GNU Classpath - The Portable Object Adapter package

The Portable Object Adapter (POA) provides more control on the request processing than it is possible when connecting objects directly to the ORB. The POA model defines a tree structure of POAs, the root POA being connected directly to the ORB. Any branch of this tree can be temporary or permanently inactivated using {@link org.omg.PortableServer.POAManager}. The same manager can control several branches in the POA tree. Also, any branch in this tree can have different processing options (policies).

The newly created POA is in holding state, just queuing requests. To start processing requests, it must be turned into the active state by its {@link org.omg.PortableServer.POAManagerOperations#activate}.

The previously monolite object implementation is now divided into object (that implements {@link org.omg.CORBA.Object}) and servant (that implements either {@link org.omg.CORBA.portable.InvokeHandler} or {@link org.omg.PortableServer.DynamicImplementation}). Frequently each object has its own servant, but it can also be a single servant per multiple objects and also default servant for POA (see {@link org.omg.PortableServer.POAOperations#set_servant}). Each object has its own Object Id, unique in the scope of the POA, where the object is connected. These Ids need not be different for objects belonging to different POAs, even if these POAs are connected to the same ORB. Under the USER_ID is assignment policy this Id can be a specified by user in {@link org.omg.PortableServer.POAOperations#activate_object_with_id}, encapsulating some meaningful information about the object. The Id of the object being currently served can be identified with {@link org.omg.PortableServer.Servant#_object_id}. This approach is used in cases when it is possible to encapsulate all object-related data into the Object Id. Such system only needs one servant, one server socket and one socket port per POA that can handle thounsands of objects.

Instead of being connected directly to the ORB, objects are now connected to one of the ORBs POAs. Since JDK 1.4 the application specific implementation base is derived from the {@link org.omg.PortableServer.Servant}, having a different name pattern (`*POA.java` instead of the previous `_*ImplBase.java`). This `*POA` suffix does not mean that these servants implement or are derived from POA. They are different classes that can be connected to one of the POAs, by instance, using {@link org.omg.PortableServer.POAOperations#servant_to_reference}. The implementation base also inherits an *Operations interface, containing definitions of the application specific methods. The application programmer writes a descendent of the implementation base, implementing these methods for the application - specific functionality.

The POA objects support the method invocation by name, using {@link org.omg.CORBA.Request}. This alternative method works without the service-specific classes that may not be available at run time.

The objects in POA can also be activated and inactivated independently. It is possible to set a listener ({@link org.omg.PortableServer.ServantActivator}) that would register the object activations ("incarnations") and deactivations ("etherializations"). The servant need not be specifyed when creating an object. Under the IMPLICIT_ACTIVATION {@link org.omg.PortableServer.ImplicitActivationPolicy} the {@link org.omg.PortableServer.ServantActivator} can provide the servant in response to the first (local or remote) call of any method on the previously incative object.

The root POA is obtained by resolving the initial reference "RootPOA" for the orb. In the simpliest case the objects can be connected directly to that root POA without creating the POA tree. The policies, used by the root POA, are defined by OMG as following:

Policy type | Accepted policy
--- | ---
{@link org.omg.PortableServer.IdAssignmentPolicy} | SYSTEM_ID (Ids are created by POA)
{@link org.omg.PortableServer.IdUniquenessPolicy} | UNIQUE_ID (single object (and Id) per servant)
{@link org.omg.PortableServer.ImplicitActivationPolicy} | IMPLICIT_ACTIVATION (if inactive, activate)
{@link org.omg.PortableServer.LifespanPolicy} | TRANSIENT (the POA objects cannot outlive POA)
{@link org.omg.PortableServer.RequestProcessingPolicy} | USE_ACTIVE_OBJECT_MAP_ONLY (the servant is provided during activation)
{@link org.omg.PortableServer.ServantRetentionPolicy} | RETAIN (retain servants for subsequent invocations)
{@link org.omg.PortableServer.ThreadPolicy} | ORB_CTRL_MODEL (single thread per request and single server socket per object)

These values are also default for the child POAs The policies are never inherited from the parent POA.

This set of policies means that each object will have a separate serving thread, separate network socket port and usually a separate servant. It is appropriate when the expected number of objects is not too large. If the expected number of objects is larger than the supportable number of threads and socket ports, the SINGLE_THREAD_MODEL {@link org.omg.PortableServer.ThreadPolicy} is used. Then all objects in POA with this policy are served in a single thread, using the same server socket, connected to a single port. If the request processing policy is additionally set to USE_DEFAULT_SERVANT, all objects of this POA share the same (default) servant.

