Home > Articles

Administration Basics

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

Creating the Database Connection Pool

Now let's introduce Tomcat to your database. We do so by creating a database connection pool that is actually a JNDI resource. JNDI is a J2EE technology that I talk more about in Chapter 21, "Administering JNDI Resources." Essentially, JNDI is a directory-like listing of resources maintained by a provider and available for lookup and access by a client. In our case, Tomcat serves as the provider, and our application (just the JSP, in this case) is the client. JNDI works by assigning resources names that the client can use to retrieve a handle to the resource itself.

When you create a JNDI-accessible database connection pool in Tomcat, you must define it in $CATALINA_HOME/conf/server.xml. As we'll see in Chapter 21, there are various places where you are allowed to define your pool. You can define it at the top level, thereby making it available to all hosts and host applications; at the virtual host level; and at the application or context level. The key determining factor here is scope: who needs your pool? Because we are just getting started, we'll keep your definition at the application level, which means we must now edit the application definition file, $CATALINA_HOME/conf/Catalina/localhost/unleashed.xml. The revised file appears in Listing 3.4.

Listing 3.4 Revised unleashed.xml

<Context path="/unleashed" docBase="unleashed" debug="0">

 <Logger className="org.apache.catalina.logger.FileLogger"
  prefix="localhost_unleashed_" suffix=".log"

 <Resource name="jdbc/unleashed" auth="Container"
 <ResourceParams name="jdbc/unleashed">


This code is a typical example of a JNDI resource definition. First, you need to define the resource itself. The fact that the type attribute is javax.sql.DataSource signals to Tomcat that it is a database connection pool. The name can be anything you want, but you typically use the prefix jdbc/ for connection pools. Also, it helps to use the database name as the rest of the name because it is clear at a glance what database this resource points to.

After the resource definition comes various parameters that apply to the resource. Obviously, each resource type has its own applicable parameters, some required and some not. For database pools, you need at least the four shown here: database user ID, database password, driver class name, and the JDBC URL that points to the database itself. Tomcat uses all four parameters to create connections to your database. The driver name is important because Tomcat will be looking in its classpath for that object. If you put the MySQL JDBC jar file in $CATALINA_HOME/common/lib, you're guaranteed that Tomcat will find it. Often, one of the biggest problems people have in setting up connection pools is in not having the JDBC jar file for their databases, having it but not in the right place (so it is not in Tomcat's classpath), or having the driver name wrong. Typically, the JDBC documentation specific to your database server will tell you what driver class name to use, as well as the format for the URL, another place where errors commonly occur. If you look at your URL, you'll notice that the syntax is basically host and database. You could also add a port number after the host, if you had your MySQL server running on something other than the default of 3306.

Also, remember to change the username and password parameters if you changed the MySQL defaults. You'll notice my password parameter is empty; that's because, again, the root user has no password by default. More parameters for database polls control their operation—minimum, maximum, rules for orphaned connections, and so on. You'll deal with all of them later, in Chapter 21.

Creating the JSP

We're two steps away from seeing whether everything works. First, we need to create a basic JSP page as shown in Listing 3.5. The logic of this page is to do a JNDI lookup on the connection pool and acquire a JDBC connection, which we can then use to retrieve the rows from the table. If you are not familiar with JDBC, just cut and paste away. Call this page resources.jsp and put it in the unleashed directory in your deployment directory.

Listing 3.5 resources.jsp

<%@ page language="java"
  import="javax.sql.*,javax.naming.*,java.sql.*" session="false"%>

 <style type="text/css">
  a { text-decoration: none }
  body { font-family: verdana, helvetica, sans serif; font-size: 10pt; }

  <h3>This is a test of a Tomcat-managed connection pool</h3>

  try {
    String jdbcname = "jdbc/unleashed";
    String cmd = "select title, url from TomcatResources order by title";
    Context ctx = new InitialContext();
    Context envCtx = (Context)ctx.lookup("java:comp/env");
    DataSource ds = (DataSource)envCtx.lookup(jdbcname);
    Connection conn = ds.getConnection();
    Statement stmt = conn.createStatement();
    ResultSet rs = stmt.executeQuery(cmd);
    ResultSetMetaData rsmd = rs.getMetaData();
    int cols = rsmd.getColumnCount();

    out.println("<table border\"1\">");
    out.println("<tr bgcolor=\"lightGrey\">");

    for (int i = 1; i <= cols; i++) {
      out.println("<td align=\"center\" width=\"200\"><b>" +
        rsmd.getColumnName(i) + "</b></td>");


    while (rs.next()) {

      for (int i = 1; i <= cols; i++) {
        out.println("<td align=\"left\">" +
            rs.getString(i) + "</td>");


  } catch(Exception e) {
    out.println("Error: " + e);



One thing that is different between this JSP page and the index.jsp created earlier is that I had to add some imports to the page declaration. I imported javax.naming.* for the JNDI classes, javax.sql.* for the JNDI data source class, and java.sql.* for the JDBC classes. Another thing to note is that rather than intermingle HTML and Java code, I just created a single code block and used the println method on the out object to spit out the necessary HTML. The out object is one provided by default to JSP pages, and it stands for the ServletOutputStream object, with which the JSP writes data to the client. Sometimes, it is easier than mixing raw HTML with the Java code, although you do have to escape double-quotes, which can be a real pain.

The basic logic in this page is to retrieve a handle, which is called an InitialContext, to the JNDI naming environment. Note that we actually do two lookups: one to get the overall JNDI context, named as java:comp/env, and one to get the particular resource, which we named before as jdbc/unleashed. Remember I said that a JNDI lookup actually returns a handle to the resource. In this case, that handle is a javax.sql.DataSource object, from which we can then retrieve the connection that is wrapped inside. Then, we can do all your regular JDBC calls. You'll notice that I wrote a rather generic section of code that prints column names and headers based on the metadata returned by the result set. It is a useful way that you can use with any kind of select statement because it creates the table dynamically based on what is returned. If you want to see whether that is true, you can just replace the SQL statement I have with something that goes against one of your own tables. (You might also have to change the resource definition to point to another database, in this case.)


If you haven't already done so, you need to start or restart Tomcat. Then, you can browse to http://localhost:8080/unleashed/resources.jsp and see the results shown in Figure 3.4.

Figure 3.4Figure 3.4 Testing resources.jsp.

If you don't get this page, you have a couple of things to check. First, make sure you do have resources.jsp in the right directory. Second, and most important, you could see an error such as that shown in Figure 3.5.

This error is probably the most common error with Tomcat connection pools: the driver indicated in your resource definition in server.xml cannot be found. Note that my JSP spits out any exceptions to the browser, but not all code is so helpful. Many times, you'll have to look in the log files or the Tomcat console window for this error. You'll typically see a big stack trace, but somewhere buried should be this message. Then, you'll know that the JDBC jar file for your database is in the wrong place or missing. Correct the problem, restart Tomcat, and you'll be fine.

Figure 3.5Figure 3.5 Error page in resources.jsp.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020