Contents

Extensible Runtime Containment and Services Protocol for JavaBeans Version 1.0

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1.0 Introduction.

Currently the JavaBeans specification (Version 1.0) contains neither conventions describing a hierarchy or logical structure of JavaBeans, nor conventions for those JavaBeans to rendezvous with, or obtain arbitrary services or facilities from, the execution environment within which the JavaBean was instantiated.

It is desirable to both provide a logical, traversable, hierarchy of JavaBeans, and further to provide a general mechanism whereby an object instantiating an arbitrary JavaBean can offer that JavaBean a variety of services, or interpose itself between the underlying system service and the JavaBean, in a conventional fashion.

In other component models there exists the concept of a relationship between a Component and its environment, or Container, wherein a newly instantiated Component is provided with a reference to its Container or Embedding Context.

The Container, or Embedding Context not only establishes the hierarchy or logical structure, but its also acts as a service provider that Components may interrogate in order to determine, and subsequently employ, the services provided by their Context.

This proposal defines such a protocol that supports extensible mechanisms that:

  • Introduce an abstraction for the environment, or context, in which a JavaBean logically functions during its lifecycle, that is a hierarchy or structure of JavaBeans.
  • Enable the dynamic addition of arbitrary services to a JavaBean’s environment.
  • Provide a single service discovery mechanism through which JavaBeans may interrogate their environment in order both to ascertain the availability of particular services and to subsequently employ those services.
  • Provide a simple mechanism to propagate an Environment to a JavaBean.
  • Provide better support for JavaBeans that are also Applets.
graphic representation of previous list points

2.0 API Specification

2.1 interface java.beans.beancontext.BeanContext

The hierarchal structure and general facilities of a BeanContext are provided for as follows:

public interface                java.beans.beancontext.BeanContext 
           extends              java.beans.beancontext.BeanContextChild, 
                      java.util.Collection, 
                         java.beans.DesignMode, 
                         java.beans.Visibility { 
 
        Object instantiateChild(String beanName) 
                  throws IOException, ClassNotFoundException; 
 
        public InputStream 
                getResourceAsStream(String           name, 
                                    BeanContextChild requestor 
                ); 
 
        public java.net.URL 
                getResource(String           name, 
                            BeanContextChild requestor 
                ); 
 
        void addBeanContextMembershipListener( 
                   BeanContextMembershipListener bcml 
          ); 
 
        void removeBeanContextMembershipListener{ 
                    BeanContextMembershipListener bcml 
        ); 
 
        public static final Object globalHierarchyLock; 
} 

Notifications of changes in the membership of a BeanContext are modeled as follows:

public interface BeanContextMembershipListener 
          extends   java.util.Listener { 
        void childrenAdded(BeanContextMembershipEvent   bcme); 
        void childrenRemoved(BeanContextMembershipEvent bcme); 
} 
 

The base class of all BeanContext related Events is defined by:

public abstract class BeanContextEvent 
          extends  java.util.EventObject { 
        public BeanContext getBeanContext(); 
 
        public synchronized void 
                setPropagatedFrom(BeanContext bc); 
 
        public synchronized BeanContext getPropagatedFrom(); 
 
        public synchronized boolean isPropagated() 
} 
 

The BeanContextMembershipEvent is defined as:

public class BeanContextMembershipEvent 
          extends BeanContextEvent { 
 
        public BeanContextMembershipEvent(BeanContext bc, 
                                          Object[]    deltas); 
 
        public BeanContextMembershipEvent(BeanContext bc, 
                                          Collection     deltas); 
 
        public int size(); 
 
        public boolean contains(Object child); 
 
        public Object[] toArray(); 
 
        public Iterator iterator(); 
} 

2.1.1 The BeanContext as a participant in nested structure

One of the roles of the BeanContext is to introduce the notion of a hierarchical nesting or structure of BeanContext and JavaBean instances. In order to model this structure the BeanContext must expose API that defines the relationships in the structure or hierarchy.

The BeanContext exposes its superstructure through implementation of the java.beans.beancontext.BeanContextChild interface (as described later). This interface allows the discovery and manipulation of a BeanContext’s nesting BeanContext, and thus introduces a facility to create a hierarchy of BeanContexts.

The BeanContext exposes its substructure through a number of interface methods modeled by the java.util.Collection interface semantics

BeanContexts are required to implement all the mandatory Collection API’s, with the following particular semantics for add() and remove():.

The add() method may be invoked in order to nest a new Object, BeanContextChild, or BeanContext within the target BeanContext. A conformant add() implementation is required to adhere to the following semantics:

  • Synchronize on the BeanContext.globalHierarchyLock.
  • Each child object shall appear only once in the set of children for a given BeanContext. If the instance is already a member of the BeanContext then the method shall return False.
  • Each valid child shall be added to the set of children of a given source BeanContext, and thus shall appear in the set of children, obtained through either the toArray(), or iterator() methods, until such time as that child is deleted from the nesting BeanContext via an invocation of remove(), removeAll(), retainAll(), or clear()
  • As the child is added to the set of nested children, and where that child implements the java.beans.beancontext.BeanContextChild interface (or BeanContextProxy, see later for details), the BeanContext shall invoke the setBeanContext() method upon that child, with a reference to itself. Upon invocation, a child may, if it is for some reason unable or unprepared to function in that BeanContext, throw a PropertyVetoException to notify the nesting BeanContext. If the child throws such an exception the BeanContext shall revoke the addition of the child (and any other partial changes made to the state of the BeanContext as a side effect of this addition operation) to the set of nested children and throw an IllegalStateException.
  • JavaBeans that implement the java.beans.Visibility interface shall be notified via the appropriate method, either dontUseGui() or okToUseGui(), of their current ability to render GUI as defined the policy of the BeanContext.
  • If the newly added child implements BeanContextChild, the BeanContext shall register itself with the child on both its VetoableChangeListener and PropertyChangeListener interfaces to monitor, at least, that BeanContextChild’s “beanContext” property.

