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Am I closing the DB connection correctly? JDBC - DBCP

Will closing the preparedStatement also close and return the connection to the connection pool?
public void insertProjectIntoDatabase(Project project) {
String insertProjectIntoDatabase =
"INSERT INTO projects(Project_Id, Project_Name, Project_StartDate, Deadline) " +
"VALUES (?, ?, ?, ?)";
try {
preparedStatement = DBCPDataSource.getConnection().prepareStatement(insertProjectIntoDatabase);
preparedStatement.setInt(1, project.getProjectId());
preparedStatement.setString(2, project.getName());
preparedStatement.setDate(3, java.sql.Date.valueOf(project.getStartDate()));
preparedStatement.setDate(4, java.sql.Date.valueOf(project.getDeadline()));
preparedStatement.execute();
preparedStatement.close();
}
catch (SQLException e)
{
System.out.println("Error happened in ProjectRepository at insertProjectIntoDatabase(): " + e.getMessage());
}
}
Bonus question:
I have created performance tests for creating a new connection each time an object needs one, Singleton connection and connection pool.
Singleton - Fastest
Creating a new connection each time - Slower (1.2s than the one above)
Connection Pool - Slowest (First connection - 2-3s slower than the one above, following tests are 0.4s slower than the one above)
I am using Apache Commons DBCP for the connection pool.
I thought using connection pools, would be just a little slower than Singleton connection.
Have I done something wrong?

You asked:
Will closing the preparedStatement also close and return the connection to the connection pool?
Start with the documentation:
Releases this Statement object's database and JDBC resources immediately instead of waiting for this to happen when it is automatically closed. It is generally good practice to release resources as soon as you are finished with them to avoid tying up database resources.
Calling the method close on a Statement object that is already closed has no effect.
Note:When a Statement object is closed, its current ResultSet object, if one exists, is also closed.
No mention of closing the connection.
Try intuition: Do we ever run more than one statement in SQL? Yes, obviously. So logically the connection needs to survive across multiple statements to be useful.
Lastly: Try it yourself, an empirical test. Call Connection#isOpen after calling Statement#close.
➥ No, closing the statement does not close the connection.
For the simplest code, learn to use try-with-resources syntax to auto-close your database resources such as result set, statement, and connection. You’ll find many examples of such code on this site, including some written by me.
As for connection pools, yes, calling close on a connection retrieved from a pool causes the connection object to be be returned to the pool. The pool may choose to re-use the connection, or the pool may choose to close the connection. (Not our concern.)
The only point to a connection pool is speed. If opening a connection to the database takes a significant amount of time, we can save that time by re-using existing connection. Generating and re-using connections is the job of a connection pool.
If a connection pool is showing the slowest results in your testing, then here is something seriously wrong with either your pool or your tests. You did not reveal to us your tests, so we cannot help there. Note: As Marmite Bomber commented, be sure your tests do not include the time needed to establish the connection pool.
Frankly, I have found in my experience that opening a database connection does not take a significant amount of time. Furthermore, the details involved in properly implementing a connection pool are complex and treacherous as evidenced by the list of failed and abandoned connection pool implementation projects. That, combined with the inherent risks such as a transaction being left open on a retrieved connection, led me to avoiding the use of connection pools. I would posit that using a connection pool before collecting proof of an actual problem is a case of premature optimization.
I suggest using an implementation of the interface DataSource as a way to mask from the rest of your code whether you are using a pool and to hide which pool implementation you are currently using. Using DataSource gives you the flexibility to to change between using or not using a connection pool, and the flexibility to change between pools. Those changes become deployment choices, with no need to change your app programming.

Pools are meant to improve performance, not degrade it. DBCP is naive, complicated, and outdated.
I don't think it's appropriate for a production application, especially when so many drivers support pooling in their DataSource natively. The entire pool gets locked the whole time a new connection attempt is made to the database. So, if something happens to your database that results in slow connections or timeouts, other threads are blocked when they try to return a connection to the pool—even though they are done using a database.
Even C3PO performs terribly.
Please try using one of the two connection pools tomcat_connection_pool or HikariCP
Now coming to your main part of the question if you have closed the connection correctly?
Whenever you use a connection pool and you fetch an available connection from the pool you need not have to close the connection that you fetched in your Dao layer. The pool manages the connections that you have created and each connection that the pool lends has a timeout associated with it before which it has to return to the pool. When the pool is shut down all the connections shutdown too.
For more information on how to configure these properties in your connection pool. Please check the links above for each of the connection pools.

