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How to reset a JDBC Connection object?

How to reset a JDBC Connection object (that is, a java.sql.Connection object)?
I have explored the concept of connection pooling. When a connection pool is used, a Connection object can be recycled. But how can a connection pool recycle a Connection object? I think that a connection needs to be "reset" (for example, if it is in a transaction, then perhaps rollback it, but there may be more things to reset) before it can be reused. But I cannot find such a "reset" method in the Java documentation about the class java.sql.Connection.

If you are talking about what you as a user of a connection should do, then that is simple: call close() on the connection. Closing the logical connection will signal to the connection pool that the connection is available for reuse, and the connection pooling manager is then responsible for performing the necessary reset, invalidation of statement handles, etc.
If you are talking about what you as the implementer of a connection pooling manager should do, that is where things become complicated.
Historically, JDBC provides no way to 'reset' a java.sql.Connection, other than by your own code (or your third-party connection pool) remembering the initial configuration, and restoring it after use, and keeping track of objects like statements and result sets and closing them when the connection is returned to the pool.
Originally, the intended way to reset connections in JDBC was for an application server to use a driver's javax.sql.ConnectionPoolDataSource as a factory for javax.sql.PooledConnection objects. These PooledConnection objects serve as handles for physical connections to be held in a connection pool (to be clear, ConnectionPoolDataSource is not a connection pool, it is a data source for a connection pool). The application server would then expose a javax.sql.DataSource handing out logical Connection objects, where on Connection.close(), the driver specific implementation of PooledConnection would take care of any necessary reset of a connection (though JDBC underspecifies what is 'necessary').
However, in practice this route is hardly ever used because support in drivers was (and often still is) spotty, inconsistent or downright incorrect, and JDBC wasn't clear enough on exactly what needed to be done when the logical connection was closed. The world has also shifted to using third-party connection pool libraries that do not use ConnectionPoolDataSource.
JDBC 4.3 (Java 9 and higher) introduced the methods Connection.beginRequest() and Connection.endRequest() to be called by connection pooling managers, which would seem to fit such pattern, but unfortunately JDBC 4.3 doesn't actually specify what kind of things an implementation should or should not do in response to beginRequest and endRequest.
In short, there is no real general way to reset a connection in JDBC.

Do you try to close() method and reinitialize Connection object again.
close()
Releases this Connection object's database and JDBC resources immediately instead of waiting for them to be automatically released

C3P0 connection provider creates new connection or reuse connection?

I am facing some use related to GET_LOCK in MySQL. I am using c3p0 connection provider. Does getConnection() method in c3P0 create new connection every time or reuse the connection?

getConnection on DataSource gets the connection from connection pool. If there isn't any available and maxPoolSize isn't reached then it opens new connection. If the maxPoolSize is reached then it waits for some connection to return to the pool. (it must be released by the thread which is using it)
After the thread releases the connection then it is returned to the pool and might be reused by some other thread.
The documentation is http://www.mchange.com/projects/c3p0/

c3p0 maintains a pool of Connections, which are reused for multiple clients.
however, you cannot make any assumption that a Connection you use will be the same Connection you will see again. you might receive any Connection from c3p0's pool, there's no guarantee or likelihood you'll see one you've already seen. and, depending on your configuration, c3p0 tests, expires, and replaces Connections behind the scenes.
if you are trying to associate a lock with a Connection in one client session and then release it in another, well, that won't work and will break things fast. really, with any explicit locking, your acquisition and release should be done in one client session, using try / finally semantics to ensure lock release.

Where to close Connection using Connection Pooling in HTTP Servlets

Which one is better approach?
a)Get connection from Connection Pool at the beginning of doXXX() method and close at the end.
Use this connection throughout doXXX() method. This way it will fetch connection only once from the connection pool but Connection will be open for the whole time it takes to execute doXXX().
b)Get connection from Connection Pool for each db operation and close.
It will fetch connection from the connection pool for every db operation but will be closed immediately.

It depends. Getting a connection from the pool, worst case could result in the expensive creation of a new connection which might be slow. Between DB calls, how long is it going to be doing stuff? In general, releasing after each operation will slow things down for the specific process, but speed things up for other processes, and vice versa. You only really have to worry about this under massive load.

Best approach for returning connection objects to HikariCP pool

I am trying to use HikariCP connection pool. I was able to get it to work and get a connection that I could use. I am not sure what is the best approach for returning the connection to the pool.
I have the following questions:
Should I close the connection when I am done, rely on idleTimeout
and maxLifetime settings or is there another call that I can use so
as not to hog the connections from the pool?
If I close the connections (instead of returning to the pool), would
that not result in additional connection objects being created
to meet the requirements of the connection pool size?
Looking for helpful suggestions.

As with most connection pools, Hikari doesn't give you an actual JDBC Connection when you ask for one. What it does instead is give you a proxy that implements the Connection interface. In the case of Hikari - it's a ConnectionProxy object.
This proxy serves a few purposes, the main of which is - take the control of opening/closing connections and statements away from you and into the connection pool. This happens automagically and you should be using your connections as usual. This includes closing them after use.
If you look at the source code for Hikari, at the ConnectionProxy class in particular, you will see that the close() method is very different from the standard one. The code reads as:
Mark the connection as closed, do cleanup, reset underlying connection state and params.
Hence, simply calling close() will just clean and return the connection to the pool.

Concurrency: implement connection timeout in custom connection pool

I have to implement my own connection pool, and I want the connection will automatically
return to the pool after some CONNECTION_TIMEOUT. How can I achive that?
Everything that comes to mind is to use ScheduledExecutorService in a separate thread and replan it each time the connection is used.
Any other ideas?

You can use the client petition for a pooled connection, for triggering some actions, for example, check the timeout for currently used connections, and realease the currently used connections which timeout has expired. You will avoid using a new thread.

If a user of your connection pool borrowed a connection then it's the user responsibility to return it to the pool. The connection cannot return itself to the pool.