By doing so the BeanContext can monitor its child and can detect when such children are removed by a 3rd party (usually another BeanContext) invoking setBeanContext(). A BeanContext may veto such a change by a 3rd party if it determines that the child is not in a state to depart membership at that time.

  • If the JavaBean(s) added implement Listener interfaces that the BeanContext is a source for, then the BeanContext may register the newly added objects via the appropriate Listener registration methods as a permissible side effect of nesting.
  • If the JavaBean(s) added are Event Sources for Event that a particular BeanContext has interest in the BeanContext may, as a side effect of adding the child, register Listeners on that child. The BeanContext should avoid using Serializable Listeners thus avoiding accidental serialization of unwanted structure when a child serializes itself.
  • Once the targetChild has been successfully processed, the BeanContext shall fire a java.beans.beancontext.BeanContextMembershipEvent, containing a reference to the newly added targetChild, to the childrenAdded() method of all the BeanContextMemebrshipListeners currently registered.
  • The method shall return true if successful.

The remove() method may be invoked in order to remove an existing child JavaBean or BeanContext from within the target BeanContext. A conformant remove() implementation is required to adhere to the following semantics:

  • Synchronize with the BeanContext.globalHierarchyLock.
  • If a particular child is not present in the set of children for the source BeanContext, the method shall return False.
  • Remove the valid targetChild from the set of children for the source BeanContext, also removing that child from any other Listener interfaces that it was implicitly registered on, for that BeanContext as a side-effect of nesting.

Subsequently, if the targetChild implements the java.beans.beancontext.BeanContextChild interface (or BeanContextProxy, see later for deatils), the BeanContext shall invoke the setBeanContext() with a null1 BeanContext value, in order to notify that child that it is no longer nested within the BeanContext.

If a particular BeanContextChild is in a state where it is not able to be un-nested from its nesting BeanContext it may throw a PropertyVetoException, upon receipt of this the BeanContext shall revoke the remove operation for this instance and throw IllegalStateException. To avoid infinite recursion, children are not permitted to repeatedly veto subsequent remove notifications. In practice, a child should attempt to resolve the condition (if temporary) that precludes it’s removal from it’s current nesting BeanContext.

  • If the targetChild implements java.beans.beancontext.BeanContextChild then the BeanContext shall de-register itself from that child’s PropertyChangeListener and VetoableChangeListener sources.
  • If the BeanContext had previously registered the object(s) removed as Listeners on events sources implemented by the BeanContext as a side effect of nesting those objects, then the BeanContext shall de-register the newly removed object from the applicable source(s) via the appropriate Listener de-registration method(s)
  • If the BeanContext had previously registered Listener(s) on the object(s) removed then the BeanContext shall remove those Listener(s) from those object(s).
  • Once the targetChild has been removed from the set of children, the BeanContext shall fire a java.beans.beancontext.BeanContextMembershipEvent, containing a reference to the targetChild just removed, to the childrenRemoved() method of all the BeanContextMembershipListeners currently registered.
  • Finally the method shall return the value true if successful.

Note that the lifetime of any child of a nesting BeanContext, is at least for the duration of that child’s containment within a given BeanContext. For simple JavaBeans that are not aware of their containment within a BeanContext, this usually implies that the JavaBean shall exist for at least the lifetime of the nesting BeanContext.

BeanContext’s are not required to implement either addAll(), removeAll(), retainAll() or clear() optional methods defined by java.util.Collection API, however if they do they must implement the semantics defined, per object, for both add() and remove() above. In the failure cases these methods shall revoke any partially applied changes to return the BeanContext to the state it was in prior to the failing composite operation being invoked, no BeanContextEvents shall be fired in the failure case as is consistent with the definition of add() and remove() above.

BeanContextMembershipListeners may be added and removed via invocations of addBeanContextMembershipListener() and removeBeanContextMembershipListener().

The toArray(), method shall return a copy of the current set of JavaBean or BeanContext instances nested within the target BeanContext, and the iterator() method shall supply a java.util.Iterator object over the same set of children.

The contains() method shall return true if the object specified is currently a child of the BeanContext.

The size() method returns the current number of children nested.

The isEmpty() method returns true if the BeanContext has no children.

Note that all the Collection methods all require proper synchronization between each other by a given implementation in order to function correctly in a multi-threaded environment, that is, to ensure that any changes to the membership of the set of JavaBeans nested within a given BeanContext are applied atomically. All implementations are required to synchronized their implementations of these methods with the BeanContext.globalHierarchyLock.

In some situations, add() and remove() (or a variant thereof) operations may occur nested, that is multiple occurrences may appear on the stack of the calling Thread simultaneously, e.g: when BeanContextChild, A, is added (or removed), it’s setBeanContext() method also adds (or removes) another BeanContextChild, B. A particular BeanContext implementation may choose to fire either two BeanContextMembershipListener notifications, one for each add()/remove() operation of B then A (in this order since B is successfully added before A), or coalesce these into a single notification containing both A, and B. Note that should A be unable to be added or removed for any reason it shall not perform, or undo, any add or remove operations upon B as a side-effect, prior to throwing a PropertyVetoException to indicate this condition, that is, it must avoid or undo any side-effect membership changes prior to rejecting any changes to its own membership status.