Reset JDBC connections of Connection Pool in Java

Scenario:
There is something called FAN notification in Oracle Database, which is used for High Availability.
Whenever I will get this notification in my Java application I want to reset all the JDBC connections object available in my Connection Pool.
But the problem is, even if I reset the connection pool objects, there will be some JDBC transaction going on using old connection objects.
My question is, can we somehow change that (which is in use) connection object with the new object and copy or transfer the transactions to new connection object without any inflight transaction.
[update]: This scenario is handled between server and database using something called Grid Link. But I want to handle it in application end, by removing all the JDBC connection objects from connection pool, these connection objects will point to new database instance which is up and running.
If it is not possible please suggest me some alternative solution.

Java Connection Pooling

I searched for connection pooling and read about it. If I understand it correctly, a connection pool is like a collection of open connections. If a connection is established or created it should be added to the connection pool, and if that connection is closed it should be removed in connection pool; while it is open I can use it again and again.
While reading these tutorials and explanations about connection pooling I have some questions:
Can a pool of connections only be used on a certain computer? Like ComputerA
cannot share its connection pool with ComputerB?
Where should connection.close() be placed?
Is it correct to use a connection ONLY when selecting/loading record? After I got the returned records/data I close the connection at finally statement. Same as adding, editing and deleting records. And while it is processing I place a progress bar so the user will have to wait for it to be completed and to do some process again, which means I will only open connection one at a time.
Thanks for the explanation. :)

Note: I assume we're talking about the java.sql.Connection interface.
Can a pool of connections only be used on a certain computer? Like ComputerA cannot share its connection pool with ComputerB?
A connection exists between a running application and a database. Naturally, two different machines can't share the same running application, so they can't share connections with a database.
Where should connection.close() be placed?
You should always make sure to call close() on a Connection instance after using it (typically in a finally block). If pooling is being used, this will actually return the connection to the pool behind the scenes. Reference: Closing JDBC Connections in Pool
Is it correct to use a connection ONLY when selecting/loading record? After I got the returned records/data I close the connection at finally statement.
Yes, that's correct. You don't want to manually hang on to a Connection reference - use it to execute SQL/DML and then check it back into the pool by calling close() in the finally block, just like you're doing.

Connection behavior - DriverManager.getConnection() and DataSource.getConnection()

If I get a connection object using DriverManager.getConnection() and DataSource.getConnection(), how they differ in behavior when .close() is called on those objects?
Before .close() method call, I got relevant Statement and ResultSet objects from these two different connections. Soon after getting these two objects, if I say connection1.close() (through DriverManager.getConnection()), it will nullify the connection object and I'm not supposed / allowed to access the relevant Statement and ResultSet objects. Correct me if I'm wrong?
Second scenario, now if I say connection2.close() (through DataSource.getConnection()), it simply returns it back to the pool. But the connection is still live. Will I be able to access the associated Statement and ResultSet objects?

If we assume a (basic) DataSource (that is: one that does not do connection pooling), then you obtain a physical connection that is the same as one obtained from DriverManager (some drivers even internally use DriverManager from the DataSource, or a DataSource from DriverManager). So those connections will behave identically.
Now if we assume a DataSource that provides connection pooling, then the DataSource itself uses a ConnectionPoolDataSource (or a similar internal mechanism) to obtain a PooledConnection. This PooledConnection manages the actual physical connection to the database.
When a user requests a connection from the DataSource, the DataSource will checkout a PooledConnection, and ask it for a Connection. The PooledConnection will then create a logical connection that uses or wraps the physical connection (eg using a Proxy). The DataSource will return that logical connection to the user.
To the user the logical connection should behave identical to a physical connection in all aspects. So when a user closes the connection, that logical connection and all dependent JDBC objects will be closed and behave identical to a physical connection close.
JDBC 4.1 section 11.1 says:
Connection pooling is completely transparent to the client: A client obtains a pooled
connection and uses it just the same way it obtains and uses a non pooled connection.
And section 11.4:
If the application attempts to reuse the logical handle, the Connection implementation
throws an SQLException.
and
For a given PooledConnection object, only the most recently produced logical Connection object will be valid. Any previously existing Connection object is automatically closed when the associated PooledConnection.getConnection method is called.
In the background however, when the logical connection is closed, the PooledConnection will signal the DataSource that it is available for reuse, and the DataSource will then return it to the connection pool, or close the PooledConnection (which closes the physical connection) if it no longer needs the connection.
The DataSource can also forcefully revoke a connection from a user (eg when a connection is checked out too long, etc), by asking the PooledConnection to close the logical connection.

