Tomcat Standard Resource Factories

Tomcat includes a series of standard resource factories that can provide services to your web applications, but give you configuration flexibility (via the <Context> element) without modifying the web application or the deployment descriptor. Each subsection below details the configuration and usage of the standard resource factories.

Of the standard resource factories, only the “JDBC Data Source” and “User Transaction” factories are mandated to be available on other platforms, and then they are required only if the platform implements the Java Enterprise Edition (Java EE) specs. All other standard resource factories, plus custom resource factories that you write yourself, are specific to Tomcat and cannot be assumed to be available on other containers.

Generic JavaBean Resources

Introduction

This resource factory can be used to create objects of any Java class that conforms to standard JavaBeans naming conventions (i.e. it has a zero-arguments constructor, and has property setters that conform to the setFoo() naming pattern. The resource factory will only create a new instance of the appropriate bean class every time a lookup() for this entry is made if the singleton attribute of the factory is set to false.

The steps required to use this facility are described below.

1. Create Your JavaBean Class

Create the JavaBean class which will be instantiated each time that the resource factory is looked up. For this example, assume you create a class com.mycompany.MyBean, which looks like this:

 

package com.mycompany;

public class MyBean {

private String foo = “Default Foo”;

public String getFoo() {

return (this.foo);

}

 

public void setFoo(String foo) {

this.foo = foo;

}

 

private int bar = 0;

public int getBar() {

return (this.bar);

}

 

public void setBar(int bar) {

this.bar = bar;

}

}

2. Declare Your Resource Requirements

Next, modify your web application deployment descriptor (/WEB-INF/web.xml) to declare the JNDI name under which you will request new instances of this bean. The simplest approach is to use a <resource-env-ref>element, like this:

 

<resource-env-ref>

<description>

Object factory for MyBean instances.

</description>

<resource-env-ref-name>

bean/MyBeanFactory

</resource-env-ref-name>

<resource-env-ref-type>

com.mycompany.MyBean

</resource-env-ref-type>

</resource-env-ref>

Be sure you respect the element ordering that is required by the DTD for web application deployment descriptors!

3. Code Your Application’s Use Of This Resource

A typical use of this resource environment reference might look like this:

Context initCtx = new InitialContext();

Context envCtx = (Context) initCtx.lookup(“java:comp/env”);

MyBean bean = (MyBean) envCtx.lookup(“bean/MyBeanFactory”);

 

writer.println(“foo = ” + bean.getFoo() + “, bar = ” +

bean.getBar());

4. Configure Tomcat’s Resource Factory

To configure Tomcat’s resource factory, add an element like this to the <Context> element for this web application.

<Context …>

…

<Resource name=”bean/MyBeanFactory” auth=”Container”

type=”com.mycompany.MyBean”

factory=”org.apache.naming.factory.BeanFactory”

bar=”23″/>

…

</Context>

Note that the resource name (here, bean/MyBeanFactory must match the value specified in the web application deployment descriptor. We are also initializing the value of the bar property, which will cause setBar(23) to be called before the new bean is returned. Because we are not initializing the foo property (although we could have), the bean will contain whatever default value is set up by its constructor.

UserDatabase Resources

Introduction

UserDatabase resources are typically configured as global resources for use by a UserDatabase realm. Tomcat includes a UserDatabaseFactoory that creates UserDatabase resources backed by an XML file – usually tomcat-users.xml

The steps required to set up a global UserDatabase resource are described below.