The instantiateChild() method is a convenience method that may be invoked to instantiate a new JavaBean instance as a child of the target BeanContext. The implementation of the JavaBean is derived from the value of the beanName actual parameter, and is defined by the java.beans.Beans.instantiate() method.

Typically, this shall be implemented by calling the appropriate java.beans.Beans.instantiate() method, using the ClassLoader of the target BeanContext. However a particular BeanContext implementation may interpose side-effects on the instantiate operation in their implementation of this method.

The BeanContextEvent is the abstract root EventObject class for all Events pertaining to changes in state of a BeanContext’s defined semantics. This abstract root class defines the BeanContext that is the source of the notification, and also introduces a mechanism to allow the propagation of BeanContextEvent subclasses through a hierarchy of BeanContexts. The setPropagatedFrom() and getPropagatedFrom() methods allows a BeanContext to identify itself as the source of a propagated Event to the BeanContext to which it subsequently propagates the Event to. This is a general propagation mechanism and should be used with care as it has significant performance implications when propagated through large hierarchies.

The BeanContextMembershipEvent describes changes that occur in the membership of a particular BeanContext instance. This event encapsulates the list of children either added to, or removed from, the membership of a particular BeanContext instance, i.e the delta in the membership set.

Whenever a successful add(), remove(), addAll(), retainAll(), removeAll(), or clear() is invoked upon a particular BeanContext instance, a BeanContextMembershipEvent is fired describing the children effected by the operation.

2.1.2 Resources.

The BeanContext defines two methods; getResourceAsStream() and getResource() which are analogous to those methods found on java.lang.ClassLoader. BeanContextChild instances nested within a BeanContext shall invoke the methods on their nesting BeanContext in preference for those on ClassLoader, to allow a BeanContext implementation to augment the semantics by interposing behavior between the child and the underlying ClassLoader semantics.

2.1.3 The BeanContext as a Service Provider

The service facilities of a BeanContext are provided as follows:

public interface BeanContextServices 
          extends   BeanContext,BeanContextServicesListener { 
 
        boolean addService(Class           serviceClass,  
                           BeanContextServiceProvider service); 
 
        boolean revokeService(Class           serviceClass, 
                              BeanContextServiceProvider bcsp, 
                              boolean         revokeNow 
        ); 
 
        boolean hasService(Class serviceClass); 
 
        Object getService(BeanContextChild      bcc, 
                          Object                requestor. 
                          Class                 serviceClass, 
                          Object                serviceSelector, 
                          BeanContextServicesRevokedListener sl 
        ) throws TooManyListenersException; 
 
        void releaseService(BeanContextChild bcc, 
                            Object           requestor, 
                            Object           service); 
 
        Iterator getCurrentServiceClasses(); 
 
        public Iterator getCurrentServiceSelectors(Class sc); 
 
        addBeanContextServicesListener( 
                BeanContextServicesListener bcsl 
        ); 
 
        removeBeanContextServicesListener( 
                BeanContextServicesListener bcsl 
     ); 
} 

The BeanContextServiceProvider interface is defined as follows:

public interface BeanContextServiceProvider { 
        Object getService(BeanContext bc, 
                          Object      requestor, 
                          Class       serviceCls, 
                          Object      serviceSelector); 
 
        void releaseService(BeanContext bc, 
                            Object      requestor, 
                            Object      service); 
 
        Iterator getCurrentServiceSelectors(BeanContext bc, 
                                            Class serviceCls); 
} 
 
The BeanContextServiceRevokedListener is defined as follows: 
 
public interface BeanContextServiceRevokedListener 
          extends java.util.EventListener { 
        void serviceRevoked( 
                BeanContextServiceRevokedEvent bcsre 
        ); 
} 

The BeanContextServicesListener is defined as follows:

public interface BeanContextServicesListener 
          extends BeanContextServiceRevokedListener { 
        void serviceAvailable( 
                BeanContextServiceAvailableEvent bcsae 
        ); 
} 

The BeanContextServiceAvailableEvent is defined as follows:

public class BeanContextServiceAvailableEvent  
          extends BeanContextEvent { 
 
        public BeanContextServiceAvailableEvent( 
                        BeanContextServices        bcs, 
                        Class                      sc 
     ); 
 
        BeanContextServices getSourceAsBeanContextServices(); 
 
        public Class getServiceClass(); 
 
        public boolean isServiceClass(Class serviceClass); 
 
        public Iterator getCurrentServiceSelectors(); 
} 

The BeanContextServiceRevokedEvent is defined as follows:

public class BeanContextServiceRevokedEvent  
          extends BeanContextEvent { 
        public BeanContextServiceRevokedEvent( 
                                BeanContextServices        bcs, 
                                Class                      sc, 
                             boolean                    invalidNow 
        ); 
        public BeanContextServices 
                        getSourceAsBeanContextServices(); 
 
        public Class getServiceClass(); 
 
        public boolean isServiceClass(Class service); 
 
        public boolean isCurrentServiceInvalidNow(); 
} 
 

The BeanContextServiceProviderBeanInfo is defined as follows:

public interface BeanContextServicesProviderBeanInfo 
       extends   java.beans.BeanInfo { 
    java.beans.BeanInfo[] getServicesBeanInfo(); 
} 
 

Apart from providing a structured hierarchy, the other major role of a BeanContext is to provide a standard mechanism for a JavaBean component to obtain context-specific facilities or services from its environment.

