## What is the purpose of the change
- Refactor getSharedClient
- see #5987
## Brief changelog
- The current lock mode
```java
locks.putIfAbsent(key, new Object());
synchronized (locks.get(key)) {
//do some thing
//remove lock
locks.remove(key);
}
```
- The situation which there is a problem:
When thread A finishes executing locks.putIfAbsent(key, new Object()),
another thread B which holding the lock executes locks.remove(key),
and then thread A executes synchronized (locks.get(key)).
- Attempt get lock by computeIfAbsent like this:
```java
synchronized (locks.computeIfAbsent(key, (k) -> new Object())) {
try{
//do some thing
} finally {
//remove lock
locks.remove(key);
}
}
```
- The situation which there is a problem:
1.When thread A finishes executing `synchronized (locks.computeIfAbsent(key,
(k) -> new Object()))` create the lock `LockA` And synchronized ,
2.Another thread B synchronized with `LockA`,
3.Thread A executes `locks.remove(key)`.
4.Thread B hold the `LockA` And synchronized.
5.Thread C executing `synchronized (locks.computeIfAbsent(key, (k) -> new
Object()))` create the lock `LockC` And synchronized .
-It seems difficult to remove a lock without any threads waiting for it exactly.
## Verifying this change
Run test cases.
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[ Full content available at: https://github.com/apache/dubbo/pull/6018 ]
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