Package com.lightstreamer.interfaces.data

Interface ItemEventListener

public interface ItemEventListener
Used by Lightstreamer Kernel to receive the ItemEvents and other kinds of events related with the Item subscription lifecycle. It can also receive asynchronous severe error notifications from the Data Adapter. The listener instance is supplied to the Data Adapter by Lightstreamer Kernel through a setListener call. The listener can manage multiple kinds of ItemEvents: ItemEvent objects, OldItemEvent objects, IndexedItemEvent objects and java.util.Map objects. The common characteristics of all these kinds of ItemEvent objects are that:
  • they contain the new values and, in some cases, the current values of the Fields of an Item; the Item name is not directly asked to the object.
  • they provide an iterator that supplies the names of all the Fields reported in the ItemEvent; the iterator may not be provided if the related Item has been subscribed by setting the needsIterator flag as false.
  • they provide a method for getting the value of a Field by name; the value can be expressed as either a String or a byte array.
When an ItemEvent, of whichever kind, has been sent to the listener, the object instance is totally owned by Lightstreamer Kernel, which may hold it for some time after the listener call has returned. For this reason, the event object should not be used anymore by the Data Adapter. Only provided that the event object is thread safe, the object can still be accessed in a read-only way. In particular, if exactly the same event contents were to be passed multiple times to the listener (for the same or different items), then passing the same event object instance repeatedly would be allowed. However, specific update methods may pose further restrictions.
The same restriction holds for any underlying object referenced to by the ItemEvent object. However, immutable objects are obviously unaffected; in particular, the latter holds for any objects used to contain values, if they are of String type; but also any byte array objects used to contain values are unaffected and can be reused if they are no longer modified.
If ItemEvents are implemented as wrappers of the data objects received from the external feed (like JMS Messages), all the above has to be carefully considered.
Similar considerations hold for the Map objects that can be used to provide Field "diff" support information (see declareFieldDiffOrder(java.lang.String, java.util.Map<java.lang.String, com.lightstreamer.interfaces.data.DiffAlgorithm[]>) and smartDeclareFieldDiffOrder(java.lang.Object, java.util.Map<java.lang.String, com.lightstreamer.interfaces.data.DiffAlgorithm[]>)) and their contents.
See Also:

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    clearSnapshot(String itemName)
    Called by a Data Adapter to signal to Lightstreamer Kernel that the current Snapshot of an Item has suddenly become empty.
    void
    declareFieldDiffOrder(String itemName, Map<String,DiffAlgorithm[]> algorithmsMap)
    Called by a Data Adapter to send to Lightstreamer kernel Field "diff" support information in relation to a specific subscribed Item.
    void
    endOfSnapshot(String itemName)
    Called by a Data Adapter to signal to Lightstreamer Kernel that no more ItemEvent belonging to the Snapshot are expected for an Item.
    void
    failure(Throwable e)
    Called by a Data Adapter to notify Lightstreamer Kernel of the occurrence of a severe problem that can compromise future operation of the Data Adapter.
    void
    smartClearSnapshot(Object itemHandle)
    Can be called, instead of clearSnapshot, by a Data Adapter that implements the extended interface SmartDataProvider, in all cases in which clearSnapshot can be called.
    void
    smartDeclareFieldDiffOrder(Object itemHandle, Map<String,DiffAlgorithm[]> algorithmsMap)
    Can be called, instead of declareFieldDiffOrder, by a Data Adapter that implements the extended interface SmartDataProvider, in all cases in which declareFieldDiffOrder can be called.
    void
    smartEndOfSnapshot(Object itemHandle)
    Can be called, instead of endOfSnapshot, by a Data Adapter that implements the extended interface SmartDataProvider, in all cases in which endOfSnapshot can be called.
    void
    smartUpdate(Object itemHandle, IndexedItemEvent event, boolean isSnapshot)
    Can be called by a Data Adapter that implements the extended interface SmartDataProvider to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an IndexedItemEvent instance.
    void
    smartUpdate(Object itemHandle, ItemEvent event, boolean isSnapshot)
    Can be called by a Data Adapter that implements the extended interface SmartDataProvider to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an ItemEvent instance.
    void
    smartUpdate(Object itemHandle, OldItemEvent event, boolean isSnapshot)
    Can be called by a Data Adapter that implements the extended interface SmartDataProvider to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an OldItemEvent instance.
    void
    smartUpdate(Object itemHandle, Map event, boolean isSnapshot)
    Can be called by a Data Adapter that implements the extended interface SmartDataProvider to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as a java.util.Map instance.
    void
    update(String itemName, IndexedItemEvent event, boolean isSnapshot)
    Called by a Data Adapter to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an IndexedItemEvent instance.
    void
    update(String itemName, ItemEvent event, boolean isSnapshot)
    Called by a Data Adapter to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an ItemEvent instance.
    void
    update(String itemName, OldItemEvent event, boolean isSnapshot)
    Called by a Data Adapter to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an OldItemEvent instance.
    void
    update(String itemName, Map event, boolean isSnapshot)
    Called by a Data Adapter to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as a java.util.Map instance.