A service, represented by a Class object, is typically a reference to either an interface, or to an implementation that is not publicly instantiable. This Class defines an interface protocol or contract between a BeanContextServiceProvider, the factory of the service, and an arbitrary object associated with a BeanContextChild that is currently nested within the BeanContext the service is registered with. Typically such protocols encapsulate some context specific or sensitive behavior that isolates a BeanContextChild’s implementation from such dependencies thus resulting in simpler implementations, greater interoperability and portability.

A BeanContextServiceProvider, is a “factory” for one or more services. It registers itself with a particular BeanContextServices via it’s adService() method, if the service is not already registered with the BeanContextServices, the BeanContextServices associates the service specified with the BeanContextServiceProvider, and fires a BeanContextServiceAvailableEvent via the serviceAvailable() method to those BeanContextServicesListeners currently registered, then returns true, otherwise false indicating that the service is already registered for that BeanContextServices.

Once registered, and until revoked, the service is available via the BeanContextServices getService() method.

The hasService() method may be used to test the presence of a particular service, and the getCurrentServices() method returns an iterator over the currently available services for that BeanContextServices.

A BeanContextChild or any arbitrary object associated with a BeanContextChild, may obtain a reference to a currently registered service from its nesting BeanContextServices via an invocation of the getService() method. The getService() method specifies; the BeanContextChild, the associated requestor, the Class of the service requested, a service dependent parameter (known as a Service Selector), and a BeanContextServicesRevokedListener used to subsequently notify the requestor that the service class has been revoked by the BeanContextServiceProvider. The Listener is registered automatically with a unicast event source per requestor and service class and is automatically unregistered when a requestor relinquishes all references of a given service class, or as a side effect of the service being “forcibly revoked” by the providing BeanContextServiceProvider.

The BeanContextServices passes this getService() invocation onto the associated BeanContextServiceProvider (if any) to be satisfied via an invocation of its getService() method. The BeanContextServiceProvider is passed the BeanContext, the Class of the service provided, the service dependent service parameter (the Service Selector) and a reference to the object requesting the service.

The reference to the BeanContext is intended to enable the BeanContextServiceProvider to distinguish service requests from multiple sources. A BeanContextServiceProvider is only permitted to retain a weak reference to any BeanContext so obtained.

The Service Selector parameter is a service dependent value used by a service requestor for a particular service in order to parameterize the service to be provided to it by the BeanContextServiceProvider. Some examples of its usage are; a parameter to a constructor for the service implementation class; a value for a particular service’s property, or as a key into a map of existing implementations.

The reference to the requestor is intended to permit the BeanContextServiceProvider to interrogate the state of the requestor in order to perform any customization or parameterization of the service, therefore this reference shall be treated as immutable by the BeanContextServicesProvider. Additionally the BeanContextServiceProvider is permitted to retain only weak and immutable reference to both the requestor and the BeanContextChild after returning from the getService() invocation.

The BeanContextServiceProvider may satisfy the request, returning a reference to an instance of the Class of the requested service (such that the reference returned shall result in the expression: <serviceRefence> instanceof <serviceClass> being true), return null, or throw an unchecked exception.

In the case when a nested BeanContextServices is requested for a particular service that it has no BeanContextServiceProvider for, then the BeanContextServices may delegate the service requested to its own nesting BeanContextServices in order to be satisfied. Thus delegation requests can propagate from the leaf BeanContextServices to the root BeanContextServices.

A BeanContextChild may query a particular BeanContextServices for a list of currently available Service Classes (via the getCurrentServiceClasses() method)and any associated Service Selectors, if a particular service Class implements a finite list of apriori values for a Service Class, via its nesting BeanContextServices.getCurrentServiceSelectors() method, which in turn obtains the currently available Service Selectors (if any) via the BeanContextServiceProvider.getCurrentServiceSelectors() method.

If the service in question does not implement a finite set of apriori values for the set of valid Service Selectors it shall return null.

A reference obtained by a BeanContextChild via getService() is valid until the reference is released by the BeanContextChild via an invocation of its nesting BeanContextServices releaseService() method, except in the case where the BeanContextServices fires a BeanContextServiceRevokedEvent and that Event’s isCurrentServiceInvalidNow() method returns true, in this case the BeanContextServices and/or the BeanContextServiceProvider that provided the service has determined that current service references are immediately invalidated, or “forcibly revoked” (this typically occurs in the following situation).

When BeanContextChild instances are removed from a particular BeanContextServices instance, they shall discard all references to any services they obtained from that BeanContextServices by appropriate invocations of releaseService(). If the un-nesting BeanContextChild is also a BeanContextServices instance, and if any of these service references have been exposed to the un-nesting BeanContextServices’s own members as a result of a delegated getService() request as defined above, the BeanContextServiecs shall fire a BeanContextServiceRevokedEvent to notify its nested children that the service(s) are “forcibly revoked”. This immediate invalidation of current references to delegated services at un-nesting is to ensure that services that are dependent upon the structure of the hierarchy are not used by requestors after their location in the structure has changed.

BeanContextChild instances receiving a “forcable revocation” of a Service Class shall not invoke releaseService() for any references it may hold of that type, since in this case, the BeanContextServiceProvider or the BeanContextServices that provided the service reference to that BeanContextChild has already invalidated all references to that service on their behalf.

