The java.sql.Connection object, which encapsulates a single connection to a particular database, forms the basis of all JDBC data-handling code. An application can maintain multiple connections, up to the limits imposed by the database system itself. A standard small office or web server Oracle installation can support 50 or so connections, while a major corporate database could host several thousand. The DriverManager.getConnection() method creates a connection:
Connection con = DriverManager.getConnection("url", "user", "password");
You pass three arguments to getConnection(): a JDBC URL, a database username, and a password. For databases that do not require explicit logins, the user and password strings should be left blank. When the method is called, the DriverManager queries each registered driver, asking if it understands the URL. If a driver recognizes the URL, it returns a Connection object. Because the getConnection() method checks each driver in turn, you should avoid loading more drivers than are necessary for your application.
The getConnection() method has two other variants that are less frequently used. One variant takes a single String argument and tries to create a connection to that JDBC URL without a username or password. The other version takes a JDBC URL and a java.util.Properties object that contains a set of name/value pairs. You generally need to provide at least username=value and password=value pairs.
When a Connection has outlived its usefulness, you should be sure to explicitly close it by calling its close() method. This frees up any memory being used by the object, and, more importantly, it releases any other database resources the connection may be holding on to. These resources (cursors, handles, and so on) can be much more valuable than a few bytes of memory, as they are often quite limited. This is particularly important in applications such as servlets that might need to create and destroy thousands of JDBC connections between restarts. Because of the way some JDBC drivers are designed, it is not safe to rely on Java's garbage collection to remove unneeded JDBC connections.
The JDBC 2.0 standard extension, discussed later in this chapter, provides a facility for connection pooling, whereby an application can maintain several open database connections and spread the load among them. This is often necessary for enterprise-level applications, such as servlets, that may be called upon to perform tens of thousands of database transactions a day.
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| 2.3. JDBC Drivers |  | 2.5. Statements |
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