Class LargeSelect<T>

  • Type Parameters:
    T - the type of the objects which are returned on a query.
    All Implemented Interfaces:
    Serializable, Runnable

    public class LargeSelect<T>
    extends Object
    implements Runnable, Serializable
    This class can be used to retrieve a large result set from a database query. The query is started and then rows are returned a page at a time. The LargeSelect is meant to be placed into the Session or User.Temp, so that it can be used in response to several related requests. Note that in order to use LargeSelect you need to be willing to accept the fact that the result set may become inconsistent with the database if updates are processed subsequent to the queries being executed. Specifying a memory page limit of 1 will give you a consistent view of the records but the totals may not be accurate and the performance will be terrible. In most cases the potential for inconsistencies data should not cause any serious problems and performance should be pretty good (but read on for further warnings).

    The idea here is that the full query result would consume too much memory and if displayed to a user the page would be too long to be useful. Rather than loading the full result set into memory, a window of data (the memory limit) is loaded and retrieved a page at a time. If a request occurs for data that falls outside the currently loaded window of data then a new query is executed to fetch the required data. Performance is optimized by starting a thread to execute the database query and fetch the results. This will perform best when paging forwards through the data, but a minor optimization where the window is moved backwards by two rather than one page is included for when a user pages past the beginning of the window.

    As the query is performed in in steps, it is often the case that the total number of records and pages of data is unknown. LargeSelect provides various methods for indicating how many records and pages it is currently aware of and for presenting this information to users.

    LargeSelect utilizes the Criteria methods setOffset() and setLimit() to limit the amount of data retrieved from the database - these values are either passed through to the DBMS when supported (efficient with the caveat below) or handled by the Torque API when it is not (not so efficient).

    As LargeSelect must re-execute the query each time the user pages out of the window of loaded data, you should consider the impact of non-index sort orderings and other criteria that will require the DBMS to execute the entire query before filtering down to the offset and limit either internally or via Torque.

    The memory limit defaults to 5 times the page size you specify, but alternative constructors and the class method setMemoryPageLimit() allow you to override this for a specific instance of LargeSelect or future instances respectively.

    Typically you will create a LargeSelect using your Criteria (perhaps created from the results of a search parameter page), page size, memory page limit, return class name (for which you may have defined a business object class before hand) and peer class and place this in user.Temp thus:

         data.getUser().setTemp("someName", largeSelect);
     

    In your template you will then use something along the lines of:

        #set($largeSelect = $data.User.getTemp("someName"))
        #set($searchop = $data.Parameters.getString("searchop"))
        #if($searchop.equals("prev"))
          #set($recs = $largeSelect.PreviousResults)
        #else
          #if($searchop.equals("goto"))
            #set($recs = $largeSelect.getPage($data.Parameters.getInt("page", 1)))
          #else
            #set($recs = $largeSelect.NextResults)
          #end
        #end
     

    ...to move through the records. LargeSelect implements a number of convenience methods that make it easy to add all of the necessary bells and whistles to your template.

    Version:
    $Id: LargeSelect.java 1879929 2020-07-16 07:42:57Z gk $
    Author:
    John D. McNally, Scott Eade
    See Also:
    Serialized Form
    • Constructor Summary

      Constructors 
      Constructor Description
      LargeSelect​(Criteria criteria, int pageSize, int memoryPageLimit, BasePeerImpl<T> peerImpl)
      Creates a LargeSelect whose results are returned as a List containing a maximum of pageSize objects of the type T at a time, maintaining a maximum of memoryPageLimit pages of results in memory.
      LargeSelect​(Criteria criteria, int pageSize, BasePeerImpl<T> peerImpl)
      Creates a LargeSelect whose results are returned as a List containing a maximum of pageSize objects of the type defined within the class named returnBuilderClassName at a time, maintaining a maximum of LargeSelect.memoryPageLimit pages of results in memory.
    • Field Detail

      • DEFAULT_PAGE_PROGRESS_TEXT_PATTERN

        public static final String DEFAULT_PAGE_PROGRESS_TEXT_PATTERN
        default MessageFormat pattern for the page progress text
        See Also:
        Constant Field Values
      • DEFAULT_RECORD_PROGRESS_TEXT_PATTERN

        public static final String DEFAULT_RECORD_PROGRESS_TEXT_PATTERN
        default MessageFormat pattern for the record progress text
        See Also:
        Constant Field Values
      • DEFAULT_MEMORY_LIMIT_PAGES

        public static final int DEFAULT_MEMORY_LIMIT_PAGES
        The default value for the maximum number of pages of data to be retained in memory.
        See Also:
        Constant Field Values
    • Constructor Detail

