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Using a JDBC database is a three step process:

- Configuring the <database> in the resin-web.xml or resin.conf
- Injecting the `DataSource` to a field with a `@javax.webbeans.Id` annotation.
- Using a `Connection` from the `DataSource` to execute the SQL.

JDBC database access is based around the Factory pattern. With JDBC, `javax.sql.DataSource` is the Factory object. The <database> configures the `DataSource` and stores it in the WebBeans directory. The servlet will retrieve the `DataSource` and use it as a factory to obtain `Connection` objects, the main workhorse for using databases.

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resin-web.xml configuration
The JDBC query servlet.
</deftable> <example> CREATE TABLE jdbc_basic_brooms ( id INTEGER PRIMARY KEY auto_increment, name VARCHAR(128), cost INTEGER ); INSERT INTO jdbc_basic_brooms (name, cost) VALUES ('firebolt', 4000) INSERT INTO jdbc_basic_brooms (name, cost) VALUES ('nimbus 2001', 500) INSERT INTO jdbc_basic_brooms (name, cost) VALUES ('nimbus 2000', 300) INSERT INTO jdbc_basic_brooms (name, cost) VALUES ('cleansweep 7', 150) INSERT INTO jdbc_basic_brooms (name, cost) VALUES ('cleansweep 5', 100) INSERT INTO jdbc_basic_brooms (name, cost) VALUES ('shooting star', 50) </example>

In Resin 3.0, the <database> tag configures the database pool and driver and saves the connection factory (DataSource) in the Resin WebBeans directory. WebBeans is an IoC (inversion-of-control) configuration system, 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:

1. JCA - Java Connection Architecture - this is a common driver interface for more than just JDBC. If possible, it's generally the best to choose.
2. ConnectionPoolDataSource - JDBC driver which has extra hooks to help Resin pool the connections.
3. Driver - old-style JDBC driver. Its main benefit is that it's generally always available as a fallback.

<example> <web-app xmlns="http://caucho.com/ns/resin"> <database jndi-name="jdbc/basic"> <driver type="com.caucho.db.jca.ConnectionFactory"> <url>resin:WEB-INF/db</url> </driver> </database> </web-app> </example>

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 `DataSource` to access JDBC. Resin allows two styles of configuration: Dependency Injection using WebBeans injection and standard servlet <init-param> configuration with JNDI lookups. The Dependency Injection style is simpler, cleaner and makes the application more testable and robust.

<example> import javax.webbeans.In; public class BasicServlet extends HttpServlet { @In private DataSource _ds; ... } </example>

Using dependency injection to configure servlets has some advantages over the init-param method:

1. The configuration is type-safe. Resin's dependency injection will check the DataSource type before injecting the value. Resin will also inform you of any conflicts, e.g. configuration of multiple matching databases.
2. The servlet initialization code is simpler. The servlet doesn't need JNDI code.
3. The configured values can be more complicated than the string-limitation of <init-param>.
4. The DataSource itself isn't tied to JNDI, although JNDI will certainly remain the primary registry.

Enabling the Dependency Injection pattern is trivial: just add the `@javax.webbeans.In` annotation to your `DataSource` field.

<servlet servlet-name="my-servlet" servlet-class="example.BasicServlet"> </servlet>

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.

Connection conn = _ds.getConnection(); try { ... } finally { conn.close(); }

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 in a `ServletException`. Splitting the servlet method simplifies the `doQuery` method, so it can concentrate on the database access.

<example> public void service(HttpServletRequest req, HttpServletResponse res) throws java.io.IOException, ServletException { PrintWriter out = res.getWriter(); try { doQuery(out); } catch (SQLException e) { throw new ServletException(e); } } private void doQuery(PrintWriter out) throws IOException, SQLException { Connection conn = _ds.getConnection(); try { String sql = "SELECT name, cost FROM jdbc_basic_brooms ORDER BY cost DESC"; Statement stmt = conn.createStatement(); ResultSet rs = stmt.executeQuery(sql); out.println("<table border='3'>"); while (rs.next()) { out.println("<tr><td>" + rs.getString(1)); out.println(" <td>" + rs.getString(2)); } out.println("</table>"); rs.close(); stmt.close(); } finally { conn.close(); } }

- Database configuration reference
- Sample configurations for several database drivers
- Using Dependency Injection for Databases
- The Dependency Injection (or Assembly Line) pattern.

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