[SNMP4J] Table Listeners and multi-threading

[Frank Fock]

pasteman8-agentpp at yahoo.com wrote:

>There is a basic question regarding the design of
>TableUtils:
>Can the getTable method be used in a runnable run()
>method? In otherwords, does getTable return only when
>the table is done?
>
>  
>
No, the TableUtils.getTable() method available in v1.0.3 asynchronously
retrieves a table. It returns immediately. For a synchronously getTable
method, please add the following code to TableUtils class (this code
will be part of v1.0.4):

  /**
   * Gets synchronously SNMP tabular data from one or more tables.
   * The data is returned row-by-row as a list of {@link TableEvent} 
instances.
   * Each instance represents a row (or an error condition). Besides the
   * target agent, the OIDs of the columnar objects have to be specified
   * for which instances should be retrieved. With a lower bound index and
   * an upper bound index, the result set can be narrowed to improve
   * performance. This method can be executed concurrently by multiple 
threads.
   *
   * @param target
   *    a <code>Target</code> instance.
   * @param columnOIDs
   *    an array of OIDs of the columnar objects whose instances should be
   *    retrieved. The columnar objects may belong to different tables.
   *    Typically they belong to tables that share a common index or 
sub-index
   *    prefix. Note: The result of this method is not defined if 
instance OIDs
   *    are supplied in this array!
   * @param lowerBoundIndex
   *    an optional parameter that specifies the lower bound index.
   *    If not <code>null</code>, all returned rows have an index 
greater than
   *    <code>lowerBoundIndex</code>.
   * @param upperBoundIndex
   *    an optional parameter that specifies the upper bound index.
   *    If not <code>null</code>, all returned rows have an index less 
or equal
   *    than <code>lowerBoundIndex</code>.
   * @return
   *    a <code>List</code> of {@link TableEvent} instances. Each instance
   *    represents successfully retrieved row or an error condition. Error
   *    conditions (any status other than {@link TableEvent#STATUS_OK})
   *    may only appear at the last element of the list.
   */
  public List getTable(Target target,
                       OID[] columnOIDs,
                       OID lowerBoundIndex,
                       OID upperBoundIndex) {

    if ((columnOIDs == null) || (columnOIDs.length == 0)) {
      throw new IllegalArgumentException("No column OIDs specified");
    }
    InternalTableListener listener = new InternalTableListener();
    TableRequest req = new TableRequest(target, columnOIDs, listener,
                                        null,
                                        lowerBoundIndex,
                                        upperBoundIndex);
    synchronized (listener) {
      req.sendNextChunk();
      try {
        listener.wait();
      }
      catch (InterruptedException ex) {
        logger.warn(ex);
      }
    }   
    return listener.getRows();
  }

  class InternalTableListener implements TableListener {
   
    private LinkedList rows = new LinkedList();
   
    public boolean next(TableEvent event) {
      rows.add(event);
      return true;
    }

    public synchronized void finished(TableEvent event) {
      if ((event.getStatus() != TableEvent.STATUS_OK) ||
          (event.getIndex() != null)) {
        rows.add(event);
      }
      notify();
    }

    public List getRows() {
      return rows;
    }
  }


Best regards,
Frank