      • LargeSelect

        public LargeSelect​(Criteria criteria,
                           int pageSize,
                           BasePeerImpl<T> peerImpl)
        Creates a LargeSelect whose results are returned as a List containing a maximum of pageSize objects of the type defined within the class named returnBuilderClassName at a time, maintaining a maximum of LargeSelect.memoryPageLimit pages of results in memory.
        Parameters:
        criteria - object used by BasePeer to build the query. In order to allow this class to utilize database server implemented offsets and limits (when available), the provided criteria must not have any limit or offset defined.
        pageSize - number of rows to return in one block.
        peerImpl - the peer that will be used to do the select operations
        Throws:
        IllegalArgumentException - if criteria uses one or both of offset and limit, if pageSize is less than 1 or the Criteria object does not contain SELECT columns.
      • LargeSelect

        public LargeSelect​(Criteria criteria,
                           int pageSize,
                           int memoryPageLimit,
                           BasePeerImpl<T> peerImpl)
        Creates a LargeSelect whose results are returned as a List containing a maximum of pageSize objects of the type T at a time, maintaining a maximum of memoryPageLimit pages of results in memory.
        Parameters:
        criteria - object used by BasePeerImpl to build the query. In order to allow this class to utilize database server implemented offsets and limits (when available), the provided criteria must not have any limit or offset defined.
        pageSize - number of rows to return in one block.
        memoryPageLimit - maximum number of pages worth of rows to be held in memory at one time.
        peerImpl - the peer that will be used to do the select operations
        Throws:
        IllegalArgumentException - if criteria uses one or both of offset and limit, if pageSize or memoryLimitPages are less than 1 or the Criteria object does not contain SELECT columns..
    • Method Detail

      • getPage

        public List<T> getPage​(int pageNumber)
                        throws TorqueException
        Retrieve a specific page, if it exists.
        Parameters:
        pageNumber - the number of the page to be retrieved - must be greater than zero. An empty List will be returned if pageNumber exceeds the total number of pages that exist.
        Returns:
        a List of query results containing a maximum of pageSize results.
        Throws:
        IllegalArgumentException - when pageNo is not greater than zero.
        TorqueException - if a sleep is unexpectedly interrupted.
      • getNextResults

        public List<T> getNextResults()
                               throws TorqueException
        Gets the next page of rows.
        Returns:
        a List of query results containing a maximum of pageSize results.
        Throws:
        TorqueException - if a sleep is unexpectedly interrupted.
      • getCurrentPageResults

        public List<T> getCurrentPageResults()
                                      throws TorqueException
        Provide access to the results from the current page.
        Returns:
        a List of query results containing a maximum of pageSize results.
        Throws:
        TorqueException - if a sleep is unexpectedly interrupted.
      • getPreviousResults

        public List<T> getPreviousResults()
                                   throws TorqueException
        Gets the previous page of rows.
        Returns:
        a List of query results containing a maximum of pageSize results.
        Throws:
        TorqueException - if a sleep is unexpectedly interrupted.
      • run

        public void run()
        A background thread that retrieves the rows.
        Specified by:
        run in interface Runnable
      • getCurrentPageNumber

        public int getCurrentPageNumber()
        Retrieve the number of the current page.
        Returns:
        the current page number.
      • getTotalRecords

        public int getTotalRecords()
        Retrieve the total number of search result records that are known to exist (this will be the actual value when the query has completed (see getTotalsFinalized()). The convenience method getRecordProgressText() may be more useful for presenting to users.
        Returns:
        the number of result records known to exist (not accurate until getTotalsFinalized() returns true).
      • getPaginated

        public boolean getPaginated()
        Provide an indication of whether or not paging of results will be required.
        Returns:
        true when multiple pages of results exist.
      • getTotalPages

        public int getTotalPages()
        Retrieve the total number of pages of search results that are known to exist (this will be the actual value when the query has completeted (see getQyeryCompleted()). The convenience method getPageProgressText() may be more useful for presenting to users.
        Returns:
        the number of pages of results known to exist (not accurate until getTotalsFinalized() returns true).
      • getPageSize

        public int getPageSize()
        Retrieve the page size.
        Returns:
        the number of records returned on each invocation of getNextResults()/getPreviousResults().
      • getTotalsFinalized

        public boolean getTotalsFinalized()
        Provide access to indicator that the total values for the number of records and pages are now accurate as opposed to known upper limits.
        Returns:
        true when the totals are known to have been fully computed.
      • getPageProgressTextPattern

        public String getPageProgressTextPattern()
        Retrieve the MessageFormat pattern for the page progress The default is
        {0} of {1,choice,0#> |1#}{2}
        Returns:
        the pattern as a string
      • setPageProgressTextPattern

        public void setPageProgressTextPattern​(String pageProgressTextPattern)
        Set the MessageFormat pattern for the page progress. The default is
        {0} of {1,choice,0#> |1#}{2}

        The pattern contains three placeholders

        • {0} - the current page
        • {1} - 0 if the total number of pages is not yet known, 1 otherwise
        • {2} - the total number of pages

        Localized example in German:

