A problem with many JDBC
drivers is that only one thread can use a Connection at any one time -- otherwise a
thread could send a query while another one is receiving results,
and this would be a bad thing for the database engine.
The PostgreSQL JDBC Driver is thread safe. Consequently, if your application uses multiple threads then you do not have to worry about complex algorithms to ensure that only one uses the database at any time.
If a thread attempts to use the connection while another one
is using it, it will wait until the other thread has finished its
current operation. If it is a regular SQL statement, then the operation consists of
sending the statement and retrieving any ResultSet (in full). If it is a Fastpath call (e.g., reading a block from a
LargeObject) then it is the time
to send and retrieve that block.
This is fine for applications and applets but can cause a performance problem with servlets. With servlets you can have a heavy load on the connection. If you have several threads performing queries then each but one will pause, which may not be what you are after.
To solve this, you would be advised to create a pool of
connections. When ever a thread needs to use the database, it
asks a manager class for a Connection. The manager hands a free
connection to the thread and marks it as busy. If a free
connection is not available, it opens one. Once the thread has
finished with it, it returns it to the manager who can then
either close it or add it to the pool. The manager would also
check that the connection is still alive and remove it from the
pool if it is dead.
So, with servlets, it is up to you to use either a single
connection, or a pool. The plus side for a pool is that threads
will not be hit by the bottle neck caused by a single network
connection. The down side is that it increases the load on the
server, as a backend process is created for each Connection. It is up to you and your
applications requirements.