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.