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18.7 Connection Pooling

503

3. Allocate new connections. 

If a connection is required, there is no idle connection available, 

and the connection limit has not been reached, then start a 

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. The 

natural approach is to use the 

wait

 method, which gives up the 

thread synchronization lock and suspends the thread until 

notify

 or 

notifyAll

 is called. Since 

notifyAll

 could stem 

from several possible sources, threads that wake up still need to 

test to see if they can proceed. In this case, the simplest way to 

accomplish this task is to recursively repeat the process of trying 

to obtain a connection.

try {

wait();

} catch(InterruptedException ie) {}

return(getConnection());

It may be that you don't want to let clients wait and would rather 

throw an exception when no connections are available and the 

connection limit has been reached. In such a case, do the follow 

ing instead:

throw new SQLException("Connection limit reached");

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.

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