Database Connection Pooling in Tomcat 

Database Connection Pooling is a great technique used by lot of application servers to optimize the performance. Database Connection creation is a costly task thus it impacts the performance of application. Hence lot of application server creates a database connection pool which are pre initiated db connections that can be leverage to increase performance.

Apache Tomcat also provide a way of creating DB Connection Pool. Let us see an example to implement DB Connection Pooling in Apache Tomcat server. We will create a sample web application with a servlet that will get the db connection from tomcat db connection pool and fetch the data using a query. We will use Eclipse as our development environment. This is not a prerequisite i.e. you may want to use any IDE to create this example.

Step 1: Create Dynamic Web Project in Eclipse

Create a Dynamic Web Project in Eclipse by selecting:
File -> New -> Project… ->Dynamic Web Project.

Step 2: Create context.xml

Apache Tomcat allow the applications to define the resource used by the web application in a file called context.xml (from Tomcat 5.x version onwards). We will create a file context.xml under META-INFdirectory.

Copy following content in the context.xml file.

    <?xml version="1.0" encoding="UTF-8"?>  <Context>  	<!-- Specify a JDBC datasource -->  	<Resource name="jdbc/testdb" auth="Container"  		type="javax.sql.DataSource" username="DB_USERNAME" password="DB_PASSWORD"  		driverClassName="oracle.jdbc.driver.OracleDriver"  		url="jdbc:oracle:thin:@xxx:1525:dbname"  		maxActive="10" maxIdle="4" />    </Context>  

In above code snippet, we have specify a database connection pool. The name of the resource isjdbc/testdb. We will use this name in our application to get the data connection. Also we specify db username and password and connection URL of database. Note that I am using Oracle as the database for this example. You may want to change this Driver class with any of other DB Providers (like MySQL Driver Class).

Step 3: Create Test Servlet and WEB xml entry

Create a file called TestServlet.java. I have created this file under package: net.viralpatel.servlet. Copy following code into it.

package net.viralpatel.servlet;    import java.io.IOException;  import java.sql.Connection;  import java.sql.ResultSet;  import java.sql.SQLException;  import java.sql.Statement;    import javax.naming.Context;  import javax.naming.InitialContext;  import javax.naming.NamingException;  import javax.servlet.ServletException;  import javax.servlet.http.HttpServlet;  import javax.servlet.http.HttpServletRequest;  import javax.servlet.http.HttpServletResponse;  import javax.sql.DataSource;    public class TestServlet extends HttpServlet {  	  	private DataSource dataSource;  	private Connection connection;  	private Statement statement;  	  	public void init() throws ServletException {  		try {  			// Get DataSource  			Context initContext  = new InitialContext();  			Context envContext  = (Context)initContext.lookup("java:/comp/env");  			dataSource = (DataSource)envContext.lookup("jdbc/testdb");    			  		} catch (NamingException e) {  			e.printStackTrace();  		}  	}    	public void doGet(HttpServletRequest req, HttpServletResponse resp)  			throws ServletException, IOException {  		  		ResultSet resultSet = null;  		try {  			// Get Connection and Statement  			connection = dataSource.getConnection();  			statement = connection.createStatement();  			String query = "SELECT * FROM STUDENT";  			resultSet = statement.executeQuery(query);  			while (resultSet.next()) {  				System.out.println(resultSet.getString(1) + resultSet.getString(2) + resultSet.getString(3));  			}  		} catch (SQLException e) {  			e.printStackTrace();  		}finally {  			try { if(null!=resultSet)resultSet.close();} catch (SQLException e)   			{e.printStackTrace();}  			try { if(null!=statement)statement.close();} catch (SQLException e)   			{e.printStackTrace();}  			try { if(null!=connection)connection.close();} catch (SQLException e)   			{e.printStackTrace();}  		}  	}  }  

In the above code we initiated the datasource using InitialContext lookup:

    Context initContext  = new InitialContext();  Context envContext  = (Context)initContext.lookup("java:/comp/env");  dataSource = (DataSource)envContext.lookup("jdbc/testdb");  

Create test servlet mapping in the web.xml file (deployment descriptor) of the web application. The web.xml file will look like:

<?xml version="1.0" encoding="UTF-8"?>  <web-app id="WebApp_ID" version="2.4"  	xmlns="http://java.sun.com/xml/ns/j2ee"  	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  	xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">  	<display-name>TomcatConnectionPooling</display-name>  	<welcome-file-list>  		<welcome-file>index.jsp</welcome-file>  	</welcome-file-list>    	<servlet>  		<servlet-name>TestServlet</servlet-name>  		<servlet-class>  			net.viralpatel.servlet.TestServlet  		</servlet-class>  	</servlet>  	<servlet-mapping>  		<servlet-name>TestServlet</servlet-name>  		<url-pattern>/servlet/test</url-pattern>  	</servlet-mapping>  </web-app>  

Now Run the web application in Tomcat using Eclipse (Alt + Shift + X, R). You will be able to see the result of the query executed.

Thus this way we can create a database pool in Tomcat and get the connections from it.