A BeanContextServiceProvider may revoke a Service Class at any time after it has registered it with a BeanContextServices by invoking its revokeService() method. Once the BeanContextServices has fired a BeanContextServiceRevokedEvent notifying the currently registered BeanContextServiceRevokedListeners and the BeanContextServicesListeners that the service is now unavailable it shall no longer satisfy any new service requests for the revoked service until (if at all) that Service Class is re-registered. References obtained by BeanContextChild requestors to a service prior to its being revoked remain valid, and therefore the service shall remain valid to satisfy those extant references, until all references to that service are released, unless in exceptional circumstances the BeanContextServiceProvider, or BeanContextServices, when revoking the service, wants to immediately terminate service to all the current references. This immediate revocation is achieved by invoking the BeanContextServices .revokeService() method with an actual parameter value of revokeNow == true. Subsequent to immediate invalidation of current service references the service implementation may throw a service specific unchecked exception in response to any attempts to continue to use the revoked service by service requestors that have erroneously retained references to the service, ignoring the earlier immediate revocation notification.

Note that in order to function correctly (when delegating service requests) in a multi-threaded environment, implementations of BeanContextServices are required to synchronize their implementations of; addService(), hasService(), getCurrentServiceClasses(), getCurrentServiceSelectors(), getService(), releaseService() and revokeService() with the BeanContext.globalHierarhyLock.

A BeanContextServicesProvider may expose the BeanInfo for the Service Classes it provides implementations for by providing a BeanInfo class that implements BeanContextServicesProviderBeanInfo. Thus exposing an array of BeanInfo’s, one for each Service Class supported. Builder tools can, for example, use this infomation to provide application developers with a palette of Servlice Classes for inclusion in an application.

2.1.4 The role of a BeanContext in Persistence

Since one of the primary roles of a BeanContext is to represent a logical nested structure of JavaBean component and BeanContext instance hierarchies, it is natural to expect that in many scenarios that hierarchy should be persistent, i.e that the BeanContext should participate in persistence mechanisms, in particular, either java.io.Serializable or java.io.Externalizable (If the latter the BeanContext is responsible for acting as the persistence container for the sub-graph of children, encoding and decoding the class information, and maintaining sub-graph equivalence after deserialization, basically the function(s) provide for serialization by ObjectOutputStream and ObjectInputStream).

In particular BeanContexts shall persist and restore their current children that implement the appropriate persistence interfaces when they themselves are made persistent or subsequently restored.

As a result of the above requirement, persistent BeanContextChild implementations are required to not persist any references to either their nesting BeanContext, or to any Delegates obtained via its nesting BeanContextServices.

BeanContexts shall, when restoring an instance of BeanContextChild from its persistence state, be required to perform the equivalent of invoking add()on the newly instantiated BeanContextChild,, in order to notify the newly restored instance of its nesting BeanContext, thus allowing that BeanContextChild to fully reestablish its dependencies on its environment.

Also note that since BeanContext implements java.beans.beancontext.BeanContextChild it shall obey the persistence requirements defined below for implementors of that interface.

2.1.5 BeanContext with associated presentation hierarchies

Although not required, many BeanContexts may be associated within a presentation hierarchy of java.awt.Containers and java.awt.Components. A Container cannot implement BeanContext directly2 but may be associated with one by implementing the BeanContextProxy interface described herein.

 
public interface BeanContextProxy { 
    BeanContext getBeanContext(); 
} 

For instances of classes that do not (or cannot in the case of Component or subclasses thereof) directly implement the BeanContext interface, but are associated with an instance of such an implementation, (via delegation) such instances may expose this association by implementing the BeanContextProxy interface. By doing so, this enables arbitrary 3rd parties, such as builder tools, to interrogate and discover the BeanContext associated with such objects for the purposes of either nesting objects within that associated BeanContext, observing changes in the membership, or obtaining services thereof.

This also permits multiple distinct objects (e.g: Containers) to share a single BeanContext. [Note though that in this case a shared BeanContext shall not implement BeanContextContainerProxy since that is a peer-to-peer relationship between a single BeanContext and the Container implementing that interface]

The value returned from getBeanContext() is constant for the lifetime of the implementing instance, that is the relationship between a BeanContextProxy and it’s associated BeanContext is static and thus may not change for the lifetime of either participant.

No class may implement both the BeanContext (or BeanContextChild) and the BeanContextProxy interfaces, they are mutually exclusive.

Some BeanContextProxy implementors may also implement java.util.Collection, or some other collection-like API (e.g java.awt.Container), in addition to, and possibly distinct from, maintaining a BeanContext based Collection.

In such cases it is possible to add, or remove, elements from either the BeanContext, via it’s Collection API’s, or the BeanContextProxy implementor using it’s own collection-like API’s (e.g: public boolean java.awt.Container.add(Component)). It is implementation dependent whether or not objects added or removed from either the BeanContext’s Collection, or the BeanContextProxy implementor’s collection are also added or removed from the corresponding object’s collection (i.e: should a Container.add() also infer a BeanContext.add() and vica-versa?). In such situations both participants (the implementor of BeanContextProxy and the BeanContext itself) are required to; 1) implement the same add/remove semantics as the other (i.e: if x.add(o) has a side effect of x.getBeanContext().add(o) then x.getBeanContext().add(o) should also a have side effect of x.add(o)), and 2) before adding/removing an object to/from the other participants collection, it should test (synchronized) if that object is/is not a member of the other participants collection before proceeding with the operation in question (this is to avoid infinite recursion between collection operations on both participants) (i.e: x.add(o) should not invoke x.getBeanContext().add(o) if x.getBeanContext().contains(o) is true and vica-versa).