1. Create/edit the XML file

The XML file is typically located at $CATALINA_BASE/conf/tomcat-users.xml however, you are free to locate the file anywhere on the file system. It is recommended that the XML files are placed in $CATALINA_BASE/conf. A typical XML would look like:

 

<?xml version=’1.0′ encoding=’utf-8′?>

<tomcat-users>

<role rolename=”tomcat”/>

<role rolename=”role1″/>

<user username=”tomcat” password=”tomcat” roles=”tomcat”/>

<user username=”both” password=”tomcat” roles=”tomcat,role1″/>

<user username=”role1″ password=”tomcat” roles=”role1″/>

</tomcat-users>

2. Declare Your Resource

Next, modify $CATALINA_BASE/conf/server.xml to create the UserDatabase resource based on your XML file. It should look something like this:

 

<Resource name=”UserDatabase”

auth=”Container”

type=”org.apache.catalina.UserDatabase”

description=”User database that can be updated and saved”

factory=”org.apache.catalina.users.MemoryUserDatabaseFactory”

pathname=”conf/tomcat-users.xml”

readonly=”false” />

The pathname attribute can be absolute or relative. If relative, it is relative to $CATALINA_BASE.

The readonly attribute is optional and defaults to true if not supplied. If the XML is writeable then it will be written to when Tomcat starts. WARNING: When the file is written it will inherit the default file permissions for the user Tomcat is running as. Ensure that these are appropriate to maintain the security of your installation.

3. Configure the Realm

Configure a UserDatabase Realm to use this resource as described in the Realm configuration documentation.

JavaMail Sessions

Introduction

In many web applications, sending electronic mail messages is a required part of the system’s functionality. The Java Mail API makes this process relatively straightforward, but requires many configuration details that the client application must be aware of (including the name of the SMTP host to be used for message sending).

Tomcat includes a standard resource factory that will create javax.mail.Session session instances for you, already configured to connect to an SMTP server. In this way, the application is totally insulated from changes in the email server configuration environment – it simply asks for, and receives, a preconfigured session whenever needed.

The steps required for this are outlined below.

1. Declare Your Resource Requirements

The first thing you should do is modify the web application deployment descriptor (/WEB-INF/web.xml) to declare the JNDI name under which you will look up preconfigured sessions. By convention, all such names should resolve to the mail sub context (relative to the standard java:comp/env naming context that is the root of all provided resource factories. A typical web.xml entry might look like this:

 

<resource-ref>

<description>

Resource reference to a factory for javax.mail.Session

instances that may be used for sending electronic mail

messages, preconfigured to connect to the appropriate

SMTP server.

</description>

<res-ref-name>

mail/Session

</res-ref-name>

<res-type>

javax.mail.Session

</res-type>

<res-auth>

Container

</res-auth>

</resource-ref>

Be sure you respect the element ordering that is required by the DTD for web application deployment descriptors!

2. Code Your Application’s Use Of This Resource

A typical use of this resource reference might look like this:

 

Context initCtx = new InitialContext();

Context envCtx = (Context) initCtx.lookup(“java:comp/env”);

Session session = (Session) envCtx.lookup(“mail/Session”);

 

Message message = new MimeMessage(session);

message.setFrom(new InternetAddress(request.getParameter(“from”)));

InternetAddress to[] = new InternetAddress[1];

to[0] = new InternetAddress(request.getParameter(“to”));

message.setRecipients(Message.RecipientType.TO, to);

message.setSubject(request.getParameter(“subject”));

message.setContent(request.getParameter(“content”), “text/plain”);

Transport.send(message);

 

Note that the application uses the same resource reference name that was declared in the web application deployment descriptor. This is matched up against the resource factory that is configured in the <Context>element for the web application as described below.

3. Configure Tomcat’s Resource Factory

To configure Tomcat’s resource factory, add an elements like this to the <Context> element for this web application.

 

<Context …>

…

<Resource name=”mail/Session” auth=”Container”

type=”javax.mail.Session”

mail.smtp.host=”localhost”/>

…

</Context>

Note that the resource name (here, mail/Session) must match the value specified in the web application deployment descriptor. Customize the value of the mail.smtp.host parameter to point at the server that provides SMTP service for your network.

Additional resource attributes and values will be converted to properties and values and passed tojavax.mail.Session.getInstance(java.util.Properties) as part of the java.util.Properties collection. In addition to the properties defined in Annex A of the JavaMail specification, individual providers may also support additional properties.

If the resource is configured with a password attribute and either a mail.smtp.user or mail.user attribute then Tomcat’s resource factory will configure and add a javax.mail.Authenticator to the mail session.

