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Posted to notifications@dubbo.apache.org by "looly (via GitHub)" <gi...@apache.org> on 2023/04/02 12:26:15 UTC
[GitHub] [dubbo] looly opened a new issue, #11986: ConcurrentHashMapUtils无法解决jdk8的循环bug
looly opened a new issue, #11986:
URL: https://github.com/apache/dubbo/issues/11986
### Environment
* Dubbo version: 3.2, 3.3
* Operating System version: Windows11
* Java version: jdk8
### Steps to reproduce this issue
[ConcurrentHashMapUtils](https://github.com/apache/dubbo/blob/3.2/dubbo-common/src/main/java/org/apache/dubbo/common/utils/ConcurrentHashMapUtils.java)
中提供的`computeIfAbsent`方法没有解决jdk8的循环问题，复现代码如下：
```java
final ConcurrentHashMap<String,Integer> map=new ConcurrentHashMap<>();
// // map.computeIfAbsent("AaAa", key->map.computeIfAbsent("BBBB",key2->42));
ConcurrentHashMapUtils.computeIfAbsent(map, "AaAa", key->map.computeIfAbsent("BBBB",key2->42));
```
[Hutool](https://github.com/dromara/hutool) 中的解决方案是：
```java
V value = map.get(key);
if (null == value) {
map.putIfAbsent(key, mappingFunction.apply(key));
value = map.get(key);
}
return value;
```
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[GitHub] [dubbo] AlbumenJ commented on issue #11986: ConcurrentHashMapUtils无法解决jdk8的循环bug
Posted by "AlbumenJ (via GitHub)" <gi...@apache.org>.
AlbumenJ commented on issue #11986:
URL: https://github.com/apache/dubbo/issues/11986#issuecomment-1493319808
死锁这个问题可以提个 PR 改下
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[GitHub] [dubbo] looly commented on issue #11986: ConcurrentHashMapUtils无法解决jdk8的循环bug
Posted by "looly (via GitHub)" <gi...@apache.org>.
looly commented on issue #11986:
URL: https://github.com/apache/dubbo/issues/11986#issuecomment-1493330046
PR see: https://github.com/apache/dubbo/issues/11986
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[GitHub] [dubbo] looly commented on issue #11986: ConcurrentHashMapUtils无法解决jdk8的循环bug
Posted by "looly (via GitHub)" <gi...@apache.org>.
looly commented on issue #11986:
URL: https://github.com/apache/dubbo/issues/11986#issuecomment-1493320931
看到原来的issue和PR：
https://github.com/apache/dubbo/pull/11326
https://github.com/apache/dubbo/issues/11294
这个PR是为了解决 https://bugs.openjdk.org/browse/JDK-8161372 这个问题，但这本质上是个死锁问题。
@AlbumenJ 没有提PR就是因为两个解决方案我都不能保证逻辑是正确的，需要review验证，这也是困扰我的地方。
稍后提个PR。
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[GitHub] [dubbo] AlbumenJ closed issue #11986: ConcurrentHashMapUtils无法解决jdk8的循环bug
Posted by "AlbumenJ (via GitHub)" <gi...@apache.org>.
AlbumenJ closed issue #11986: ConcurrentHashMapUtils无法解决jdk8的循环bug
URL: https://github.com/apache/dubbo/issues/11986
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[GitHub] [dubbo] looly commented on issue #11986: ConcurrentHashMapUtils无法解决jdk8的循环bug
Posted by "looly (via GitHub)" <gi...@apache.org>.
looly commented on issue #11986:
URL: https://github.com/apache/dubbo/issues/11986#issuecomment-1493319150
但是有issue反馈说这种方式获取的value有可能为null，我不确定这种写法是否存在问题。
issue中提出的替代方案是：
```java
V value = map.get(key);
if (null == value) {
value = mappingFunction.apply(key);
final V res = map.putIfAbsent(key, mappingFunction.apply(key));
if(null != res){
// issues#I6RVMY
// 如果旧值存在，说明其他线程已经赋值成功，putIfAbsent没有执行，返回旧值
return res;
}
// 如果旧值不存在，说明赋值成功，返回当前值
}
return value;
```
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[GitHub] [dubbo] AlbumenJ commented on issue #11986: ConcurrentHashMapUtils无法解决jdk8的循环bug
Posted by "AlbumenJ (via GitHub)" <gi...@apache.org>.
AlbumenJ commented on issue #11986:
URL: https://github.com/apache/dubbo/issues/11986#issuecomment-1493319213
ConcurrentHashMapUtils 这个主要是解决性能的问题
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