Links User Guide Reference Apache Tomcat Development | | Introduction |
Tomcat 6 provides a JNDI InitialContext implementation
instance for each web application running under it, in a manner that is
compatible with those provided by a
Java2 Enterprise Edition application
server.
The J2EE standard provides a standard set of elements in
the /WEB-INF/web.xml file to reference resources; resources
referenced in these elements must be defined in an application-server-specific
configuration.
For Tomcat 6, these entries in per-web-application
InitialContext are configured in the
<Context> elements that
can be specified in either $CATALINA_BASE/conf/server.xml or,
preferably, the per-web-application context XML file (
META-INF/context.xml).
Tomcat 6 maintains a separate namespace of global resources for the
entire server. These are configured in the
<GlobalNameingResources> element of
$CATALINA_BASE/conf/server.xml. You may expose these resources to
web applications by using
<ResourceLink> elements.
The resources defined in these elements
may be referenced by the following elements in the web application deployment
descriptor (/WEB-INF/web.xml) of your web application:
<env-entry> - Environment entry, a
single-value parameter that can be used to configure how the application
will operate.<resource-ref> - Resource reference,
which is typically to an object factory for resources such as a JDBC
DataSource, a JavaMail Session, or custom
object factories configured into Tomcat 6.<resource-env-ref> - Resource
environment reference, a new variation of resource-ref
added in Servlet 2.4 that is simpler to configure for resources
that do not require authentication information.
The InitialContext is configured as a web application is
initially deployed, and is made available to web application components (for
read-only access). All configured entries and resources are placed in
the java:comp/env portion of the JNDI namespace, so a typical
access to a resource - in this case, to a JDBC DataSource -
would look something like this:
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// Obtain our environment naming context
Context initCtx = new InitialContext();
Context envCtx = (Context) initCtx.lookup("java:comp/env");
// Look up our data source
DataSource ds = (DataSource)
envCtx.lookup("jdbc/EmployeeDB");
// Allocate and use a connection from the pool
Connection conn = ds.getConnection();
... use this connection to access the database ...
conn.close();
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See the following Specifications for more information about programming APIs
for JNDI, and for the features supported by Java2 Enterprise Edition (J2EE)
servers, which Tomcat emulates for the services that it provides:
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| Configuring JNDI Resources |
Each available JNDI Resource is configured based on inclusion of the
following elements in the
<Context> element:
- <Environment> -
Configure names and values for scalar environment entries that will be
exposed to the web application through the JNDI
InitialContext (equivalent to the inclusion of an
<env-entry> element in the web application
deployment descriptor).
- <Resource> -
Configure the name and data type of a resource made available to the
application (equivalent to the inclusion of a
<resource-ref> element in the web application
deployment descriptor).
- <ResourceLink> -
Add a link to a resource defined in the global JNDI context. Use resource
links to give a web application access to a resource defined in
the<GlobalNamingResources>
child element of the <Server>
element.
- <Transaction> -
Add a resource factory for instantiating the UserTransaction object
instance that is available at
java:comp/UserTransaction.
Any number of these elements may be nested inside a
<Context> element (to be
associated only with that particular web application).
In addition, the names and values of all <env-entry>
elements included in the web application deployment descriptor
(/WEB-INF/web.xml) are configured into the initial context as
well, overriding corresponding values from the
<Context> element
only if allowed by the corresponding
<Environment> element (by setting the override
attribute to "true").
Global resources can be defined in the server-wide JNDI context, by adding
the resource elements described above to the
<GlobalNamingResources>
child element of the <Server>
element and using a
<ResourceLink> to
include it in the per-web-application context.
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| Tomcat Standard Resource Factories |
Tomcat 6 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.
See Adding Custom
Resource Factories for information about how to create, install,
configure, and use your own custom resource factory classes with
Tomcat 6.
NOTE - 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 Java2 Enterprise Edition (J2EE) 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 |
0. 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
create a new instance of the appropriate bean class every time a
lookup() for this entry is made.
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:
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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;
}
}
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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:
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<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>
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WARNING - Be sure you respect the element ordering
that is required by the DTD for web application deployment descriptors!
See the
Servlet
Specification for details.
3. Code Your Application's Use Of This Resource
A typical use of this resource environment reference might look
like this:
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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());
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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.
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<Context ...>
...
<Resource name="bean/MyBeanFactory" auth="Container"
type="com.mycompany.MyBean"
factory="org.apache.naming.factory.BeanFactory"
bar="23"/>
...
</Context>
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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.
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| JavaMail Sessions |
0. 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 6 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 subcontext (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:
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<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>
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WARNING - Be sure you respect the element ordering
that is required by the DTD for web application deployment descriptors!
See the
Servlet
Specification for details.
2. Code Your Application's Use Of This Resource
A typical use of this resource reference might look like this:
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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);
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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.
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<Context ...>
...
<Resource name="mail/Session" auth="Container"
type="javax.mail.Session"
mail.smtp.host="localhost"/>
...
</Context>
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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.
4. Install the JavaMail libraries
Download the JavaMail API. The JavaMail API requires the Java Activation
Framework (JAF) API as well. The Java Activation Framework can be downloaded
from Sun's site.
This download includes 2 vital libraries for the configuration;
activation.jar and mail.jar. Unpackage both distributions and place
them into $CATALINA_HOME/lib so that they are available to
Tomcat during the initialization of the mail Session Resource.
Note: placing these jars in both $CATALINA_HOME/lib and a
web application's lib folder will cause an error, so ensure you have
them in the $CATALINA_HOME/lib location only.
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.
WARNING - 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 the
mail.smtp.host parameter to be the host name of an SMTP server
on your network.
