<!--$Id: db_get.html,v 1.1 2003/02/15 04:55:50 zarzycki Exp $--> <!--Copyright 1997-2002 by Sleepycat Software, Inc.--> <!--All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB: Db.get</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++"> </head> <body bgcolor=white> <a name="2"><!--meow--></a> <table width="100%"><tr valign=top> <td> <h1>Db.get</h1> </td> <td align=right> <a href="../api_java/c_index.html"><img src="../images/api.gif" alt="API"></a><a href="../reftoc.html"><img src="../images/ref.gif" alt="Ref"></a> </td></tr></table> <hr size=1 noshade> <tt> <h3><pre> import com.sleepycat.db.*; <p> public int get(DbTxn txnid, Dbt key, Dbt data, int flags) throws DbException; public int pget(DbTxn txnid, Dbt key, Dbt pkey, Dbt data, int flags) throws DbException; </pre></h3> <h1>Description</h1> <p>The Db.get method retrieves key/data pairs from the database. The byte array and length of the data associated with the specified <b>key</b> are returned in the structure to which <b>data</b> refers. <p>In the presence of duplicate key values, Db.get will return the first data item for the designated key. Duplicates are sorted by insert order, except where this order has been overridden by cursor operations. <b>Retrieval of duplicates requires the use of cursor operations.</b> See <a href="../api_java/dbc_get.html">Dbc.get</a> for details. <p>When called on a database that has been made into a secondary index using the <a href="../api_java/db_associate.html">Db.associate</a> method, the Db.get and Db.pget methods return the key from the secondary index and the data item from the primary database. In addition, the Db.pget method returns the key from the primary database. In databases that are not secondary indices, the Db.pget interface will always fail and return EINVAL. <p>If the operation is to be transaction-protected, the <b>txnid</b> parameter is a transaction handle returned from <a href="../api_java/txn_begin.html">DbEnv.txn_begin</a>; otherwise, null. <p>The <b>flags</b> value must be set to 0 or one of the following values: <p><dl compact> <p><dt><a name="Db.DB_CONSUME">Db.DB_CONSUME</a><dd>Return the record number and data from the available record closest to the head of the queue, and delete the record. The cursor will be positioned on the deleted record. The record number will be returned in <b>key</b>, as described in <a href="../api_java/dbt_class.html">Dbt</a>. The data will be returned in the <b>data</b> parameter. A record is available if it is not deleted and is not currently locked. The underlying database must be of type Queue for Db.DB_CONSUME to be specified. <p><dt><a name="Db.DB_CONSUME_WAIT">Db.DB_CONSUME_WAIT</a><dd>The Db.DB_CONSUME_WAIT flag is the same as the Db.DB_CONSUME flag, except that if the Queue database is empty, the thread of control will wait until there is data in the queue before returning. The underlying database must be of type Queue for Db.DB_CONSUME_WAIT to be specified. <p><dt><a name="Db.DB_GET_BOTH">Db.DB_GET_BOTH</a><dd>Retrieve the key/data pair only if both the key and data match the arguments. <p>When used with the Db.pget version of this interface on a secondary index handle, return the secondary key/primary key/data tuple only if both the primary and secondary keys match the arguments. It is an error to use the Db.DB_GET_BOTH flag with the Db.get version of this interface and a secondary index handle. <p><dt><a name="Db.DB_SET_RECNO">Db.DB_SET_RECNO</a><dd>Retrieve the specified numbered key/data pair from a database. Upon return, both the <b>key</b> and <b>data</b> items will have been filled in. <p>The <b>data</b> field of the specified <b>key</b> must be a byte array large enough to hold a logical record number (that is, an int). This record number determines the record to be retrieved. <p>For Db.DB_SET_RECNO to be specified, the underlying database must be of type Btree, and it must have been created with the DB_RECNUM flag. </dl> <p>In addition, the following flags may be set by bitwise inclusively <b>OR</b>'ing them into the <b>flags</b> parameter: <p><dl compact> <p><dt><a name="Db.DB_DIRTY_READ">Db.DB_DIRTY_READ</a><dd>Read modified but not yet committed data. Silently ignored if the <a