The operations, supported by POA are defined separately in {@link org.omg.PortableServer.POAOperations}.

### The typical POA usage scenarios

#### POA converts servant to the object reference

In the simpliest case, the servant implementation is connected to POA by {@link org.omg.PortableServer.POAOperations#servant_to_reference}, the returned object being a target of remote and local invocations. It may be converted into the stringified reference, registered with the naming service, used locally or, when serving or invoking local or remote method, passed as a parameter or return value having the CORBA Object type. The object obtains Id from POA and is activated due default implicit activation policy. This scenario is supported by the default policy set and is used in the most of the "hello world" examples.

#### Servant provides to the object reference

The servant can be connected to an ORB by {@link org.omg.PortableServer.Servant#_this_object(org.omg.CORBA.ORB)}, obtaining the object reference. The overridable {@link org.omg.PortableServer.Servant#_default_POA()} specifies POA to that the servant will be connected. The default method connects to the root poa. IDL compilers frequently generate the `_this(ORB)` metod for servants for getting the object reference that is already narrowed to the exact object type.

#### Explicit activation with POA assigned ids

The objects are activated by calling the {@link org.omg.PortableServer.POAOperations#activate_object} on the POA with the object in question. The POA allocates, assigns, and returns a unique identity value for the object. This scenario requires the SYSTEM_ID {@link org.omg.PortableServer.IdAssignmentPolicy}.

#### Explicit Activation with User-assigned Ids

The POA supports an explicit activation operation, {@link org.omg.PortableServer.POAOperations#activate_object_with_id}, that associates a servant with the user-defined Object Id. This scenario requires the USER_ID {@link org.omg.PortableServer.IdAssignmentPolicy}. The servant manager may be or may not be used.

#### References before activation

It may be useful to create references for objects before activating them. Such reference can be created using {@link org.omg.PortableServer.POAOperations#create_reference} or {@link org.omg.PortableServer.POAOperations#create_reference_with_id}, both methods also requiring to give the object repository id. Such object may be later activated either by {@link org.omg.PortableServer.POAOperations#activate_object_with_id} or automatically, if the IMPLICIT_ACTIVATION policy applies.

#### Multiple Ids per servant

If the MULTIPLE_ID policy applies, the servant may be activated many times. Under this policy, {@link org.omg.PortableServer.POAOperations#servant_to_reference} and {@link org.omg.PortableServer.POAOperations#servant_to_id} during each call create a new object and object reference for the used servant.

#### One servant for all objects

If the USE_DEFAULT_SERVANT policy applies, that default servant serves all objects, belonging this POA. This approach is used when there is very little data associated with each object, so little that the data can be encoded in the Object Id. Also, it may be needed when a very large number of objects is expected. If the RETAIN applies, it is possible to activate an object explicitly setting the servant other than default. If NO_RETAIN applies, the default servant will serve all known an unknown objects for that POA.

#### Single Servant, Many Objects and Types

Combining USER_ID, USE_DEFAULT_SERVANT and RETAIN, it is possible to create and serve objects "on the fly". The servant must determine the object type (for instance, from the value of the agreed attribute, shared by all supported types, or from the Object Id) and be able to handle the method, named in request. If the names and parameter lists of the object methods are also created "on the fly", the requests to such object can still be submitted using {@link org.omg.CORBA.Request}. This method is used when the created object represents some entity in the complex database.

#### The ServantLocator finds a servant for each call

The {@link org.omg.PortableServer.ServantLocator} is used by POAs that combinine NON_RETAIN and USE_SERVANT_MANAGER policies. It provides a new or reused servant every time the invocation is made. The servant locator must provide a servant in response of calling {@link org.omg.PortableServer.ServantLocatorOperations#preinvoke}. This method has access the the Id of the object being served and the name of the method being called. It must return the appropriate instance of the servant or throw an exception, forwarding the request to another object (usually in another server). After the invocation, a {@link org.omg.PortableServer.ServantLocatorOperations#postinvoke} is called. It should be not assumed that the call of `preinvoke` will be followed by the call of the `postinvoke`; in multithreaded environment these calls are not serialized in this way. If the `preinvoke` has to tell something this-call-specific to the `postinvoke`, it must use the provided cookie holder. The `preinvoke/postinoke` are also called to provide a servant during each local invocation on the objects, belonging to the described POA.

All these scenarios must work with the current GNU Classpath release. @author Audrius Meskauskas, Lithuania (AudriusA@Bioinformatics.org)