How class.forName loads the Database Driver in JDBC

When Class.forName() is called with an argument like this:

Class.forName("org.gjt.mm.mysql.Driver");

the classloader attempts to load and link the Driver class in the “org.gjt.mm.mysql” package and if successful, the static initializer is run. The MySQL Driver static initializer looks like this:

static {
  try {
    java.sql.DriverManager.registerDriver(new Driver());
  } catch (SQLException E) {
    throw new RuntimeException("Can't register driver!");
  }
}

So it calls a static method in the java.sql.DriverManager class which apparently registers a copy of itself when it loads.

 To understand the why you have to look at the next line in the initial code example:

Connection con = DriverManager.getConnection(url,
  "myLogin", "myPassword");

The DriverManager class returns a database connection given a JDBC URL string, a username and a password. In order to create that connection, the DriverManager class has to know which database driver you want to use. It does that by iterating over the array (internally a Vector) of drivers that have registered with it (ie: the registerDriver(Driver driver) method illustrated above) and calls the acceptsURL(url)) method on each driver in the array, effectively asking the driver to tell it whether or not it can handle the JDBC URL.

First the DriverManager tries to use each driver in the order it was registered. (The drivers listed in jdbc.drivers are always registered first.) It will skip any drivers that are untrusted code unless they have been loaded from the same source as the code that is trying to open the connection.

It tests the drivers by calling the method Driver.connect on each one in turn, passing them the URL that the user originally passed to the method DriverManager.getConnection. The first driver that recognizes the URL makes the connection.

All DriverManager methods are declared static, which means that they operate on the class as a whole rather than on particular instances. In fact, the constructor for DriverManager is declared private to prevent users from instantiating it. Logically, there is one instance of the DriverManager class.