## What is the purpose of the change Resolve concurrent bug (or NullPointerException) in DubboProtocol.java
## Brief changelog The statement "locks.get(key)" will get NULL after "locks.remove(key);", and then "synchronized(null)" will throw NullPointerException. Obviously, this two statement, "locks.putIfAbsent(key, new Object());" and "locks.get(key)", are not an atomic operation. I can't find a better way to reduce lock granularity, maybe we have to sacrifice some performance to ensure safety here. ## Verifying this change XXXXX Follow this checklist to help us incorporate your contribution quickly and easily: - [x] Make sure there is a [GITHUB_issue](https://github.com/apache/dubbo/issues) field for the change (usually before you start working on it). Trivial changes like typos do not require a GITHUB issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue. - [ ] Format the pull request title like `[Dubbo-XXX] Fix UnknownException when host config not exist #XXX`. Each commit in the pull request should have a meaningful subject line and body. - [ ] Write a pull request description that is detailed enough to understand what the pull request does, how, and why. - [ ] Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in [dubbo samples](https://github.com/apache/dubbo-samples) project. - [ ] Run `mvn clean install -DskipTests=false` & `mvn clean test-compile failsafe:integration-test` to make sure unit-test and integration-test pass. - [ ] If this contribution is large, please follow the [Software Donation Guide](https://github.com/apache/dubbo/wiki/Software-donation-guide). [ Full content available at: https://github.com/apache/dubbo/pull/6102 ] This message was relayed via gitbox.apache.org for [email protected]
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