docs/application-resources.adoc

= Application Resources
:index-group: Unrevised
:jbake-date: 2018-12-05
:jbake-type: page
:jbake-status: published


== Resources

TomEE provides a simple but powerful way to define resources that can be
injected into managed components inside your application, or looked up
via JNDI. To use a resource, it needs to be defined in the `tomee.xml`
configuration file, a `resources.xml` file within an application, or as
a system property. Defining a resource in `tomee.xml` will make it
available server-wide, whereas defining the resource within a
`resources.xml` file makes it available to a specific application.

As a simple example, a JMS queue can be defined within `tomee.xml` with
the following configuration.

[source,xml]
----
<tomee>
    <Resource id="MyQueue" type="javax.jms.Queue"/>
</tomee>
----

Once the resource has been defined, the server will create an instance
of the resource during startup, and it will be available to be injected
into managed components using the `@Resource` annotation, as shown
below. The `name` attribute on the `@Resource` annotation should match
the `id` attribute on the `Resource` tag.

[source,java]
----
public class JmsClient {

    @Resource(name="MyQueue")
    private Queue queue;

    public void sendMessage() {
        // implementation here...
    }

}
----

As an alternative to defining a resource in XML, resources can also be
defined using system properties:

[source,properties]
----
MyQueue = new://Resource?type=javax.jms.Queue
----

Resources, or attributes for resources specified using system properties
will override definitions specified in `tomee.xml`. Server-wide
resources can be looked up in JNDI under the following name:
openejb:Resources/resource id.

== Defining Resources

The `<Resource>` tag has a number of attributes, and a resource may also
have a number of fields that can be configured by adding properties to
the body of the `Resource` tag.

For example, a DataSource resource needs a JDBC driver, URL, username
and password to be able to connect to a database. That would be
configured with the following syntax. Notice the key/value pair syntax
for the properties within the `<Resource>` tag.

[source,xml]
----
<Resource id="DB" type="DataSource">
  JdbcDriver  com.mysql.jdbc.Driver
  JdbcUrl     jdbc:mysql://localhost/test
  UserName    test
  Password    password
</Resource>
----

Specifying the key/value pairs specific to a Resource can also be done
when defining the resource via system properties. This is done be
specifying an additional property for each key/value pair, using the
resource ID as a prefix: `<resourceId>.<propertyName>=<value>`. The
system properties equivalent of the resource above is:

[source,java]
----
p.setProperty("DB", "new://Resource?type=DataSource");
p.setProperty("DB.JdbcDriver", "com.mysql.jdbc.Driver");
p.setProperty("DB,JdbcUrl", "jdbc:mysql://localhost/test");
p.setProperty("DB.UserName", "test");
p.setProperty("DB.Password", "password");
----

The `<Resource>` tag has a number of attributes which control the way
that the resource get created.

* type

A type that TomEE knows. The type is associated with a provider that
knows how to create that type, and also any default properties that the
resource should have if they are not specified in the resource
definition. See service-jar.xml for an example set of service providers
that come with TomEE.

* provider

Explicitly specifies a provider to create the resource, using defaults
for any properties not specified.

* class-name

The fully qualified class that creates the resource. This might the
resource class itself, which is created by calling the constructor, or a
factory class that provides a specific factory method to create the
resource.

* factory-name

The name of the method to call to create the resource. If this is not
specified, the constructor for the class specified by class-name will be
used.

* constructor

Specifies a comma separated list of constructor arguments. These can be
other services, or attributes on the resource itself.

== Custom resources

TomEE allows you to define resources using your own Java classes, and
these can also be injected into managed components in the same way as
known resource types are.

So the following simple resource

[source,java]
----
public class Configuration {

    private String url;
    private String username;
    private int poolSize;

    // getters and setters
}
----

Can be defined in `tomee.xml` using the following configuration (note
the `class-name` attribute):

[source,xml]
----
<Resource id="config" class-name="org.superbiz.Configuration">
    url http://localhost
    username tomee
    poolSize 20
</Resource>
----

This resource must be available in TomEE's system classpath - i.e. it
must be defined in a .jar within the `lib/` directory.

== Field and properties

As shown above, a resource class can define a number of fields, and
TomEE will attempt to apply the values from the resource definition onto
those fields.

