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A configuration in WEB-INF/web.xml
makes a datasource available to the application.
The following example configures a PostgreSQL driver, more example
configurations are in the Third-party section
of the documentation.
and are used to
enable the servlet.
<web-app xmlns="http://caucho.com/ns/resin"
xmlns:resin="http://caucho.com/ns/resin/core">
<database jndi-name="jdbc/HogwartsStore"/>
<driver>
<type>org.postgresql.Driver</type>
<url>jdbc:postgresql://127.0.0.1:5432/hogwarts_store</url>
<user>web</user>
<password>i19web</password>
</driver>
</database>
...
<servlet servlet-name="example.TestDatabase" servlet-class="example.TestDatabase"/>
<servlet-mapping url-pattern="/TestDatabase" servlet-name="example.TestDatabase"/>
...
</web-app>
The DataSource is obtained using the jndi-name specified when configuring the database
resource. The JNDI lookup of DataSource is done only once, the
DataSource is stored in the _dataSource member variable so
that it can be used for each request. If it was not stored, there would be an
impact on performance
from having to do the lookup each time.
package example;
import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.naming.*;
import javax.sql.*;
public class TestDatabase
extends HttpServlet
{
private final static Logger log = Logger.getLogger(TestDatabase.class.getName());
private final static String DATASOURCE_NAME = "jdbc/HogwartsStore";
private DataSource _dataSource;
public void setDataSource(DataSource dataSource)
{
_dataSource = dataSource;
}
public DataSource getDataSource()
{
return _dataSource;
}
public void init()
throws ServletException
{
if (_dataSource == null) {
try {
Context env = (Context) new InitialContext().lookup("java:comp/env");
_dataSource = (DataSource) env.lookup(DATASOURCE_NAME);
if (_dataSource == null)
throw new ServletException("`" + DATASOURCE_NAME + "' is an unknown DataSource");
} catch (NamingException e) {
throw new ServletException(e);
}
}
}
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException
{
...
}
}
The DataSource is a factory that is used to obtain connections when they are
needed. The following is a sample design pattern for getting new database
connections from within a servlet.. The try ... finally block is very
important. Without the close in the finally block, Resin's database pool can
lose connections.
This is so important, it is worth repeating. The try
... finally block is very important. Without the close in the finally block,
Resin's database pool can lose connections. If you fail to close() a
connection, Resin cannot know that it is not being used anymore, and cannot
allocate it for another request. Eventually, you may run out of connections.
...
public class TestDatabase
extends HttpServlet
{
public void setDataSource(DataSource dataSource) ...
public DataSource getDataSource() ...
public void init() ...
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException
{
res.setContentType("text/html");
PrintWriter out = res.getWriter();
Connection conn = null;
try {
conn = getDataSource().getConnection();
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("select NAME, PRICE from BROOMS");
out.println("Brooms:<br>");
while (rs.next()) {
out.print(rs.getString(1));
out.print(" ");
out.print(rs.getInt(2));
out.println("<br>");
}
rs.close();
stmt.close();
}
catch (SQLException e) {
throw new ServletException(e);
}
finally {
try {
if (conn != null)
conn.close();
}
catch (SQLException e) {
log.log(Level.WARNING, ex.toString(), ex);
}
}
}
}
The example as it stands is tied to a single data-source name,
"jdbc/HogwartsStore". This limits configuration options, using a different
DataSource requires a change to the code.
Fortunately, the code as it stands supports
Dependency Injection
on servers (like Resin) that support it.
For example, to have the servlet use the "jdbc/DiagonAlley" DataSource requires
only a small change in web.xml.
<servlet servlet-name="example.TestDatabase" servlet-class="example.TestDatabase">
<init>
<data-source>${jndi:lookup("jdbc/DiagonAlley")}</data-source>
</init>
</servlet>
<servlet-mapping url-pattern="/TestDatabase" servlet-name="example.TestDatabase"/>
The init block causes Resin to call setDataSource before
the init() method is called on the servlet. The servlet does
not do a jndi lookup for the DataSource, because it already has one.
Copyright (c) 1998-2009 Caucho Technology, Inc. All rights reserved.
caucho® ,
resin® and
quercus®
are registered trademarks of Caucho Technology, Inc.
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