Client to use the slave when the master is down in MySQL replication
Since you have Master/Slave, you may want to implement the use of two things

Circular Replication
Database Virtual IP (aka DBVIP)

Circular Replication
Circular Replication is nothing more than first setting up Master/Slave then performing the same steps using the Slave as the Master's Master and the Master as the Slave's Slave. It just entails

explicitly user a different server_id on each DB Server
enabling binary logging on both DB servers
making sure the replication user is defined on both DB servers

Database Virtual IP (aka DBVIP)
There are products you can download and install to setup a DBVIP. One such product I use is ucarp. Another product is Linux Heartbeat. I normally do not use such things with MySQL Circular Replication or Master/Slave. Why?
Since those products can perform automatic failover, you do not want to do that in the event a Slave is some number of seconds behind in replication lag.
You should perform manual failovers.
Here is a poor man's approach to implementing DBVIP management.
Suppose you have this setup

DB Server1 has IP 10.1.2.30
DB Server2 has IP 10.1.2.40
You want to use DBVIP 10.1.2.50

Create the Script called /usr/local/sbin/MyAppDBVIP like this
echo echo 10.1.2.50 &gt; /usr/local/sbin/MyAppDBVIP

Create the Script called /usr/local/sbin/dbvip-up
DBVIP=`/usr/local/sbin/MyAppDBVIP`
ip addr add ${DBVIP}/24 dev eth1

Create the Script called /usr/local/sbin/dbvip-down
DBVIP=`/usr/local/sbin/MyAppDBVIP`
ip addr del ${DBVIP}/24 dev eth1

Make sure all scripts are executable
chmod +x /usr/local/sbin/MyAppDBVIP
chmod +x /usr/local/sbin/dbvip-up
chmod +x /usr/local/sbin/dbvip-down

Make sure these script exist on both DB Servers
Simply run dbvip on whichever server you choose. .
So the failover process and protocol are the following:

Run dbvip-down on the DB Server that has the DBVIP. If you cannot
Run dbvip-up on the DB Server that you want to have the DBVIP
Just remember you should not run dbvip-up on both machines
After running dbvip-up, restart apache, JBoss, or any other app server contacting MySQL via the old Master