++%size] += i + " ";
It is:
这是：
21: aload_2
22: iload_3
23: iinc 3, 1
26: iload_1
27: irem
28: aload_2
29: iload_3
30: iinc 3, 1
33: iload_1
34: irem
35: aaload
36: iload_3
37: invokedynamic #5, 0 // makeConcatWithConstants:(Ljava/lang/String;I)Ljava/lang/String;
42: aastore
Where the last aaloadis the actual load from the array. However, the part
最后一个aaload是数组的实际负载。然而，该部分
21: aload_2 // load the array reference
22: iload_3 // load 'i'
23: iinc 3, 1 // increment 'i' (doesn't affect the loaded value)
26: iload_1 // load 'size'
27: irem // compute the remainder
Which roughly corresponds to the expression array[i++%size](minus the actual load and store), is in there twice. This is incorrect, as the spec says in jls-15.26.2:
大致对应于表达式array[i++%size]（减去实际加载和存储），在那里两次。这是不正确的，正如jls-15.26.2 中的规范所说：
A compound assignment expression of the form
E1 op= E2is equivalent toE1 = (T) ((E1) op (E2)), whereTis the type ofE1, except thatE1is evaluated only once.
形式的复合赋值表达式
E1 op= E2等价于E1 = (T) ((E1) op (E2))，其中T是 的类型E1，只是E1只计算一次。
So, for the expression array[i++%size] += i + " ";, the part array[i++%size]should only be evaluated once. But it is evaluated twice (once for the load, and once for the store).
因此，对于表达式array[i++%size] += i + " ";，该部分array[i++%size]应该只计算一次。但它会被评估两次（一次用于负载，一次用于存储）。
So yes, this is a bug.
所以是的，这是一个错误。
Some updates:
一些更新：
The bug is fixed in JDK 11 and there will be a back-port to JDK 10(but not JDK 9, since it no longer receives public updates).
该错误已在 JDK 11 中修复，并且将有一个到 JDK 10的后向端口（但不是 JDK 9，因为它不再接收公共更新）。
Aleksey Shipilev mentions on the JBS page(and @DidierL in the comments here):
Aleksey Shipilev 在JBS 页面上提到（和@DidierL 在这里的评论中）：
Workaround: compile with
-XDstringConcat=inline
解决方法：编译
-XDstringConcat=inline
That will revert to using StringBuilderto do the concatenation, and doesn't have the bug.
这将恢复用于进行StringBuilder连接，并且没有错误。
