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Application Resources

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.

<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.

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:

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.

<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:

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

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):

<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:

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:

<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>.

<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:

<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:

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.

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