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| JDBC Data Sources |
0. Introduction
Many web applications need to access a database via a JDBC driver,
to support the functionality required by that application. The J2EE
Platform Specification requires J2EE Application Servers to make
available a DataSource implementation (that is, a connection
pool for JDBC connections) for this purpose. Tomcat 6 offers exactly
the same support, so that database-based applications you develop on
Tomcat using this service will run unchanged on any J2EE server.
For information about JDBC, you should consult the following:
NOTE - The default data source support in Tomcat
is based on the DBCP connection pool from the
Commons
project. 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 jdbc subcontext (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:
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<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>
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WARNING - Be sure you respect the element ordering
that is required by the DTD for web application deployment descriptors!
See the
Servlet
Specification for details.
3. Code Your Application's Use Of This Resource
A typical use of this resource reference might look like this:
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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();
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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.
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<Context ...>
...
<Resource name="jdbc/EmployeeDB"
auth="Container"
type="javax.sql.DataSource"
username="dbusername"
password="dbpassword"
driverClassName="org.hsql.jdbcDriver"
url="jdbc:HypersonicSQL:database"
maxActive="8"
maxIdle="4"/>
...
</Context>
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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 the driverClassName 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.dbcp.BasicDataSourceFactory) are
as follows:
- driverClassName - Fully qualified Java class name
of the JDBC driver to be used.
- maxActive - The maximum number of active instances
that can be allocated from this pool at the same time.
- maxIdle - The maximum number of connections that
can sit idle in this pool at the same time.
- maxWait - 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.
- password - Database password to be passed to our
JDBC driver.
- url - Connection URL to be passed to our JDBC driver.
(For backwards compatibility, the property
driverName
is also recognized.)
- user - Database username to be passed to our
JDBC driver.
- 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.
For more details, please refer to the commons-dbcp documentation.
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| Adding Custom Resource Factories |
If none of the standard resource factories meet your needs, you can write
your own factory and integrate it into Tomcat 6, and then configure the use
of this factory in the
<Context> element for
the web application. In the example below, we will create a factory that only
knows how to create com.mycompany.MyBean beans from the
Generic JavaBean Resources example
above.
1. Write A Resource Factory Class
You must write a class that implements the JNDI service provider
javax.naming.spi.ObjectFactory inteface. Every time your
web application calls lookup() on a context entry that is
bound to this factory, the getObjectInstance() method is
called, with the following arguments:
- Object obj - The (possibly null) object containing
location or reference information that can be used in creating an object.
For Tomcat, this will always be an object of type
javax.naming.Reference, which contains the class name of
this factory class, as well as the configuration properties (from the
<Context> for the
web application) to use in creating objects to be returned.
- Name name - The name to which this factory is bound
relative to
nameCtx, or null if no name
is specified.
- Context nameCtx - The context relative to which the
name parameter is specified, or null if
name is relative to the default initial context.
- Hashtable environment - The (possibly null)
environment that is used in creating this object. This is generally
ignored in Tomcat object factories.
To create a resource factory that knows how to produce MyBean
instances, you might create a class like this:
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package com.mycompany;
import java.util.Enumeration;
import java.util.Hashtable;
import javax.naming.Context;
import javax.naming.Name;
import javax.naming.NamingException;
import javax.naming.RefAddr;
import javax.naming.Reference;
import javax.naming.spi.ObjectFactory;
public class MyBeanFactory implements ObjectFactory {
public Object getObjectInstance(Object obj,
Name name, Context nameCtx, Hashtable environment)
throws NamingException {
// Acquire an instance of our specified bean class
MyBean bean = new MyBean();
// Customize the bean properties from our attributes
Reference ref = (Reference) obj;
Enumeration addrs = ref.getAll();
while (addrs.hasMoreElements()) {
RefAddr addr = (RefAddr) addrs.nextElement();
String name = addr.getType();
String value = (String) addr.getContent();
if (name.equals("foo")) {
bean.setFoo(value);
} else if (name.equals("bar")) {
try {
bean.setBar(Integer.parseInt(value));
} catch (NumberFormatException e) {
throw new NamingException("Invalid 'bar' value " + value);
}
}
}
// Return the customized instance
return (bean);
}
}
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In this example, we are unconditionally creating a new instance of
the com.mycompany.MyBean class, and populating its properties
based on the parameters included in the <ResourceParams>
element that configures this factory (see below). You should note that any
parameter named factory should be skipped - that parameter is
used to specify the name of the factory class itself (in this case,
com.mycompany.MyBeanFactory) rather than a property of the
bean being configured.
For more information about ObjectFactory, see the
JNDI 1.2 Service
Provider Interface (SPI) Specification.
You will need to compile this class against a class path that includes
all of the JAR files in the $CATALINA_HOME/lib directory. When you are through,
place the factory class (and the corresponding bean class) unpacked under
$CATALINA_HOME/lib, or in a JAR file inside
$CATALINA_HOME/lib. In this way, the required class
files are visible to both Catalina internal resources and your web
application.
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>
| | | | |
WARNING - Be sure you respect the element ordering
that is required by the DTD for web application deployment descriptors!
See the
Servlet
Specification for details.
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 elements like this to the
<Context> element for
this web application.
| | | |
<Context ...>
...
<Resource name="bean/MyBeanFactory" auth="Container"
type="com.mycompany.MyBean"
factory="com.mycompany.MyBeanFactory"
bar="23"/>
...
</Context>
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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.
You will also note that, from the application developer's perspective,
the declaration of the resource environment reference, and the programming
used to request new instances, is identical to the approach used for the
Generic JavaBean Resources example. This illustrates one of the
advantages of using JNDI resources to encapsulate functionality - you can
change the underlying implementation without necessarily having to
modify applications using the resources, as long as you maintain
compatible APIs.
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