  • Method Details

    • update

      void update(@Nonnull String itemName, @Nonnull ItemEvent event, boolean isSnapshot)
      Called by a Data Adapter to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an ItemEvent instance.
      The Adapter should ensure that, after an unsubscribe call for the Item has returned, no more update calls are issued, until requested by a new subscription for the same Item. This assures that, upon a new subscription for the Item, no trailing events due to the previous subscription can be received by the Kernel. Note that the method is nonblocking; moreover, it only takes locks to first order mutexes; so, it can safely be called while holding a custom lock.
      Parameters:
      itemName - The name of the Item whose values are carried by the ItemEvent.
      event - An ItemEvent instance.
      isSnapshot - true if the ItemEvent carries the Item Snapshot.
      See Also:

    • update

      void update(@Nonnull String itemName, @Nonnull OldItemEvent event, boolean isSnapshot)
      Called by a Data Adapter to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an OldItemEvent instance.
      The Adapter should ensure that, after an unsubscribe call for the Item has returned, no more update calls are issued, until requested by a new subscription for the same Item. This assures that, upon a new subscription for the Item, no trailing events due to the previous subscription can be received by the Kernel. Note that the method is nonblocking; moreover, it only takes locks to first order mutexes; so, it can safely be called while holding a custom lock.
      Parameters:
      itemName - The name of the Item whose values are carried by the ItemEvent.
      event - An OldItemEvent instance.
      isSnapshot - true if the ItemEvent carries the Item Snapshot.
      See Also:

    • update

      void update(@Nonnull String itemName, @Nonnull Map event, boolean isSnapshot)
      Called by a Data Adapter to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as a java.util.Map instance.
      The Adapter should ensure that, after an unsubscribe call for the Item has returned, no more update calls are issued, until requested by a new subscription for the same Item. This assures that, upon a new subscription for the Item, no trailing events due to the previous subscription can be received by the Kernel. Note that the method is nonblocking; moreover, it only takes locks to first order mutexes; so, it can safely be called while holding a custom lock.
      If the event object is of type java.util.HashMap, then Lightstreamer Kernel, in order to improve performances, may synchronize on the object lock and modify the object contents while processing it. As a consequence, in this case, the event object, once passed to the listener, cannot be accessed anymore by the Data Adapter, even for reading (it can still be passed multiple times to the listener, if needed).
      Parameters:
      itemName - The name of the Item whose values are carried by the ItemEvent.
      event - A java.util.Map instance, in which Field names are associated to Field values. A Field value can be null or missing if the Field is not to be reported in the event.
      Each value can be expressed as either a String or a byte array; see ItemEvent.getValue(String) for details.
      isSnapshot - true if the ItemEvent carries the Item Snapshot.
      See Also:

    • update

      void update(@Nonnull String itemName, @Nonnull IndexedItemEvent event, boolean isSnapshot)
      Called by a Data Adapter to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an IndexedItemEvent instance.
      The Adapter should ensure that, after an unsubscribe call for the Item has returned, no more update calls are issued, until requested by a new subscription for the same Item. This assures that, upon a new subscription for the Item, no trailing events due to the previous subscription can be received by the Kernel. Note that the method is nonblocking; moreover, it only takes locks to first order mutexes; so, it can safely be called while holding a custom lock.
      Parameters:
      itemName - The name of the Item whose values are carried by the ItemEvent.
      event - An IndexedItemEvent instance.
      isSnapshot - true if the ItemEvent carries the Item Snapshot.
      See Also:

    • smartUpdate

      void smartUpdate(@Nonnull Object itemHandle, @Nonnull ItemEvent event, boolean isSnapshot)
      Can be called by a Data Adapter that implements the extended interface SmartDataProvider to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an ItemEvent instance.
      This method should not be called while holding custom locks. After that an unsubscribe call for the Item has returned, further calls of this method for the itemHandler received with the last subscription operation are still allowed and will be just discarded.
      Parameters:
      itemHandle - Identifies the Item whose values are carried by the ItemEvent. It must be the same object received in the subscribe call.
      event - An ItemEvent instance.
      isSnapshot - true if the ItemEvent carries the Item Snapshot.
      See Also:

    • smartUpdate

      void smartUpdate(@Nonnull Object itemHandle, @Nonnull OldItemEvent event, boolean isSnapshot)
      Can be called by a Data Adapter that implements the extended interface SmartDataProvider to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an OldItemEvent instance.
      This method should not be called while holding custom locks. After that an unsubscribe call for the Item has returned, further calls of this method for the itemHandler received with the last subscription operation are still allowed and will be just discarded.
      Parameters:
      itemHandle - Identifies the Item whose values are carried by the ItemEvent. It must be the same object received in the subscribe call.
      event - An OldItemEvent instance.
      isSnapshot - true if the ItemEvent carries the Item Snapshot.
      See Also:

    • smartUpdate

      void smartUpdate(@Nonnull Object itemHandle, @Nonnull Map event, boolean isSnapshot)
      Can be called by a Data Adapter that implements the extended interface SmartDataProvider to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as a java.util.Map instance.
      This method should not be called while holding custom locks. After that an unsubscribe call for the Item has returned, further calls of this method for the itemHandler received with the last subscription operation are still allowed and will be just discarded.
      If the event object is of type java.util.HashMap, then Lightstreamer Kernel, in order to improve performances, may synchronize on the object lock and modify the object contents while processing it. As a consequence, in this case, the event object, once passed to the listener, cannot be accessed anymore by the Data Adapter, even for reading (it can still be passed multiple times to the listener, if needed).
      Parameters:
      itemHandle - Identifies the Item whose values are carried by the ItemEvent. It must be the same object received in the subscribe call.
      event - A java.util.Map instance, in which Field names are associated to Field values. A Field value can be null or missing if the Field is not to be reported in the event.
      Each value can be expressed as either a String or a byte array; see ItemEvent.getValue(String) for details.
      isSnapshot - true if the ItemEvent carries the Item Snapshot.
      See Also:

    • smartUpdate

      void smartUpdate(@Nonnull Object itemHandle, @Nonnull IndexedItemEvent event, boolean isSnapshot)
      Can be called by a Data Adapter that implements the extended interface SmartDataProvider to send an ItemEvent to Lightstreamer Kernel when the ItemEvent is implemented as an IndexedItemEvent instance.
      This method should not be called while holding custom locks. After that an unsubscribe call for the Item has returned, further calls of this method for the itemHandler received with the last subscription operation are still allowed and will be just discarded.
      Parameters:
      itemHandle - Identifies the Item whose values are carried by the ItemEvent. It must be the same object received in the subscribe call.
      event - An IndexedItemEvent instance.
      isSnapshot - true if the ItemEvent carries the Item Snapshot.
      See Also:

    • endOfSnapshot

      void endOfSnapshot(@Nonnull String itemName)
      Called by a Data Adapter to signal to Lightstreamer Kernel that no more ItemEvent belonging to the Snapshot are expected for an Item. This call is optional, because the Snapshot completion can also be inferred from the isSnapshot flag in the update and smartUpdate calls. However, this call allows Lightstreamer Kernel to be informed of the Snapshot completion before the arrival of the first non-Snapshot event. Calling this function is recommended if the Item is to be subscribed in DISTINCT mode. In case the Data Adapter returned false to isSnapshotAvailable for the same Item, this function should not be called.
      The Adapter should ensure that, after an unsubscribe call for the Item has returned, a possible pending endOfSnapshot call related with the previous subscription request is no longer issued. This assures that, upon a new subscription for the Item, no trailing events due to the previous subscription can be received by the Kernel. Note that the method is nonblocking; moreover, it only takes locks to first order mutexes; so, it can safely be called while holding a custom lock.
      Parameters:
      itemName - The name of the Item whose Snapshot has been completed.
      See Also:

    • smartEndOfSnapshot

      void smartEndOfSnapshot(@Nonnull Object itemHandle)
      Can be called, instead of endOfSnapshot, by a Data Adapter that implements the extended interface SmartDataProvider, in all cases in which endOfSnapshot can be called.
      This method should not be called while holding custom locks. After that an unsubscribe call for the Item has returned, further calls of this method for the itemHandler received with the last subscription operation are still allowed and will be just discarded.
      Parameters:
      itemHandle - Identifies an Item whose Snapshot has been completed.
      See Also:

    • clearSnapshot

      void clearSnapshot(@Nonnull String itemName)
      Called by a Data Adapter to signal to Lightstreamer Kernel that the current Snapshot of an Item has suddenly become empty. More precisely:
      • for subscriptions in MERGE mode, the current state of the Item will be cleared, as though an update with all fields valued as null were issued;
      • for subscriptions in COMMAND mode, the current state of the Item will be cleared, as though a DELETE event for each key were issued;
      • for subscriptions in DISTINCT mode, a suitable notification that the Snapshot for the Item should be cleared will be sent to all the clients currently subscribed to the Item (clients based on some old client library versions may not be notified); at the same time, the current recent update history kept by the Server for the Item will be cleared and this will affect the Snapshot for new subscriptions;
      • for subscriptions in RAW mode, there will be no effect.
      Note that this is a real-time event, not a Snapshot event; hence, in order to issue this call, it is not needed that the Data Adapter has returned true to DataProvider.isSnapshotAvailable(java.lang.String) for the specified Item; moreover, if invoked while the Snapshot is being supplied, the Kernel will infer that the Snapshot has been completed.
      The Adapter should ensure that, after an unsubscribe call for the Item has returned, a possible pending clearSnapshot call related with the previous subscription request is no longer issued. This assures that, upon a new subscription for the Item, no trailing events due to the previous subscription can be received by the Kernel. Note that the method is nonblocking; moreover, it only takes locks to first order mutexes; so, it can safely be called while holding a custom lock.
      Parameters:
      itemName - The name of the Item whose Snapshot has become empty.
      See Also:

    • smartClearSnapshot

      void smartClearSnapshot(@Nonnull Object itemHandle)
      Can be called, instead of clearSnapshot, by a Data Adapter that implements the extended interface SmartDataProvider, in all cases in which clearSnapshot can be called.
      This method should not be called while holding custom locks. After that an unsubscribe call for the Item has returned, further calls of this method for the itemHandler received with the last subscription operation are still allowed and will be just discarded.
      Parameters:
      itemHandle - Identifies an Item whose Snapshot has become empty.
      See Also:

    • declareFieldDiffOrder

      void declareFieldDiffOrder(@Nonnull String itemName, @Nonnull Map<String,DiffAlgorithm[]> algorithmsMap)
      Called by a Data Adapter to send to Lightstreamer kernel Field "diff" support information in relation to a specific subscribed Item. By default, the Server can decide whether or not to try to apply an available algorithm to compute the difference between a value and the previous one in order to send the client this difference, for "delta delivery" purpose. The applicability of an algorithm depends on the capability of the client, the suitability of the values, and the throughput efficiency achieved. With this method, it is possible to enforce which algorithms to try and in which order, on a field-by-field basis. The available algorithms are specified in the DiffAlgorithm enumerative class.
      The declared "diff" order is a static property of fields, hence it cannot change throughout all the duration of the subscription. For this reason, this method should be invoked before any update is sent. However, since the available Fields may not be fully predetermined and new Fields may be added at any time, it is possible to invoke this method multiple times, interspersed with update events, with a cumulative behavior.
      As a consequence, any attempt to declare the "diff" order of a Field for which it has been already specified will be ignored. Moreover, if a Field is used in an update before sending any "diff" order information, it will stick to the Server default, and any subsequent specification of a "diff" order for that field will be ignored.
      By specifying "diff" algorithms consistent with the expected values, a significant outbound bandwidth reduction can be achieved. On the other hand, if unsuitable values are sent (for instance, strings that are not valid JSON representations for a Field with the DiffAlgorithm.JSONPATCH algorithm specified), a significant computational overhead with no useful effect may be added.
      Parameters:
      itemName - The name of the Item whose "diff" order information is carried by this notification.
      algorithmsMap - A java.util.Map instance, in which Field names are associated to ordered arrays of "diff" algorithms. Omitted fields or null arrays will add no information. On the other hand, an empty array can be supplied to mean that no "diff" algorithm is admitted for a field.

    • smartDeclareFieldDiffOrder

      void smartDeclareFieldDiffOrder(@Nonnull Object itemHandle, @Nonnull Map<String,DiffAlgorithm[]> algorithmsMap)
      Can be called, instead of declareFieldDiffOrder, by a Data Adapter that implements the extended interface SmartDataProvider, in all cases in which declareFieldDiffOrder can be called.
      This method should not be called while holding custom locks. After that an unsubscribe call for the Item has returned, further calls of this method for the itemHandler received with the last subscription operation are still allowed and will be just discarded.
      Parameters:
      itemHandle - Identifies an Item whose "diff" order information is carried by this notification.
      algorithmsMap - A java.util.Map instance, in which Field names are associated to ordered arrays of "diff" algorithms. Omitted fields or null arrays will add no information. On the other hand, an empty array can be supplied to mean that no "diff" algorithm is admitted for a field.
      See Also:

    • failure

      void failure(@Nullable Throwable e)
      Called by a Data Adapter to notify Lightstreamer Kernel of the occurrence of a severe problem that can compromise future operation of the Data Adapter. This causes the whole Server to exit, so that an external recovery mechanism may come into action.
      Parameters:
      e - any java.lang.Throwable object, with the description of the problem.