4. Install the JavaMail libraries

Unpackage the distribution and place mail.jar into $CATALINA_HOME/lib so that it is available to Tomcat during the initialization of the mail Session Resource. Note: placing this jar in both $CATALINA_HOME/lib and a web application’s lib folder will cause an error, so ensure you have it in the $CATALINA_HOME/lib location only.

5. Restart Tomcat

For the additional JAR to be visible to Tomcat, it is necessary for the Tomcat instance to be restarted.

Example Application

The /examples application included with Tomcat contains an example of utilizing this resource factory. It is accessed via the “JSP Examples” link. The source code for the servlet that actually sends the mail message is in/WEB-INF/classes/SendMailServlet.java.

The default configuration assumes that there is an SMTP server listing on port 25 on localhost. If this is not the case, edit the <Context> element for this web application and modify the parameter value for themail.smtp.host parameter to be the host name of an SMTP server on your network.

JDBC Data Sources

Introduction

Many web applications need to access a database via a JDBC driver, to support the functionality required by that application. The Java EE Platform Specification requires Java EE Application Servers to make available aDataSource implementation (that is, a connection pool for JDBC connections) for this purpose. Tomcat offers exactly the same support, so that database-based applications you develop on Tomcat using this service will run unchanged on any Java EE server.

The default data source support in Tomcat is based on the DBCP connection pool from the Commonsproject. However, it is possible to use any other connection pool that implements javax.sql.DataSource, by writing your own custom resource factory, as described below.

1. Install Your JDBC Driver

Use of the JDBC Data Sources JNDI Resource Factory requires that you make an appropriate JDBC driver available to both Tomcat internal classes and to your web application. This is most easily accomplished by installing the driver’s JAR file(s) into the $CATALINA_HOME/lib directory, which makes the driver available both to the resource factory and to your application.

2. Declare Your Resource Requirements

Next, modify the web application deployment descriptor (/WEB-INF/web.xml) to declare the JNDI name under which you will look up preconfigured data source. By convention, all such names should resolve to the jdbcsubcontext (relative to the standard java:comp/env naming context that is the root of all provided resource factories. A typical web.xml entry might look like this:

<resource-ref>

<description>

Resource reference to a factory for java.sql.Connection

instances that may be used for talking to a particular

database that is configured in the <Context>

configurartion for the web application.

</description>

<res-ref-name>

jdbc/EmployeeDB

</res-ref-name>

<res-type>

javax.sql.DataSource

</res-type>

<res-auth>

Container

</res-auth>

</resource-ref>

Be sure you respect the element ordering that is required by the DTD for web application deployment descriptors!

3. Code Your Application’s Use Of This Resource

A typical use of this resource reference might look like this:

 

Context initCtx = new InitialContext();

Context envCtx = (Context) initCtx.lookup(“java:comp/env”);

DataSource ds = (DataSource)

envCtx.lookup(“jdbc/EmployeeDB”);

 

Connection conn = ds.getConnection();

… use this connection to access the database …

conn.close();

Note that the application uses the same resource reference name that was declared in the web application deployment descriptor. This is matched up against the resource factory that is configured in the <Context>element for the web application as described below.

4. Configure Tomcat’s Resource Factory

To configure Tomcat’s resource factory, add an element like this to the <Context> element for the web application.

<Context …>

…

<Resource name=”jdbc/EmployeeDB”

auth=”Container”

type=”javax.sql.DataSource”

username=”dbusername”

password=”dbpassword”

driverClassName=”org.hsql.jdbcDriver”

url=”jdbc:HypersonicSQL:database”

maxTotal=”8″

maxIdle=”4″/>

…

</Context>

Note that the resource name (here, jdbc/EmployeeDB) must match the value specified in the web application deployment descriptor.

This example assumes that you are using the HypersonicSQL database JDBC driver. Customize thedriverClassName and driverName parameters to match your actual database’s JDBC driver and connection URL.

