[CONF] Apache Felix: iPOJO Advanced Tutorial (page edited)

confluence Mon, 27 Aug 2007 07:36:41 -0700

Page Edited : FELIX : iPOJO Advanced Tutorial

iPOJO Advanced Tutorial has been edited by Clement Escoffier (Aug 27, 2007).

(View changes)

Content:

This tutorial presented some iPOJO features. It shows:

How a component can provide two services
How to attach a service property to provided service
How a service property can be dynamically updated by component code
How to configure instances
How a service dependency can filter providers
How creating instance for a component not contained in the same bundle
How configuring instances
How to use the lifecycle controller handler

The sources of this tutorial is available here. This tutorial used the iPOJO Eclipse plugin.

Context

This tutorial uses a very simple application. Customers use a Vendor service to buy hot dog or pop corn according to the availability of these two providers. Both the pop corn vendor and the hot dog vendor implement (and provide) the vendor service. The hot dog vendor depends on two other services for getting ingredient. It depends on a bun service and a wiener service.

Writing a component providing two services

The hot dog vendor requires at the same time the bun service and the wiener service. In our application these two services are provided by one component. This component can be implemented as following:

public class BunWienerProvider implements BunService, WienerService {
    
    public void getBun() {
        System.out.println("Get a bun");
    }


    public void getWiener() {
        System.out.println("Get a wiener");
    }
}

This class just implements the two service interfaces. Then, the descriptor of this component is:

<ipojo>
<component classname="org.apache.felix.ipojo.example.vendor.provider.BunWienerProvider" name="buns_and_wieners" factory="false">
	<provides/>
</component>

<instance component="buns_and_wieners"/>
</ipojo>

First, we declare a component type for our component. The component type contains the implementation class. The attribute "classname" contains simply the qualified name of the component implementation. The "name" attribute is the component type name. It is only used to refer to this type.
The attribute "factory=false" allows avoiding factory exposition. A component type publishing a factory provides a way to create instance of this type from outside this descriptor. In our case, when want to guaranty that only one instance will be created.

The "<provides/>" element means that the component provide services. IPOJO will manage automatically service publication and providing at runtime. If not describes, iPOJO publish all implemented interfaces by the implementation class. In our case, it will publish the BunService and WienerService interfaces.
Finally, we create on instance of our component. The instance contains the component attribute describing the component type to use. We use the component type name to target the wanted component type.

At runtime, the bundle containing this component will create an instance. This instance provides the BunService and the WienerService.

Publishing a service property

The hot dog vendor just provides the Vendor service. To provide this service, it used a bun service and a wiener service. The following code snippet shows a very simple implementation of this component:

public class HotDogVendor implements VendorService {
    
    private BunService bunProvider;
    private WienerService wienerProvider;
    
    public String getName() {
        return "The Best Hot Dogs";
    }

    public String sell() {
        bunProvider.getBun();
        wienerProvider.getWiener();
        return "sell an hotdog";
    }
}

The two fields are used to inject the two required services. At runtime, iPOJO injects automatically a BunService provider in the "bunProvider" field and a WienerService provider in the "wienerProvider" field. The implementation uses these field as other field (as illustrated in the sell method).
The metadata of this type are very simple:

<ipojo>
<component classname="org.apache.felix.ipojo.example.vendor.hotdog.HotDogVendor" name="HD" factory="false">
	<provides/>
	<requires field="bunProvider"/>
	<requires field="wienerProvider"/>
</component>

<instance component="HD"/>
</ipojo>

The component type declares a provided service (the Vendor Service). Then, the component declares the two service dependencies. However, we would like to add a service property on the Vendor service describing the sale product (here hotdog). To achieve this, we just need to add a property element in the "provides" tags as following:

<ipojo>
<component classname="org.apache.felix.ipojo.example.vendor.hotdog.HotDogVendor" name="HD" factory="false">
	<provides>
		<property name="product" type="string" value="hotdog"/>
	</provides>
	<requires field="bunProvider"/>
	<requires field="wienerProvider"/>
</component>

<instance component="HD"/>
</ipojo>

IPOJO publishes the "product" property in the "vendor" service registration. This property has the "hotdog" value.

Publishing a 'dynamic' property

The bun service and the wiener service can expose service properties too. These service properties describe the stock of the ingredient. At each time the service is used, the property is decreased.

To achieve this, we modify the current implementation to add a field representing the property:

public class BunProvider implements BunService, WienerService {
    
    private int bunStock;
    
    private int wienerStock;

    public void getBun() {
        bunStock = bunStock - 1;
    }

    public void getWiener() {
        wienerStock = wienerStock - 1;
    }
}

Then the component type metadata must describe this property :

<ipojo>
<component classname="org.apache.felix.ipojo.example.vendor.provider.BunProvider" name="buns_and_wieners" factory="false">
	<provides>
		<property name="buns" field="bunStock" value="10"/>
		<property name="wieners" field="wienerStock" va	lue="10"/>
	</provides>
</component>

<instance component="buns_and_wieners"/>
</ipojo>

In the "provides" element, two properties are added. This property contains a "field" attribute aiming to attach the service property with a field of the implementation class. Then a default value is given. In the code, the property fields will obtain the initial value (10). Then at each time the fields are modified, the service property is updated (and the OSGi™ service registration is updated).