It is important to note that if an object that implements BeanContextProxy is added to , or removed from, a BeanContext, that in addition to the operation performed on that object, the same operation should be performed on the BeanContext returned from BeanContextProxy.getBeanContext(). That is an implementor of BeanContextProxy shall be treated as though it directly implemented BeanContext by any nesting BeanContext. (and vica-versa if the operation is applied to the BeanContext its shall also be applied to the corresponding BeanContextProxy)

The following interface is defined to allow a BeanContext to expose a reference to an associated Container to enable it’s BeanContextChild members to add, or remove, their associated Component objects to/from that Container or to inspect some state on the Container.

public interface BeanContextContainerProxy { 
        Container getContainer() 
} 

When a BeanContextChild with an associated Component is added to a BeanContext with an associated Container there are three models of interaction that can occur in relation to the nesting of the Component in the Container as a result:

  • If the associated Component was added to the associated Container via a Container API, then the nesting of the BeanContextChild in the BeanContext is a side effect of that and no further action is required.
  • If the Component and Container are not nested then the nesting BeanContext may as a side effect cause the Component associated with the BeanContextChild to be added to it’s associated Container.

OR

  • If the Component and Container are not nested then the BeanContextChild being nested may as a side effect may cause it’s Component to be associated with the Container associated with the nesting BeanContext.

Thus, for greatest interoperability a BeanContextChild shall always check if its Compoent’s parent is the BeanContext Container, and if it is not, then it may add itself if appropriate. Thus a BeanContextChild may function correctly under all scenarios.

The BeanContextChild is responsible for initially causing itself to eligible to be displayed via an invocation of show() [note that the BeanContextChild may also subsequently repeatedly hide() and show() itself].

The nesting BeanContext, or its associated Container, may subsequently hide() or show() the BeanContextChild’s Component arbitrarily, but it is strongly recommended that it treat that Component as immutable in all other respects with the exception of registering Listeners to obtain event notifications, or where other Component/Container specific protocols permit or require the Container to alter the state of its Component containees. An example of such a permitted interaction would be where a property such as background or foreground color were propagated from Container to Component.

Once a BeanContextChild has been un-nested from it’s BeanContext, it’s associated Component (if any) shall be removed from that BeanContext’s Container as a side effect of the removal operation, this is the responsibility of the BeanContext (typically if the BeanContextChild has been moved to another BeanContext with an associated Container via an invocation of it’s setBeanContext() method, the Component will already have been re-parented as a side effect of that operation by the time the original BeanContext is notified of the change via a PropertyChangeEvent from the child, however the check should be made and the Component removed if it has not already occurred).

To avoid infinite recursion, both a BeanContext and a BeanContextChild that also are associated with a Container and Component nesting relationship should avoid undoing any changes applied to the Component by the other party in the relationship. In general the BeanContext is responsible for the appearance, visibility and relative layout of the BeanContextChild’s Component, and the BeanContextChild is responsible for the Component’s state and content pertaining to the application functionality it is implementing.

The value returned from the getContainer() method is constant for the lifetime of the implementing BeanContext, that is the relationship between a BeanContext and a Container is static for the lifetime of both participants.

In addition the following interface is also defined:

public interface BeanContextChildComponentProxy { 
    Component getComponent(); 
} 

A BeanContext or a BeanContextChild may implement this interface to expose the GUI Component that it is associated with to it’s nesting BeanContext. A BeanContext may use this method to establish the relationship between references to instances of Component and BeanContextChild that are known to it, where a BeanContextChild and Component are not implemented by the same object instance (that is the BeanContextChild delegates its Component implementation to a distinct object rather than inheriting from Component]. A BeanContext may interrogate the Component reference it obtains from a nested BeanContextChild in order to determine its state, and it may also register Listeners for particular events, however it is strongly recommended that the BeanContext treat the reference as generally immutable to avoid changing the Component state.

The value returned from the getComponent() method is a constant for the lifetime of that BeanContextChild.

In the situation where a BeanContext has an associated Container, but does not wish to expose that Container by implementing the BeanContextContainerProxy interface, but wishes to handle the nesting of an arbitrary BeanContextChild’s associated Component (exposed by the BeanContextChild either implementing the BeanContextChildComponentProxy interface or as direct subclass of Component) the BeanContext is permitted to add/remove that Component to/from its associated Container.In such cases the BeanContextChild and it’s associated Component implementation shall not interfere with this action.

If a class implements both BeanContextChildComponentProxy and BeanContextContainerProxy then the object returned by both getComponent() and getContainer() shall be the same object.