The configuration properties for Tomcat’s standard data source resource factory (org.apache.tomcat.dbcp.dbcp2.BasicDataSourceFactory) are as follows:

• driverClassName – Fully qualified Java class name of the JDBC driver to be used.
• username – Database username to be passed to our JDBC driver.
• password – Database password to be passed to our JDBC driver.
• url – Connection URL to be passed to our JDBC driver. (For backwards compatibility, the propertydriverName is also recognized.)
• initialSize – The initial number of connections that will be created in the pool during pool initialization. Default: 0
• maxTotal – The maximum number of connections that can be allocated from this pool at the same time. Default: 8
• minIdle – The minimum number of connections that will sit idle in this pool at the same time. Default: 0
• maxIdle – The maximum number of connections that can sit idle in this pool at the same time. Default: 8
• maxWaitMillis – The maximum number of milliseconds that the pool will wait (when there are no available connections) for a connection to be returned before throwing an exception. Default: -1 (infinite)

Some additional properties handle connection validation:

• validationQuery – SQL query that can be used by the pool to validate connections before they are returned to the application. If specified, this query MUST be an SQL SELECT statement that returns at least one row.
• validationQueryTimeout – Timeout in seconds for the validation query to return. Default: -1 (infinite)
• testOnBorrow – true or false: whether a connection should be validated using the validation query each time it is borrowed from the pool. Default: true
• testOnReturn – true or false: whether a connection should be validated using the validation query each time it is returned to the pool. Default: false

Another The optional evictor thread is responsible for shrinking the pool by removing any connections which are idle for a long time. The evictor does not respect minIdle. Note that you do not need to activate the evictor thread if you only want the pool to shrink according to the configured maxIdle property.

 

The evictor is disabled by default and can be configured using the following properties:

timeBetweenEvictionRunsMillis – The number of milliseconds between consecutive runs of the evictor. Default: -1 (disabled)

numTestsPerEvictionRun – The number of connections that will be checked for idleness by the evitor during each run of the evictor. Default: 3

minEvictableIdleTimeMillis – The idle time in milliseconds after which a connection can be removed from the pool by the evictor. Default: 30*60*1000 (30 minutes)

testWhileIdle – true or false: whether a connection should be validated by the evictor thread using the validation query while sitting idle in the pool. Default: false

 

Another optional feature is the removal of abandoned connections. A connection is called abandoned if the application does not return it to the pool for a long time. The pool can close such connections automatically and remove them from the pool. This is a workaround for applications leaking connections.

The abandoning feature is disabled by default and can be configured using the following properties:

removeAbandoned – true or false: whether to remove abandoned connections from the pool. Default: false

removeAbandonedTimeout – The number of seconds after which a borrowed connection is assumed to be abandoned. Default: 300

logAbandoned – true or false: whether to log stack traces for application code which abandoned a statement or connection. This adds serious overhead. Default: false

 

Finally there are various properties that allow further fine tuning of the pool behaviour:

defaultAutoCommit – true or false: default auto-commit state of the connections created by this pool. Default: true

defaultReadOnly – true or false: default read-only state of the connections created by this pool. Default: false

defaultTransactionIsolation – This sets the default transaction isolation level. Can be one of NONE,READ_COMMITTED, READ_UNCOMMITTED, REPEATABLE_READ, SERIALIZABLE. Default: no default set

poolPreparedStatements – true or false: whether to pool PreparedStatements and CallableStatements. Default: false

maxOpenPreparedStatements – The maximum number of open statements that can be allocated from the statement pool at the same time. Default: -1 (unlimited)

defaultCatalog – The name of the default catalog. Default: not set

connectionInitSqls – A list of SQL statements run once after a Connection is created. Separate multiple statements by semicolons (;). Default: no statement

connectionProperties – A list of driver specific properties passed to the driver for creating connections. Each property is given as name=value, multiple properties are separated by semicolons (;). Default: no properties

accessToUnderlyingConnectionAllowed – true or false: whether accessing the underlying connections is allowed. Default: false

 

For more details, please refer to the commons-dbcp documentation.