Using a JDBC database is a three step process:
DataSourcefrom the global JNDI resource map.
DataSourceto execute the SQL.
JDBC database access is based around the Factory pattern.
javax.sql.DataSource is the Factory object.
The <database> configures the
DataSource and stores it
in the JNDI resource map. The servlet will retrieve the
DataSource and use it as a factory to obtain
Connection objects, the main workhorse for using databases.
In Resin 3.0, the <database> tag configures the database pool and driver and saves the connection factory (DataSource) in JNDI. JNDI is just a global lookup tree available to all classes, making it straightforward to separate resource configuration from the application code.
The <driver> tag configures the database driver. The database vendor will make the driver classes available and describe the configuration variables. The thirdparty database page describes several important database configurations.
The <type> tag is the most important driver configuration item. It specifies the main Java driver class. For many drivers, you will have a choice of different drivers following different internal JDBC APIs. If you have a choice, you should try the drivers in the following order, after checking your database vendor's recommendations:
The <url> specifies the location of the database. Each database driver will have a unique URL formal. In this case, the <url> specifies a directory for the database files. Other databases may specify a host and port.
The specific driver for this example,
com.caucho.db.jca.ConnectionFactory is a simple database
intended for examples and testing.
The servlet is configured with a
access JDBC. Resin allows two styles of configuration: Dependency Injection
using bean-style setters and standard
servlet <init-param> configuration. The Dependency Injection style is
simpler, while the <init-param> style will work on
all servlet engines. By creating a separate
a servlet can take advantage of Resin's Dependency Injection and still
be fully compatible with other servlet engines.
Using dependency injection to configure servlets has some advantages over the init-param method:
Enabling the Dependency Injection pattern is trivial: just
setDataSource method as in the example above.
The most important pattern when using JDBC is the
following try/finally block. All database access should follow this pattern.
Because connections are pooled, it's vital to close the connection no
matter what kind of exceptions may be thrown So the
conn.close() must be in a finally block.
The full example splits the database access into two methods to
clarify the roles. The
service retrieves the output
writer from the servlet response and wraps any checked exceptions
ServletException. Splitting the servlet method
doQuery method, so it can concentrate
on the database access.