JNDI-j2ee

 

Database Connection Pool (DBCP) Configurations

The default database connection pool implementation in Apache Tomcat relies on the libraries from the Apache Commons project. The following libraries are used:

• Commons DBCP
• Commons Pool

These libraries are located in a single JAR at $CATALINA_HOME/lib/tomcat-dbcp.jar. However, only the classes needed for connection pooling have been included, and the packages have been renamed to avoid interfering with applications.

DBCP 1.3 provides support for JDBC 3.0.

Installation

See the DBCP documentation for a complete list of configuration parameters.

Preventing database connection pool leaks

A database connection pool creates and manages a pool of connections to a database. Recycling and reusing already existing connections to a database is more efficient than opening a new connection.

There is one problem with connection pooling. A web application has to explicitly close ResultSet's, Statement's, and Connection's. Failure of a web application to close these resources can result in them never being available again for reuse, a database connection pool "leak". This can eventually result in your web application database connections failing if there are no more available connections.

There is a solution to this problem. The Apache Commons DBCP can be configured to track and recover these abandoned database connections. Not only can it recover them, but also generate a stack trace for the code which opened these resources and never closed them.

To configure a DBCP DataSource so that abandoned database connections are removed and recycled add the following attribute to the Resource configuration for your DBCP DataSource:

	removeAbandoned="true"

When available database connections run low DBCP will recover and recycle any abandoned database connections it finds. The default is false.

Use the removeAbandonedTimeout attribute to set the number of seconds a database connection has been idle before it is considered abandoned.

	removeAbandonedTimeout="60"

The default timeout for removing abandoned connections is 300 seconds.

The logAbandoned attribute can be set to true if you want DBCP to log a stack trace of the code which abandoned the database connection resources.

	logAbandoned="true"

The default is false.

MySQL DBCP Example

0. Introduction

Versions of MySQL and JDBC drivers that have been reported to work:

• MySQL 3.23.47, MySQL 3.23.47 using InnoDB,, MySQL 3.23.58, MySQL 4.0.1alpha
• Connector/J 3.0.11-stable (the official JDBC Driver)
• mm.mysql 2.0.14 (an old 3rd party JDBC Driver)

Before you proceed, don't forget to copy the JDBC Driver's jar into $CATALINA_HOME/lib.

1. MySQL configuration

Ensure that you follow these instructions as variations can cause problems.

Create a new test user, a new database and a single test table. Your MySQL user must have a password assigned. The driver will fail if you try to connect with an empty password.

	mysql> GRANT ALL PRIVILEGES ON *.* TO javauser@localhost -> IDENTIFIED BY 'javadude' WITH GRANT OPTION; mysql> create database javatest; mysql> use javatest; mysql> create table testdata ( -> id int not null auto_increment primary key, -> foo varchar(25), -> bar int);

Note: the above user should be removed once testing is complete!

Next insert some test data into the testdata table.

	mysql> insert into testdata values(null, 'hello', 12345); Query OK, 1 row affected (0.00 sec) mysql> select * from testdata; +----+-------+-------+ | ID | FOO | BAR | +----+-------+-------+ | 1 | hello | 12345 | +----+-------+-------+ 1 row in set (0.00 sec) mysql>

2. Context configuration

 META-INF/context.xml:

<?xml version="1.0" encoding="UTF-8"?>
<Context reloadable="true">
    <!-- maxActive: Maximum number of database connections in pool. Make sure 
        you configure your mysqld max_connections large enough to handle all of your 
        db connections. Set to -1 for no limit. -->

    <!-- maxIdle: Maximum number of idle database connections to retain in pool. 
        Set to -1 for no limit. See also the DBCP documentation on this and the minEvictableIdleTimeMillis 
        configuration parameter. -->

    <!-- maxWait: Maximum time to wait for a database connection to become available 
        in ms, in this example 10 seconds. An Exception is thrown if this timeout 
        is exceeded. Set to -1 to wait indefinitely. -->

    <!-- username and password: MySQL username and password for database connections -->

    <!-- driverClassName: Class name for the old mm.mysql JDBC driver is org.gjt.mm.mysql.Driver 
        - we recommend using Connector/J though. Class name for the official MySQL 
        Connector/J driver is com.mysql.jdbc.Driver. -->

    <!-- url: The JDBC connection url for connecting to your MySQL database. -->

    <Resource name="jdbc/TestDB" auth="Container" type="javax.sql.DataSource"
        maxActive="100" maxIdle="30" maxWait="10000" 
        username="javauser"
        password="javadude" 
        driverClassName="com.mysql.jdbc.Driver"
        url="jdbc:mysql://localhost:3306/javatest" />

</Context>

3. web.xml configuration

Now create a WEB-INF/web.xml for this test application.

	<web-app 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" version="2.4"> <description>MySQL Test App</description> <resource-ref> <description>DB Connection</description> <res-ref-name>jdbc/TestDB</res-ref-name> <res-type>javax.sql.DataSource</res-type> <res-auth>Container</res-auth> </resource-ref> </web-app>

4. Test code

Now create a simple test.jsp page for use later.

<%@ taglib uri="http://java.sun.com/jsp/jstl/sql" prefix="sql" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<sql:query var="rs" dataSource="jdbc/TestDB">
select id, foo, bar from testdata
</sql:query>

<html>
  <head>
    <title>DB(JNDI) Test</title>
  </head>
  <body>

  <h2>Results</h2>
  
<c:forEach var="row" items="${rs.rows}">
    Foo: ${row.foo}<br/>
    Bar: ${row.bar}<br/>
</c:forEach>

  </body>
</html>

That JSP page makes use of JSTL's SQL and Core taglibs. You can get it from Apache Tomcat Taglibs - Standard Tag Library project — just make sure you get a 1.1.x release. Once you have JSTL, copyjstl.jar and standard.jar t(

Standard 1.1

JSTL 1.1

Servlet 2.4, JavaServer Pages 2.0

download

)o your web app's WEB-INF/lib directory.

Finally deploy your web app into $CATALINA_BASE/webapps either as a warfile called DBTest.war or into a sub-directory called DBTest

Once deployed, point a browser at http://localhost:8080/DBTest/test.jsp to view the fruits of your hard work.