HBaseAdmin has the constructor:
As shown in above code, HBaseAdmin will get a cached HConnection or create a new HConnection and use this HConnection to connect to Master. Then, HBaseAdmin will delete the HConnection when connecting to master fail as follows:
The above code will invoke HConnectionManager#deleteStaleConnection to delete the HConnection from global HConnection cache. The risk is that the deleted HConnection might be sharing by other threads, such as HTable or HTablePool. Then, these threads which sharing the deleted HConnection will get closed HConnection exception:
If users use HTablePool, the situation will become worse because closing HTable will only return HTable to HTablePool which won't reduce the reference count of the closed HConnection. Then, the closed HConnection will always be used before clearing HTablePool. In 0.94, some modules such as Rest server are using HTablePool, therefore may suffer from this problem.