Prentice Hall and Sun Microsystems. Personal use only; do not redistribute.

A.18 JDBC and Database Connection Pooling

555

background thread to allocate a new connection. Then, wait for 

the first available connection, whether or not it is the newly allo 

cated one.

if ((totalConnections() < maxConnections) &&

!connectionPending) { // Pending = connecting in bg

makeBackgroundConnection();

try {

wait(); // Give up lock and suspend self.

} catch(InterruptedException ie) {}

return(getConnection()); // Try again.

4. Wait for a connection to become available. 

This situation occurs when there is no idle connection and 

you've reached the limit on the number of connections. This 

waiting should be accomplished without continual polling. It is 

best to use the 

wait

 method, which gives up the thread syn 

chronization lock and suspends the thread until 

notify

 or 

notifyAll

 is called. 

try {

wait();

} catch(InterruptedException ie) {}

return(getConnection());

5. Close connections when required.

Note that connections are closed when they are garbage col 

lected, so you don't always have to close them explicitly. But, 

you sometimes want more explicit control over the process.

public synchronized void closeAllConnections() {

// The closeConnections method loops down Vector, calling

// close and ignoring any exceptions thrown.

closeConnections(availableConnections);

availableConnections = new Vector();

closeConnections(busyConnections);

busyConnections = new Vector();

}

Connection pool timing results

Condition

Average Time

Slow modem connection to database, 10 

11 seconds

initial connections, 50 max connections 

(

ConnectionPoolServlet

)

Slow modem connection to database, 

22 seconds

recycling a single connection 

(

ConnectionPoolServlet2

)

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