Recently
while interacting with newbies or say IT aspirants when being asked about
simple JDBC questions most of the people were at sea. Question was simple
“While
getting JDBC connection in java application what we do is first we call
Class.forname(“<driver-name”) and then we call DriverManager.getConnection
with appropriate arguments. We do not get any value from Class.forname and
return it to DriverManager so how this works.
Class.forName("driver-name");
DriverManager.getConnection(url, user, password);
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Most of
the people those who answered simply said it will load JDBC driver. Next
question was “What does that mean”. Long pause and then again “Will load the
driver” J
So now
let’s see what happens when we call Class.forname on JDBC driver for MySQL db.
Class.forName("com.mysql.jdbc.Driver");
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Following
is code snippet from MySQL driver class.
So when
we call Class.forname(“com.mysql.jdbc.Driver”) static block of this class will
be called
static {
try {
java.sql.DriverManager.registerDriver(new Driver());
}
catch
(java.sql.SQLException E) {
throw new RuntimeException("Can't
register driver!");
}
if (DEBUG) {
Debug.trace("ALL");
}
}
|
Looking
at the code we understand that Class.forname will call registerDriver method of
DriverManager class and will pass instance of Driver class for MySQL to
DriverManager. When DriverManger.registerDriver is called it will check for
null and if it is not null then it will add this driver in list of registered
drivers.
public static synchronized void
registerDriver(java.sql.Driver driver) throws SQLException
{
/* Register the
driver if it has not already been added to our list */
if(driver != null) {
registeredDrivers.addIfAbsent(new DriverInfo(driver));
}
else {
// This is for
compatibility with the original DriverManager
throw new
NullPointerException();
}
println("registerDriver:
"
+ driver);
}
|
When
getConnection(url, user, password) is called it will internally call
getConnection(url, driverinfo, classLoaded) which will iterate through
registered drivers and call connect method on Driver for which which will
create and return connection.
for(DriverInfo aDriver : registeredDrivers)
{
// If the caller
does not have permission to load the driver then skip it.
if(isDriverAllowed(aDriver.driver,
callerCL))
{
try {
println(" trying " +
aDriver.driver.getClass().getName());
Connection
con = aDriver.driver.connect(url, info);
if (con != null) {
// Success!
println("getConnection
returning " + aDriver.driver.getClass().getName());
return (con);
}
} catch (SQLException ex)
{
if (reason == null) {
reason
= ex;
}
}
}
else {
println(" skipping: " +
aDriver.getClass().getName());
}
}
|
So this
is how all happens…Happy coding.. J