2.2 interface java.beans.beancontext.BeanContextChild3

Simple JavaBeans that do not require any support or knowledge of their environment shall continue to function as they do today. However both JavaBeans that wish to utilize their containing BeanContext, and BeanContexts that may be nested, require to implement a mechanism that enables the propagation of the reference to the enclosing BeanContext through to cognizant JavaBeans and nested BeanContexts, the interface proposed is:

 
public interface java.beans.beancontext.BeanContextChild { 
        void        setBeanContext(BeanContext bc) 
                                throws PropertyVetoException; 
 
        BeanContext getBeanContext(); 
 
        void addPropertyChangeListener 
                (String name, PropertyChangeListener pcl); 
  
     void removePropertyChangeListener 
                (String name, PropertyChangeListener pcl); 
  
        void addVetoableChangeListener 
                (String name, VetoableChangeListener pcl); 
  
     void removeVetoableChangeListener 
                (String name, VetoableChangeListener pcl); 
 
} 

The expected usage is that some 3rd party shall invoke one of the appropriate methods defined on BeanContext (by virtue of its inheritance from Collection) in order to add a BeanContextChild to the membership of the target BeanContext. As a consequence the BeanContext shall attempt to set the BeanContextChild’s “beanContext” property by invoking its setter method, setBeanContext(). Only a BeanContext may call a BeanContextChild’s setBeanContext() method, since this is the mechanism that a BeanContext uses to notify a child that it is now has a new BeanContext value. Since this property is not directly settable or customizable by a user in the context of an application construction tool the BeanInfo for a BeanContextChild should set the hidden state for this property in order for builder tools to avoid presenting the property to the user for customization.

A BeanContextChild object may throw a PropertyVetoException, to notify the nesting BeanContext that it is unable to function/be nested within that particular BeanContext. Such a veto shall be interpreted by a BeanContext as an indication that the BeanContextChild has determined that it is unable to function in that particular BeanContext and is final.

During the un-nesting of a BeanContextChild from its BeanContext, it is possible for the child, or a 3rd party listening to the child’s “beanContext” property for PropertyVetoEvents, to throw a PropertyVetoException to notify the caller that it is not in a state to be un-nested. In order to bound this interaction a BeanContextChild, or 3rd party, may veto the initial un-nesting notification, but may not veto any subsequent notifications, and must, upon receipt of such notifications, amend its state accordingly to prepare itself to be subsequently un-nested.

Note that classes that implement this interface, also act as an Event Source for (sub)interface(s) of java.beans.PropertyChangeListener, and are required to update their state accordingly and subsequently fire the appropriate java.beans.PropertyChangeEvent with propertyName = “beanContext”, oldValue = the reference to the previous nesting BeanContext, and newValue = the reference to the new nesting BeanContext, to notify any Listeners that its nesting BeanContext has changed value.

BeanContextChild instances, or nested BeanContexts in the process of terminating themselves, shall invoke the remove() method on their nesting BeanContext in order to withdraw themselves from the hierarchy prior to termination.

2.2.1 Important Persistence considerations

Instances of BeanContextChild nested within an BeanContext, will typically define fields or instance variables that will contain references to their nesting BeanContext instance, and possibly services obtained from that BeanContextServices instance via its getService() method.

In order to ensure that the act of making such an instance persistent does not erroneously persist objects from the instances nesting environment, such instances shall be required to define such fields, or instance variables as either transient, or to implement custom persistence methods that avoid persisting such state.

This requirement is crucial since operations such as cutting and pasting object instances through a clipboard via object serialization will not function correctly if the act of serializing the target object also serializes much of the entire source environment it is nested within.

3.0 Overloading java.beans.instantiate() static method

Since java.beans.instantiate() is the current mechanism for (re)instantiating JavaBeans we need to extend or overload the syntax and semantics of this method in order to accommodate the introduction of the BeanContext abstraction. The extension proposed is:

public static Object instantiate(ClassLoader cl, 
                                                String        beanName, 
                                                BeanContext beanContext); 

This method behaves has it is currently defined in the JavaBeans specification, but in addition to these existing semantics, when a non-null BeanContext is specified then the method invokes the add() method on the beanContext actual parameter with the value of the targetChild actual parameter = a reference to the newly instantiated JavaBean component.4

4.0 Providing better support for Beans that are also Applets

The current implementation of java.beans.instantiate() contains minimal support for instantiating JavaBeans that are also Applets. In particular, this method will currently construct an AppletContext and AppletStub for the newly instantiated JavaBean, set the stub on the newly instantiated Applet, and init() the Applet if it has not already been invoked.

Unfortunately this does not provide sufficient support in order to allow most Applets to be fully functional, since the AppletContext and AppletStub created by java.beans.instantiate(), are no-ops. This is a direct consequence of the lack of sufficient specification of how to construct AppletContext and AppletStub implementations in the existing Applet API’s. Furthermore, even if such specifications existed we would require an API that propagated a number of Applet attributes such as its Codebase, Parameters, AppletContext, and Documentbase into java.beans.instantiate() in order for it to subsequently instantiate the appropriately initialized objects.

Since key to supporting fully functional Applets is to provide them with fully functional AppletContext and AppletStub instances, the design goal is to provide a mechanism to provide this state to instantiate() so that it may carry out the appropriate initialization and binding5, therefore the proposed interface is:

public static Object  
                        instantiate(ClassLoader                                 cl, 
                                          String                beanName, 
                                          BeanContext           bCtxt, 
                                          AppletInitializer                                                                                                                     ai 
                        ); 
 
public interface AppletInitializer { 
        void initialize(Applet newApplet, BeanContext bCtxt); 
        void activate(Applet newApplet); 
} 

If the newly instantiated JavaBean component is an instance of java.applet.Applet then the new constructed Applet, (Bean) will be passed to the AppletInitializer via a call to initialize().

Compliant implementations of AppletInitializer.initialize() shall:

  • Associate the newly instantiated Applet with the appropriate AppletContext.
  • Instantiate an AppletStub() and associate that AppletStub with the Applet via
  • an invocation of setStub().
  • If BeanContext parameter is null, then it shall associate the Applet with its appropriate Container by adding that Applet to its Container via an invocation of add(). If the BeanContext parameter is non-null, then it is the responsibility of the BeanContext to associate the Applet with its Container during the subsequent invocation of its addChildren() method.

