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 could cause severe confusion.
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 thread uses the database at a 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 the operation is a regular SQL
statement, then the operation consists of sending the statement and
ResultSet (in full). If it
is a fast-path call (e.g., reading a block
from a large object) then it consists of
sending and retrieving the respective data.
This is fine for applications and applets but can cause a
performance problem with servlets. If you have several threads
performing queries then each but one will pause.
To solve this, you are advised to create a pool of connections.
When ever a thread needs to use the database, it asks a manager
class for a
Connection object. 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 using the connection, it returns it to the manager
which 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. The down side of a connection pool is
that it increases the load on the server because a new session is
created for each
Connection object. It is
up to you and your applications' requirements.