为什么数组[idx ++] + =“a”在Java 8中增加一次idx，在Java 9和10中增加两次？

本文翻译自：Why does array[idx++]+=“a” increase idx once in Java 8 but twice in Java 9 and 10?

For a challenge, a fellow code golfer wrote the following code : 对于挑战，一位代码高尔夫球手编写了以下代码：

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: 在Java 8中运行此代码时，我们得到以下结果：

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: 在Java 10中运行此代码时，我们得到以下结果：

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? 编号完全取决于使用Java 10.那么这里发生了什么？ Is it a bug in Java 10? 这是Java 10中的错误吗？

Follow ups from the comments: 从评论中跟进：

The issue appears when compiled with Java 9 or later (we found it in Java 10). 使用Java 9或更高版本编译时会出现此问题（我们在Java 10中找到它）。 Compiling this code on Java 8, then running in Java 9 or any later version, including Java 11 early access, gives the expected result. 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，可以得到预期的结果。
This kind of code is non-standard, but is valid according to the spec. 这种代码是非标准的，但根据规范有效。 It was found by Kevin Cruijssen in a discussion in a golfing challenge , hence the weird use case encountered. 凯文·克鲁伊森（ Kevin Cruijssen）在高尔夫挑战赛的讨论中发现了这一点，因此遇到了奇怪的用例。
Didier L found out that the issue can be reproduced with the much smaller and more understandable code: Didier L发现可以使用更小，更易理解的代码重现该问题：
```
 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: 在Java 8中编译时的结果：
```
 evaluated 
```
Result when compiled in Java 9 and 10: 在Java 9和10中编译时的结果：
```
 evaluated evaluated 
```
The issue seems to be limited to the string concatenation and assignment operator ( += ) with an expression with side effect(s) as the left operand, like in array[test()]+="a" , array[ix++]+="a" , test()[index]+="a" , or test().field+="a" . 问题似乎仅限于字符串连接和赋值运算符（ += ），其中带有副作用的表达式作为左操作数，如array[test()]+="a" ， array[ix++]+="a" ， test()[index]+="a" ，或test().field+="a" 。 To enable string concatenation, at least one of the sides must have type String . 要启用字符串连接，至少有一个边必须具有String类型。 Trying to reproduce this on other types or constructs failed. 尝试在其他类型或构造上重现此操作失败。

#1楼

参考：https://stackoom.com/question/3QfAQ/为什么数组-idx-a-在Java-中增加一次idx-在Java-和-中增加两次

#2楼

This is a bug in javac starting from JDK 9 (which made some changes with regard to string concatenation, which I suspect is part of the problem), as confirmed by the javac team under the bug id JDK-8204322 . 这是从JDK 9开始的javac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下所证实的那样。 If you look at the corresponding bytecode for the line: 如果查看该行的相应字节码：

array[i++%size] += i + " ";

It is: 它是：

  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 aaload is the actual load from the array. 最后一个aaload是数组的实际负载。 However, the part 但是，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. 大致对应于表达式array[i++%size] （减去实际的加载和存储），在那里两次。 This is incorrect, as the spec says in jls-15.26.2 : 这是不正确的，正如规范在jls-15.26.2中所说：

A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T) ((E1) op (E2)) , where T is the type of E1 , except that E1 is evaluated only once. 形式E1 op= E2的复合赋值表达式等效于E1 = (T) ((E1) op (E2)) ，其中T是E1的类型， 除了E1仅被评估一次。

So, for the expression array[i++%size] += i + " "; 因此，对于表达式array[i++%size] += i + " "; , the part array[i++%size] should only be evaluated once. ，零件array[i++%size]只应评估一次。 But it is evaluated twice (once for the load, and once for the store). 但它被评估两次（一次用于加载，一次用于商店）。

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 ). 该错误在JDK 11中得到修复，并且将有一个JDK 10的后端口（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页面上提及（和评论中的@DidierL）：

Workaround: compile with -XDstringConcat=inline 解决方法：使用-XDstringConcat=inline编译

That will revert to using StringBuilder to do the concatenation, and doesn't have the bug. 这将恢复使用StringBuilder进行连接，并且没有错误。

xfxf996

发布了0 篇原创文章 · 获赞 137 · 访问量 84万+

私信关注