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This tutorial presents some iPOJO features and 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 to create instance(s) for a component not contained in the same bundle
  • How configuring instances
  • How to use the lifecycle controller handler

The sources of this tutorial are available here. This tutorial uses the iPOJO Eclipse plugin.

Context

This tutorial is based on a very simple application; customers are using a vendor service to buy hot dog or pop corn according to the availability of appropriate providers. Both of the vendors implement (and provide) the vendor service. The hot dog vendor depends on two other services to get the ingredients. 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 services are provided by the same component. This component can be implemented as follows:

 
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. Its descriptor 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>

In the descriptor we declare a component type for our component which contains the implementation class. The "classname" attribute contains 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 "factory=false" attribute disables factory exposition. A component type publishing a factory provides a way to create instance of this type from outside this descriptor. In our case, we want to guarantee that only one instance will be created.

IPOJO manages service publication and providing automatically at runtime. The "<provides/>" element means that the component provides services. If this element is not present, iPOJO will publish all implemented interfaces by the implementation class. In our case, it will publish only BunService and WienerService interfaces.

Finally, we create one 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 which provides the BunService and the WienerService.

Publishing a service property

The hot dog vendor only provides the Vendor service. To provide this service, it uses 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 field attributes in the "requires" elements are used to inject the 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 fields the same way it would have used any other fields (as illustrated in the sell method).
This type of metadatas 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 (using the "requires" element). 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: 

<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 then 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 also expose service properties. In our case, these service properties will describe the stock of ingredients. 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;
    }
}

The component type metadata must also be modified in order to 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 each time the fields are modified, the service property is updated (as well as the OSGi™ service registration).

Configuring instances

In the previous example, the properties were configured in the component type description. It is also possible to customize any value in the instance declaration. This way, 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 providers are publishing their 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 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 provider does no more matches with the LDAP filter, the provider is no more used, and another (matching with the filter) is looked for. 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, the "sell" method is called in a constructor, and the constructor can only be called only if an object of the class is created. With iPOJO there are two different way to "activate" an instance as soon as it becomes valid.
The first one uses the lifecycle callback (described 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 as 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 add "immediate=true" in the component descriptor. Then as soon as the vendor service is available, the object is created.

Note: There is a difference between immediate component and component with a 'validate' lifecycle callback. Indeed, the callback is call at each time the instance becomes valid and calls the constructor only if no object already exists. On the other side, the immediate component's constructor is called each time.

Creating instances from an external component type

In the previous section we have declared a customer component type, which does not have the "factory=false" attribute. This feature allows separate deployment from instance creation.
Another metadata file can be used to declare instances from the customer types, this descriptor being 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>

Once deployed, this bundle looks for the required factory. If it's not available the bundle waits for the factory to be created. When this bundle is stopped, all instances are destroyed.

Using the lifecycle controller

Sometimes you want to invalidate your instance in the code (for example: to unregister a service). That's possible with the lifecycle controller handler.
Imagine the popcorn vendor with a corn stock. Each time it sells some popcorns, its stock decreases. When the stock reaches 0, it cannot sell popcorns any more (so the vendor service needs to be withdrawn).

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"; 
    }
}

Once the field is set to "false", the instance is invalidated (the vendor service is no more available). To configure the controller, you can use the 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 by setting the field to true.

Conclusion

This small tutorial has presented some of the main iPOJO features. If you have comments or questions, do not hesitate to send email to: clement.escoffier@gmail.com.

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