Concurrent use of same JDBC connection by multiple threads
I ran the following set of tests using a AWS RDS Postgres database, and Java 11:
Create a table with 11M rows, each row containing a single TEXT column, populated with a random 100-char string
Pick a random 5 character string, and search for partial-matches of this string, in the above table
Time how long the above query takes to return results. In my case, it takes ~23 seconds. Because there are very few results returned, we can conclude that the majority of this 23 seconds is spent waiting for the DB to run the full-table-scan, and not in sending the request/response packets
Run multiple queries in parallel (with different keywords), using different connections. In my case, I see that they all complete in ~23 seconds. Ie, the queries are being efficiently parallelized
Run multiple queries on parallel threads, using the same connection. I now see that the first result comes back in ~23 seconds. The second result comes back in ~46 seconds. The third in ~1 minute. etc etc. All the results are functionally correct, in that they match the specific keyword queried by that thread
To add on to what Joni mentioned earlier, his conclusion matches the behavior I'm seeing on Postgres as well. It appears that all "correctness" is preserved, but all parallelism benefits are lost, if multiple queries are sent on the same connection at the same time.
Since the JDBC spec doesn't give guarantees of concurrent execution, this question can only be answered by testing the drivers you're interested in, or reading their source code.
In the case of MySQL Connector/J, all methods to execute statements lock the connection with a synchronized block. That is, if one thread is running a query, other threads using the connection will be blocked until it finishes.