How is driver class located in JDBC4

Question

One of the great additions in version 4 of JDBC You don't have to explicitly load the driver by calling Class.forName anymore. When your application attempts to connect the database for the first time, DriverManager automatically loads the driver found in the application CLASSPATH.

My question is how? What if there are multiple drivers in the classpath?

One thing I can guess is that on parsing the connection URL whether driver needed is of JDBC or ODBC can be figured out but how can one say out of multiple jdbc compliant drivers which one is to be selected for the database I am using? (lets say I am using MySql and I need MySql-Connector driver). Is there any static mapping of such database drivers in JVM?

Answer 1

Every JDBC 4 compliant driver has a file in its jar called META-INF/services/java.sql.Driver, in that file it will list its implementation(s) of java.sql.Driver. When you request a connection, DriverManager will use the ServiceLoader to find all(!) copies of META-INF/services/java.sql.Driver in the classpath and will then load all classes listed. When a java.sql.Driver class is loaded, it has to register itself with the DriverManager, so the DriverManager loads all classes using the service loader, and each Driver implementation registers itself.

When you request a connection from DriverManager, the DriverManager will iterate over all registered drivers asking them for a Connection. The driver will use the JDBC url to check if it's a protocol it supports (eg Jaybird/Firebird JDBC checks if the url starts with "jdbc:firebirdsql:" or "jdbc:firebird:"). If the driver does not support the protocol, it will return null, if it does support the protocol it will either return an established connection, or it will throw an SQLException (eg if you made an error in the URL, or it couldn't connect). If all drivers return null (none support the protocol), then DriverManager will throw an SQLException with error "No suitable driver found for <url>"

So, having multiple drivers on the classpath does not matter as long as they support different protocols, however if there are multiple drivers for the same database (or at least: same protocol prefixes), it will use the first in the list of drivers. Depending on the Java version, if that driver fails with an SQLException, it will continue with the next driver (at least Java 5 and later), or stop trying and throw the exception (I believe this was in Java 1.4 or maybe even earlier).