DataAccessors

Java Interface

 /**
  * A DataAccessor provides access to physical resources by creating DataObjects
  * representing the resource, based on a url and optionally data about a previous access
  * and other parameters.
  */
public interface DataAccessor {

	/**
	 * Get a DataObject for the specified url. The resulting DataObject's ID may differ
         * from the specified url due to normalization schemes, following of redirected URLs, etc. 
	 *
         * An AccessData instance can optionally be specified with which the DataAccessor can store
         * and retrieve information about previous accesses to resources. This is mostly useful
         * for DataCrawlers who want to be able to incrementally scan a DataSource.
         * When an AccessData instance is specified, the resulting DataObject can be null,
         * indicating that the binary resource has not been modified since the last access.
         * 
         * A DataAccessor is always required to store something in the AccessData when a
         * url is accessed, so that afterwards AccessData.isKnownId will return true.
         * 
         * Specific DataAccessor implementations may accept additional parameters through the params Map.
         * 
	 * @param url         The url used to address the resource.
         * @param dataSource  The source that will be registered as the source of the DataObject.
         * @param accessData  Optional database containing information about previous accesses.
	 * @param params      Optional additional parameters needed to access the physical resource.
	 * @return A DataObject for the specified URI, or null when an AccessData instance has been
         * specified and the binary resource has not been modified since the last access.
         * @throws UrlNotFoundException when the binary resource could not be found
         * @throws IOException When any other kind of I/O error occurs.
	 */
	public DataObject get(String url, DataSource source,
            AccessData accessData, Map<?,?> params) throws UrlNotFoundException, IOException;
}