        Seite {0} von {1,choice,0#mehr als |1#}{2}
        Parameters:
        pageProgressTextPattern -
      • getRecordProgressTextPattern

        public String getRecordProgressTextPattern()
        Retrieve the MessageFormat pattern for the record progress The default is
        {0} - {1} of {2,choice,0#> |1#}{3}
        Returns:
        the pattern as a string
      • setRecordProgressTextPattern

        public void setRecordProgressTextPattern​(String recordProgressTextPattern)
        Set the MessageFormat pattern for the record progress. The default is
        {0} - {1} of {2,choice,0#> |1#}{3}

        The pattern contains four placeholders

        • {0} - number of the first record on the page
        • {1} - number of the last record on the page
        • {2} - 0 if the total number of records is not yet known, 1 otherwise
        • {3} - the total number of records

        Localized example in German:

        Datensätze {0} bis {1} von {2,choice,0#mehr als |1#}{3}
        Parameters:
        recordProgressTextPattern -
      • setMemoryPageLimit

        public void setMemoryPageLimit​(int memoryPageLimit)
        Sets the multiplier that will be used to compute the memory limit when a constructor with no memory page limit is used - the memory limit will be this number multiplied by the page size.
        Parameters:
        memoryPageLimit - the maximum number of pages to be in memory at one time.
      • getMemoryPageLimit

        public int getMemoryPageLimit()
        Retrieves the multiplier that will be used to compute the memory limit when a constructor with no memory page limit is used - the memory limit will be this number multiplied by the page size.
        Returns:
        memoryPageLimit the maximum number of pages to be in memory at one time.
      • getPageProgressText

        public String getPageProgressText()
        A convenience method that provides text showing progress through the selected rows on a page basis.
        Returns:
        progress text in the form of "1 of > 5" where ">" can be configured using setMoreIndicator().
      • getCurrentPageSize

        public int getCurrentPageSize()
                               throws TorqueException
        Provides a count of the number of rows to be displayed on the current page - for the last page this may be less than the configured page size.
        Returns:
        the number of records that are included on the current page of results.
        Throws:
        TorqueException - if invoking the populateObjects() method runs into problems or a sleep is unexpectedly interrupted.
      • getFirstRecordNoForPage

        public int getFirstRecordNoForPage()
        Provide the record number of the first row included on the current page.
        Returns:
        The record number of the first row of the current page.
      • getLastRecordNoForPage

        public int getLastRecordNoForPage()
                                   throws TorqueException
        Provide the record number of the last row included on the current page.
        Returns:
        the record number of the last row of the current page.
        Throws:
        TorqueException - if invoking the populateObjects() method runs into problems or a sleep is unexpectedly interrupted.
      • getRecordProgressText

        public String getRecordProgressText()
                                     throws TorqueException
        A convenience method that provides text showing progress through the selected rows on a record basis.
        Returns:
        progress text in the form of "26 - 50 of > 250" where ">" can be configured using setMoreIndicator().
        Throws:
        TorqueException - if invoking the populateObjects() method runs into problems or a sleep is unexpectedly interrupted.
      • getNextResultsAvailable

        public boolean getNextResultsAvailable()
        Indicates if further result pages are available.
        Returns:
        true when further results are available.
      • getPreviousResultsAvailable

        public boolean getPreviousResultsAvailable()
        Indicates if previous results pages are available.
        Returns:
        true when previous results are available.
      • hasResultsAvailable

        public boolean hasResultsAvailable()
        Indicates if any results are available.
        Returns:
        true of any results are available.
      • invalidateResult

        public void invalidateResult()
                              throws TorqueException
        Clear the query result so that the query is re-executed when the next page is retrieved. You may want to invoke this method if you are returning to a page after performing an operation on an item in the result set.
        Throws:
        TorqueException - if a sleep is interrupted.
      • getSearchParam

        public String getSearchParam​(String name)
        Retrieve a search parameter. This acts as a convenient place to store parameters that relate to the LargeSelect to make it easy to get at them in order to re-populate search parameters on a form when the next page of results is retrieved - they in no way effect the operation of LargeSelect.
        Parameters:
        name - the search parameter key to retrieve.
        Returns:
        the value of the search parameter.
      • getSearchParam

        public String getSearchParam​(String name,
                                     String defaultValue)
        Retrieve a search parameter. This acts as a convenient place to store parameters that relate to the LargeSelect to make it easy to get at them in order to re-populate search parameters on a form when the next page of results is retrieved - they in no way effect the operation of LargeSelect.
        Parameters:
        name - the search parameter key to retrieve.
        defaultValue - the default value to return if the key is not found.
        Returns:
        the value of the search parameter.
      • setSearchParam

        public void setSearchParam​(String name,
                                   String value)
        Set a search parameter. If the value is null then the key will be removed from the parameters.
        Parameters:
        name - the search parameter key to set.
        value - the value of the search parameter to store.
      • removeSearchParam

        public void removeSearchParam​(String name)
        Remove a value from the search parameters.
        Parameters:
        name - the search parameter key to remove.
      • toString

        public String toString()
        Provide something useful for debugging purposes.
        Overrides:
        toString in class Object
        Returns:
        some basic information about this instance of LargeSelect.