Class LOBStateTracker


  • class LOBStateTracker
    extends java.lang.Object
    An object that tracks the state of large objects (LOBs) for the current row in a result set.

    A LOB's state is either unpublished or published. When a LOB is published, it means that the end-user has been given a reference to the LOB object. This implies that the LOB cannot be automatically freed/released when the result set position changes (i.e. typically rs.next()), because the LOB object must be kept valid/alive until the transaction is ended or the LOB object is explicitly freed.

    This class covers two types of functionality regarding LOBs;

    • Keep track of whether a LOB column has been published or not.
    • Release LOB locators on the server.
    Both functionalities will be disabled if the server doesn't support locators. If locators are enabled, they will be freed when checkCurrentRow(org.apache.derby.client.am.Cursor) is called.

    The tracker has a notion of current row. The current row is changed by calling checkCurrentRow. The owner of the tracker is repsonsible for invoking the method at the correct time, and only when the cursor is positioned on a valid data row. The method must be invoked before the cursor changes the position. Note that calling the method discardState makes checkCurrentRow ignore all LOBs on the subsequent call.

    • Field Summary

      Fields 
      Modifier and Type Field Description
      private int[] columns
      1-based column indexes for the LOBs to track.
      private boolean doRelease
      Tells whether locators shall be released.
      private boolean[] isBlob
      Tells whether the LOB is Blob or a Clob.
      private int[] lastLocatorSeen
      The last locator values seen when releasing.
      static LOBStateTracker NO_OP_TRACKER
      Instance to use when there are no LOBs in the result set, or when the server doesn't support locators.
      private boolean[] published
      Tells whether the LOB colum has been published for the current row.
    • Constructor Summary

      Constructors 
      Constructor Description
      LOBStateTracker​(int[] lobIndexes, boolean[] isBlob, boolean doRelease)
      Creates a LOB state tracker for the specified configuration.
    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      (package private) void checkCurrentRow​(Cursor cursor)
      Checks the current row, updating state and releasing locators on the server as required.
      (package private) void discardState()
      Discards all recorded dynamic state about LOBs.
      (package private) void markAsPublished​(int index)
      Marks the specified column of the current row as published, which implies that the tracker should not release the associated locator.
      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
    • Field Detail

      • NO_OP_TRACKER

        public static final LOBStateTracker NO_OP_TRACKER
        Instance to use when there are no LOBs in the result set, or when the server doesn't support locators.
      • columns

        private final int[] columns
        1-based column indexes for the LOBs to track.
      • isBlob

        private final boolean[] isBlob
        Tells whether the LOB is Blob or a Clob.
      • published

        private final boolean[] published
        Tells whether the LOB colum has been published for the current row.
      • doRelease

        private final boolean doRelease
        Tells whether locators shall be released. This will be false if locators are not supported by the server.
    • Constructor Detail

      • LOBStateTracker

        LOBStateTracker​(int[] lobIndexes,
                        boolean[] isBlob,
                        boolean doRelease)
        Creates a LOB state tracker for the specified configuration.
        Parameters:
        lobIndexes - the 1-based indexes of the LOB columns
        isBlob - whether the LOB is a Blob or a Clob
        doRelease - whether locators shall be released
        See Also:
        NO_OP_TRACKER
    • Method Detail

      • checkCurrentRow

        void checkCurrentRow​(Cursor cursor)
                      throws SqlException
        Checks the current row, updating state and releasing locators on the server as required.

        This method should only be called once per valid row in the result set.

        Parameters:
        cursor - the cursor object to use for releasing the locators
        Throws:
        SqlException - if releasing the locators on the server fails
      • discardState

        void discardState()
        Discards all recorded dynamic state about LOBs.

        Typically called after connection commit or rollback, as those operations will release all locators on the server automatically. There is no need to release them from the client side in this case.

      • markAsPublished

        void markAsPublished​(int index)
        Marks the specified column of the current row as published, which implies that the tracker should not release the associated locator.

        Columns must be marked as published when a LOB object is created on the client, to avoid releasing the corresponding locator too early.

        Parameters:
        index - 1-based column index