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using dependency-injection with jdbc databases


This tutorial describes the standard pattern for using a database in Resin, following the Dependency-Injection/Inversion-of-Control pattern using WebBeans annotations.

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.

Files in this tutorial

WEB-INF/resin-web.xmlresin-web.xml configuration
WEB-INF/classes/example/BasicServlet.javaThe JDBC query servlet.

Database Schema

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)

Database Configuration

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.
<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>

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.

com.caucho.db.jca.ConnectionFactory

The specific driver for this example, com.caucho.db.jca.ConnectionFactory is a simple database intended for examples and testing.

Servlet Initialization

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.

import javax.webbeans.In;

public class BasicServlet extends HttpServlet {
  @In private DataSource _ds;

  ...
}

Dependency Injection configuration

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.

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

Using the Database

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 try ... 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.

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();
  }
}

See also


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