You want to access a database via JDBC.

Use Class.forName( ) and DriverManager.getConnection( ).

While DB and friends have their place, most of the modern database action is on relational databases, and accordingly Java Database action is on JDBC. So the bulk of this chapter is devoted to JDBC.

This is not the place for a tutorial on relational databases. I’ll assume that you know a little bit about the Structured Query Language (SQL), the universal language used to control relational databases. SQL has queries like “SELECT * from userdb”, which means to select all columns (the *) from all rows (entries) in a database table named userdb (all rows are selected because there is no “where” clause on the SELECT statement). SQL also has updates like INSERT, DELETE, CREATE, and DROP. If you need more information on SQL or relational databases, there are many good books that will introduce you to the topic in more detail.

JDBC has two Levels, JDBC 1 and JDBC 2. Level 1 is included in all JDBC implementation and drivers; Level 2 is optional, and requires a Level 2 driver. This chapter concentrates on common features, primarily Level 1.

The first step in using JDBC 1 is to load your database’s driver. This is performed using some Java JVM magic. The class java.lang.Class has a method called forName( ) that takes a string containing the full Java name for a class and loads the class, returning a Class object describing it. This is typically used in introspection (see Chapter 25), but can be used anytime to ensure that a class has been correctly configured into your CLASSPATH. This is the use that we’ll see here. And, in fact, part of the challenge of installing JDBC drivers is ensuring that they are in your CLASSPATH, both at compile time and at deployment time. But wait, there’s more! In addition to checking your CLASSPATH, this method also registers the driver with another class called the DriverManager . How does it work? Each valid JDBC driver has a bit of method-like code called a static initializer . This is used whenever the class is loaded -- just what the doctor ordered! So the static block registers the class with the DriverManager when you call Class.forName( ) on the driver class.

For the curious, the static code block in a Driver called BarFileDriver looks something like this:

/** Static code block, to initialize with the DriverManager. */
static {
    try {
         DriverManager.registerDriver(new BarFileDriver(  ));
    } catch (SQLException e) {
         DriverManager.println("Can't load driver" + 
                               "darwinsys.sql.BarFileDriver");
    }
}

Example 20-5 shows a bit of code that tries to load two drivers. The first is the JDBC-to-ODBC bridge described in the Introduction. The second is one of the commercial drivers from Oracle.

import java.awt.*;
import java.sql.*;

/** Load some drivers.  */
public class LoadDriver {

    public static void main(String[] av) {
        try {

            // Try to load the jdbc-odbc bridge driver
            // Should be present on Sun JDK implementations.
            Class c = Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
            System.out.println("Loaded " + c);

            // Try to load an Oracle driver.
            Class d = Class.forName("oracle.jdbc.driver.OracleDriver");
            System.out.println("Loaded " + d);
        } catch (ClassNotFoundException ex) {
            System.err.println(ex);
        }
    }
}

As expected, the first load succeeds and the second fails, since I don’t have Oracle installed on my notebook:

daroad.darwinsys.com$ java LoadDriver
Loaded class sun.jdbc.odbc.JdbcOdbcDriver
java.lang.ClassNotFoundException: oracle/jdbc/driver/OracleDriver
daroad.darwinsys.com$

It is also possible to preregister a driver using the -D option to load it into the System Properties; in this case, you can skip the Class.forName( ) step:

java -Djdbc.drivers=com.acmewidgets.AcmeDriver:foo.bar.OhMyDriver MyClass

Once you have registered the driver, you are ready to connect to the database.