Compliant implementations of AppletInitializer.activate() shall mark the Applet as active, and may optionally also invoke the Applet’s start() method.

Note that if the newly instantiated JavaBean is not an instance of Applet, then the AppletInitializer interface is ignored.

5.0 Standard/Suggested Conventions for BeanContext Services

5.0.1 BeanContexts that support InfoBus.

The InfoBus technology is a standard extension package that is intended to facilitate the rendezvous and exchange of dynamic self describing data, based upon a publish and subscribe abstraction, between JavaBean Components within a single Java Virtual Machine.

A BeanContext that exposes an InfoBus to its nested BeanContextChild shall do so by exposing a service via the hasService() and getService() methods of type javax.infobus.InfoBus.

Thus BeanContextChild implementations may locate a common InfoBus implementation for their current BeanContext by using this mechanism to rendezvous with that InfoBus instance.

The Infobus 1.2 specification shall define a convenience mechanism provided by the InfoBus class to simplify the discovery mechanism for BeanContextChild instances nested within a particular instance of BeanContextServices.

5.0.2 BeanContexts that support printing

A BeanContext that wishes to expose printing facilities to its descendants may delegate a reference of (sub)type java.awt.PrintJob.

As the Java Network Printing Interface evolves additional specifications will be provided to expose it’s facilities via the services mechanism.

5.0.3 BeanContext Design/Runtime mode support.

JavaBeans support the concepts of “design”-mode, when JavaBeans are being manipulated and composed by a developer in an Application Builder or IDE, and “Run”-mode, when the resulting JavaBeans are instantiated at runtime as part of an Applet, Application or some other executable abstraction.

In the first version of the specification, the “mode” or state, that is “design”-time or “run”-time was a JVM global attribute. This is insufficient since, for example, in an Application Builder environment, there may be JavaBeans that function, in “run”-mode, as part of the Application Builder environment itself, as well as the JavaBeans that function, in “design”-mode, under construction by the developer using the Application Builder to compose an application.

Therefore we require the ability to scope this “mode” at a granularity below that of JVM global.

The BeanContext abstraction, as a “Container” or “Context” for one or more JavaBeans provides appropriate mechanism to better scope this “mode”.

Thus BeanContext’s that wish to expose and propagate this “mode” to its descendants may delegate a reference of type java.beans.DesignMode:

public interface java.beans.DesignMode { 
        void    setDesignTime(boolean isDesignTime); 
        boolean isDesignTime(); 
} 

Additionally, BeanContexts delegating such a reference shall be required to fire the appropriate java.beans.propertyChangeEvent, with propertyName = “designTime”, with the appropriate values for oldValue and newValue, when the “mode” changes value.

Note that it is illegal for instances of BeanContextChild to call setDesignTime() on instances of BeanContext that they are nested within.

5.0.4 BeanContext Visibility support.

JavaBeans with associated presentation, or GUI, may be instantiated in environments where the ability to present that GUI is either not physically possible (when the hardware is not present), or is not appropriate under the current conditions (running in a server context instead of a client).

The first version of the JavaBeans Specification introduced the java.beans.Visibility interface in order to provide a mechanism for JavaBeans to have their “visible” state, or ability to render a GUI, controlled from their environment.

BeanContexts that wish to enforce a particular policy regarding the ability of their children to present GUI, shall use the java.beans.Visibility interface to control their children.

5.0.5 Determining Locale from a BeanContext

BeanContexts may have a locale associated with them, in order to associate and propagate this important attribute across the JavaBeans nested therein.

Therefore, BeanContexts, shall be required to fire the appropriate java.beans.PropertyChangeEvent, with propertyName = “locale”, oldValue = the reference to the previous value of the Locale delegate, and newValue = the reference to the new value of the Locale delegate, in order to notify its Listeners of any change in Locale.

Setting and getting the value of the Locale on the BeanContext is implementation dependent.

6.0 Support classes

In order to ease the implementation of this relatively complex protocol a “helper” classes are provided; java.beans.beancontext.BeanContextChildSupport, java.beans.beancontext.BeanContextSupport, and java.beans.beancontext.BeanContextServicesSupport. These classes are designed to either be subclassed, or delegated implicitly by another object, and provides fully compliant (extensible) implementations of the protocols embodied herein.

1Note, if the remove() was invoked as a result of the BeanContext receiving an unexpected PropertyChan- geEvent notification as a result of a 3rd party invoking setBeanContext() then the remove implementation shall not invoke setBeanContext(null) on that child as part of the remove() semantics, since to do so would overwrite the value previously set by the 3rd party.
2Unfortunately because of method name collisions between Component and Collection a Component can- not implement BeanContext or Collection directly and must model the capability with a “HasA” rather than an “IsA” relationship.
3I don’t like this name much but I am struggling for a better alternative! (we are stuck with it)
4Note: Since simple JavaBeans have no knowledge of a BeanContext, it is not advisable to introduce such instances into the hierarchy since there is no mechanism for these simple JavaBeans to remove them- selves from the hierarchy and thus subsequently be garbage collected.
5AppletContext objects expose a list of Applet objects they “contain”, unfortunately the current Applet or AppletStub API’s as defined, provide no mechanism for the AppletContext to discover its Applets from its AppletStubs, or for an AppletStub to inform its AppletContext of its Applet. Therefore we will have to assume that this binding/discovery can occur in order for this mechanism to be worthwhile in java.beans.instantiate().

 


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