Configuring instances

In the previous example, the properties were configured in the component type description. It is possible to customized value in the instance declaration. Then, each instance can obtain different values.

<ipojo>
<component classname="org.apache.felix.ipojo.example.vendor.provider.BunProvider" name="buns_and_wieners" factory="false">
	<provides>
		<property name="buns" field="bunStock" value="10"/>
		<property name="wieners" field="wienerStock" va	lue="10"/>
	</provides>
</component>

<instance component="buns_and_wieners">
	<property name="buns" value="9"/>
	<property name="wieners" value="8"/>
</instance>
</ipojo>

The previous metadata shows how to push a configuration in instance declarations. The instance declaration contains two property elements containing the name of the value of the property. Instance configuration override component type initial value. If a property does not have an initial value, the instance must provide a value for this unvalued property.

Using filter in service requirement

Now that bun and wiener provider publishes its remaining stock, the hot dog provider can look for a bun service and a wiener service with a non empty stock. To achieve this, we simply must describe an LDAP filter in the service dependency description. The following xml snipped shows this metadata:

<ipojo>
<component classname="org.apache.felix.ipojo.example.vendor.hotdog.HotDogVendor" name="HD" factory="false">
	<provides>
		<property name="product" type="string" value="hotdog"/>
	</provides>
	<requires field="bunProvider" filter="(buns>=1)"/>
	<requires field="wienerProvider" filter="(wieners>=1)"/>
</component>

<instance component="HD"/>
</ipojo>

When a used provider does no more matches with the LDAP filter, the provider is no more used, and another (matching with the filter) is looked. If no provider fulfilling the constraint is found, the instance becomes invalid and waits a matching provider.

Note: when an instance becomes invalid, provided services are withdraw from the service registry.

Immediate component instance

Now that we get the hot dog provider, we are going to implement customers. A customer simply looks for a vendor service and buys a product.

public class Customer {
    
    private VendorService vendor;
    
    private String name;
    
    public Customer() {
        System.out.println("Customer " + name + " bought " +  vendor.sell() + " from " + vendor.getName());
    }

The previous code shows a possible implementation of a customer. However, sell method is called in a constructor. The constructor is called only if an object of the class is created. With iPOJO there is two different way to "activate" an instance as soon it becomes valid. The first one use lifecycle callback (describe in the previous tutorial). The second one is by declaring the component as an immediate component. An immediate component instance creates an object of its implementation as soon it becomes valid.

<ipojo>
<component classname="org.apache.felix.ipojo.example.vendor.customer.Customer" factory="customer" immediate="true">
	<requires field="vendor"/>
	<properties>
		<property field="name"/>
	</properties>
</component>
<instance component="customer">
	<property name="name" value="my_customer"/>
</instance>
</ipojo>

To declare a component immediate, just write "immediate=true". Then as soon as the vendor service is available, it creates the object.

Note: There is a difference beetween immedaite componenet and component with a 'validate' lifecycle callback. Indeed, the callback is call at each time the instance becomes valid. IT call the constructor only if no object already exist. On the other side, the immediate component called the constructor only once time.

Creating instances from an external component type

In the previous section we have declared a customer component type. This component type does not have the "factory=false" attribute. This allows another descriptor to use this type. This feature allows deploying separately implementation from instance creation.
So another metadata file can declare instances from the customer types. This descriptor is contained in another bundle. The following descriptor creates 10 customer instances:

<ipojo>
<instance component="customer">
	<property name="name" value="customer-1"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-2"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-3"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-4"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-5"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-6"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-7"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-8"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-9"/>
</instance>
<instance component="customer">
	<property name="name" value="customer-10"/>
</instance>
</ipojo>

When deployed this bundle looks for the required factory. If not available it waits for it apparition. When this bundle is stopped, all instances are destroyed.

Using the lifecycle controller

Sometimes you want to invalidate your instance from your code (to unregister a service...). That's possible with the lifecycle controller handler.
Imagine the popcorn vendor with a corn stock. At each time he sells popcorn, its stock decreases. When the stock reach 0, it can no more sell popcorn (so, the vendor service need to be withdrawed).
The following implementation uses a field to control the lifecycle.

public class PopCornVendor implements VendorService {
    
    private int m_corn_stock = 5;
    private boolean m_can_sell = true;
    
    public String getName() {
        return "Eat my pop corn !"; 
    }

    public String sell() {
        m_corn_stock = m_corn_stock - 1;
        if (m_corn_stock == 0 && m_can_sell) {
            m_can_sell = false;
        }
        return "popcorn"; 
    }
}

When the field is set to false, the instance is invalidated (the vendor service is no more available). To configure the controller, just use following metadata :

<ipojo>
<component classname="org.apache.felix.ipojo.example.vendor.popcorn.PopCornVendor" name="popcorn" factory="false" architecture="true">
	<provides/>
	<controller field="m_can_sell"/>
</component>

<instance component="popcorn"/>
</ipojo>

The instance can be re-validated if the field is set to true.

Conclusion

This small tutorial has presented some iPOJO features. If you have comments or questions, do not hesitate to send me an email to [[EMAIL PROTECTED]].

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