connection1.close() (through DriverManager.getConnection()),
This will close the physical connection established to the database and all the resources viz. Resultset, Statement, Connection are released. So, you cannot access them after the connection is closed.
connection2.close() (through DataSource.getConnection())
This is DataSource-implementation dependent and so the behavior need not be consistent across different DataSource implementations. Also, within a given DataSource implementation, the connection's actual life cycle is dependent on various other parameters that it is strongly recommended to not differentiate this Connection from the one obtained through DriverManager.
If you really want the data held in the ResultSet to be available after the Statement and Connection are closed, you can take a look at CachedRowSet if that fits your usecase.

The client-side caching might be dependent on the driver being used to connection not sure.
But Some drivers specifically prevent you from using a statement or resultset once the connection has been closed. Others maintain the resultset on the client side

JDBC Connection Pooling: Connection Reuse?

As per my understanding, JDBC Connection Pooling (at a basic level) works this way:
create connections during app initialization and put in a cache
provide these cached connections on demand to the app
a separate thread maintains the Connection Pool, performing activities like:
discard connections that have been used (closed)
create new connections and add to the cache to maintain a specific count of connections
But, whenever I hear the term "connection reuse" in a JDBC Connection Pooling discussion, I get confused. When does the connection reuse occurs?
Does it means that Connection Pool provides the same connection for two different database interactions (without closing it)? Or, is there a way to continue using a connection even after it gets closed after a DB call?

Connection pooling works by re-using connections. Applications "borrow" a connection from the pool, then "return" it when finished. The connection is then handed out again to another part of the application, or even a different application.
This is perfectly safe as long as the same connection is not is use by two threads at the same time.
The key point with connection pooling is to avoid creating new connections where possible, since it's usually an expensive operation. Reusing connections is critical for performance.

The connection pool does not provide you with the actual Connection instance from the driver, but returns a wrapper. When you call 'close()' on a Connection instance from the pool, it will not close the driver's Connection, but instead just return the open connection to the pool so that it can be re-used (see skaffman's answer).

Connection pooling reuses connections.
Here is how apache dbcp works underline.
Connection poolableConnection= apacheDbcpDataSource.getConnection();
Apache DBCP implementation returns connection wrapper which is of type PoolableConnection.
poolableConnection.close();
PoolableConnection.close() inspects if actual underlying connection is closed or not, if not then it returns this PoolableConnection instance into connection pool (GenericObjectPool in this case).
if (!isUnderlyingConectionClosed) {
// Normal close: underlying connection is still open, so we
// simply need to return this proxy to the pool
try {
genericObjectPool.returnObject(this); //this is PoolableConnection instance in this case
....
}

My understanding is the same as stated above and, thanks to a bug, I have evidence that it's correct. In the application I work with there was a bug, an SQL command with an invalid column name. On execution an exception is thrown. If the connection is closed then the next time a connection is gotten and used, with correct SQL this time, an exception is thrown again and the error message is the same as the first time though the incorrect column name doesn't even appear in the second SQL. So the connection is obviously being reused. If the connection is not closed after the first exception is thrown (because of the bad column name) then the next time a connection is used everything works just fine. Presumably this is because the first connection hasn't been returned to the pool for reuse. (This bug is occurring with Jave 1.6_30 and a connection to a MySQL database.)