As an alternative to this, you can also add a properties field as shown
below, and this will have any used properties from the resource
configuration set added to it. So as an alternative to the above code,
you could do:

[source,java]
----
public class Configuration {

    private Properties properties;
    
    public Properties getProperties() {
        return properties;
    }
    
    public void setProperties(final Properties properties) {
        this.properties = properties;
    }

}
----

Using the same resource definition:

[source,xml]
----
<Resource id="config" class-name="org.superbiz.Configuration">
    url http://localhost
    username tomee
    poolSize 20
</Resource>
----

the url, username and poolSize values will now be available in the
properties field, so for example, the username property could be
accessed via properties.getProperty("username");

== Application resources

Resources can also be defined within an application, and optionally use
classes from the application's classpath. To define resources in a .war
file, include a `WEB-INF/resources.xml`. For an ejb-jar module, use
`META-INF/resources.xml`.

The format of `resources.xml` uses the same `<Resource>` tag as
`tomee.xml`. One key difference is the root element of the XML is
`<resources>` and not `<tomee>`.

[source,xml]
----
<resources>
    <Resource id="config" class-name="org.superbiz.Configuration">
        url http://localhost
        username tomee
        poolSize 20
    </Resource>
</resources>
----

This mechanism allows you to package your custom resources within your
application, alongside your application code, rather than requiring a
.jar file in the `lib/` directory.

Application resources are bound in JNDI under
openejb:Resource/appname/resource id.

== Additional resource properties

Resources are typically discovered, created, and bound to JNDI very
early on in the deployment process, as other components depend on them.
This may lead to problems where the final classpath for the application
has not yet been determined, and therefore TomEE is unable to load your
custom resource.

The following properties can be used to change this behavior.

* Lazy

This is a boolean value, which when true, creates a proxy that defers
the actual instantiation of the resource until the first time it is
looked up from JNDI. This can be useful if the resource's classpath
until the application is started (see below), or to improve startup time
by not fully initializing resources that might not be used.

* UseAppClassLoader

This boolean value forces a lazily instantiated resource to use the
application classloader, instead of the classloader available when the
resources were first processed.

* InitializeAfterDeployment

This boolean setting forces a resource created with the Lazy property to
be instantiated once the application has started, as opposed to waiting
for it to be looked up. Use this flag if you require the resource to be
loaded, irrespective of whether it is injected into a managed component
or manually looked up.

By default, all of these settings are `false`, unless TomEE encounters a
custom application resource that cannot be instantiated until the
application has started. In this case, it will set these three flags to
`true`, unless the `Lazy` flag has been explicitly set.

== Initializing resources

=== constructor

By default, if no factory-name attribute and no constructor attribute is
specified on the `Resource`, TomEE will instantiate the resource using
its no-arg constructor. If you wish to pass constructor arguments,
specify the arguments as a comma separated list:

[source,xml]
----
<Resource id="config" class-name="org.superbiz.Configuration" constructor="id, poolSize">
    url http://localhost
    username tomee
    poolSize 20
</Resource>
----

=== factory-name method

In some circumstances, it may be desirable to add some additional logic
to the creation process, or to use a factory pattern to create
resources. TomEE also provides this facility via the `factory-name`
method. The `factory-name` attribute on the resource can reference any
no argument method that returns an object on the class specified in the
`class-name` attribute.

For example:

[source,java]
----
public class Factory {

    private Properties properties;

    public Object create() {
    
         MyResource resource = new MyResource();
         // some custom logic here, maybe using this.properties
         
         return resource;
    }
    
    public Properties getProperties() {
        return properties;
    }
    
    public void setProperties(final Properties properties) {
        this.properties = properties;
    }

}

<resources>
    <Resource id="MyResource" class-name="org.superbiz.Factory" factory-name="create">
        UserName tomee
    </Resource>
</resources>
----

=== @PostConstruct / @PreDestroy

As an alternative to using a factory method or a constructor, you can
use `@PostConstruct` and `@PreDestroy` methods within your resource class
(note that you cannot use this within a different factory class) to
manage any additional creation or cleanup activities. TomEE will
automatically call these methods when the application is started and
destroyed. Using `@PostConstruct` will effectively force a lazily loaded
resource to be instantiated when the application is starting - in the
same way that the `InitializeAfterDeployment` property does.

[source,java]
----
public class MyClass {

    private Properties properties;
    
    public Properties getProperties() {
        return properties;
    }
    
    public void setProperties(final Properties properties) {
        this.properties = properties;
    }
    
    @PostConstruct
        public void postConstruct() throws MBeanRegistrationException {
            // some custom initialization
        }
    }

}
----

== Examples

The following examples demonstrate including custom resources within
your application:

* resources-jmx-example
* resources-declared-in-webapp