href="../api_java/db_open.html#DB_DIRTY_READ">Db.DB_DIRTY_READ</a> flag was not specified when the underlying database was opened. <p><dt><a name="Db.DB_MULTIPLE">Db.DB_MULTIPLE</a><dd>Return multiple data items. The buffer to which the <b>data</b> argument refers is filled with the specified key's data items. If all of the data items associated with the key cannot fit into the buffer, the size field of the <b>data</b> argument is set to the length needed for the specified items, and a <a href="../api_java/memp_class.html">DbMemoryException</a> is thrown. The buffer to which the <b>data</b> argument refers should be large relative to the page size of the underlying database, aligned for unsigned integer access, and be a multiple of 1024 bytes in size. <p>The Db.DB_MULTIPLE flag may only be used alone, or with the Db.DB_GET_BOTH and Db.DB_SET_RECNO options. The Db.DB_MULTIPLE flag may not be used when accessing databases made into secondary indices using the <a href="../api_java/db_associate.html">Db.associate</a> method. <p>See <a href="../api_java/dbt_bulk_class.html">DbMultipleDataIterator</a> for more information. <p><dt><a name="Db.DB_RMW">Db.DB_RMW</a><dd>Acquire write locks instead of read locks when doing the retrieval. Setting this flag can eliminate deadlock during a read-modify-write cycle by acquiring the write lock during the read part of the cycle so that another thread of control acquiring a read lock for the same item, in its own read-modify-write cycle, will not result in deadlock. <p>Because the Db.get interface will not hold locks across Berkeley DB interface calls in non-transactional environments, the <a href="../api_java/dbc_get.html#DB_RMW">Db.DB_RMW</a> flag to the Db.get call is meaningful only in the presence of transactions. </dl> <p> If the specified key is not in the database, the Db.get method will return Db.DB_NOTFOUND. If the database is a Queue or Recno database and the specified key exists, but was never explicitly created by the application or was later deleted, the Db.get method will return Db.DB_KEYEMPTY. Otherwise, the Db.get method throws an exception that encapsulates a non-zero error value on failure. <h1>Errors</h1> <p>The Db.get method may fail and throw an exception encapsulating a non-zero error for the following conditions: <p><dl compact> <p><dt>Db.DB_SECONDARY_BAD<dd>A secondary index references a nonexistent primary key. </dl> <p><dl compact> <p><dt>EINVAL<dd>An invalid flag value or parameter was specified. <p>A record number of 0 was specified. <p>The <a href="../api_java/env_open.html#DB_THREAD">Db.DB_THREAD</a> flag was specified to the <a href="../api_java/db_open.html">Db.open</a> method and none of the <a href="../api_java/dbt_class.html#DB_DBT_MALLOC">Db.DB_DBT_MALLOC</a>, <a href="../api_java/dbt_class.html#DB_DBT_REALLOC">Db.DB_DBT_REALLOC</a> or <a href="../api_java/dbt_class.html#DB_DBT_USERMEM">Db.DB_DBT_USERMEM</a> flags were set in the <a href="../api_java/dbt_class.html">Dbt</a>. <p>The Db.pget interface was called with a <a href="../api_java/db_class.html">Db</a> handle that does not refer to a secondary index. </dl> <p>If the operation was selected to resolve a deadlock, the Db.get method will fail and throw a <a href="../api_java/deadlock_class.html">DbDeadlockException</a> exception. <p>If the requested item could not be returned due to insufficient memory, the Db.get method will fail and throw a <a href="../api_java/memp_class.html">DbMemoryException</a> exception. <p>The Db.get method may fail and throw an exception for errors specified for other Berkeley DB and C library or system methods. If a catastrophic error has occurred, the Db.get method may fail and throw a <a href="../api_java/runrec_class.html">DbRunRecoveryException</a>, in which case all subsequent Berkeley DB calls will fail in the same way. <h1>Class</h1> <a href="../api_java/db_class.html">Db</a> <h1>See Also</h1> <a href="../api_java/db_list.html">Databases and Related Methods</a> </tt> <table width="100%"><tr><td><br></td><td align=right> <a href="../api_java/c_index.html"><img src="../images/api.gif" alt="API"></a><a href="../reftoc.html"><img src="../images/ref.gif" alt="Ref"></a> </td></tr></table> <p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font> </body> </html>