The Db.get method retrieves key/data pairs from the database. The byte array and length of the data associated with the specified key are returned in the structure to which data refers.
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. Retrieval of duplicates requires the use of cursor operations. See Dbc.get for details.
When called on a database that has been made into a secondary index using the Db.associate method, the Db.get method returns the key from the secondary index and the data item from the primary database. In addition, the second method signature returns the key from the primary database. In databases that are not secondary indices, the second method signature will always fail.
|data ||The data Dbt operated on. |
|flags ||must be set to 0 or one of the following values: |
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 key , as described in Dbt. The data will be returned in the data 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.
Retrieve the key/data pair only if both the key and data match the arguments.
When used with the second method signature version of this method 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 first method signature version of this method and a secondary index handle.
Retrieve the specified numbered key/data pair from a database. Upon return, both the key and data items will have been filled in.
The data field of the specified key 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.
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.
In addition, the following flags may be set by bitwise inclusively OR
'ing them into the flags
Enclose the Db.get call within a transaction. If the call succeeds, changes made by the operation will be recoverable. If the call fails, the operation will have made no changes. This flag may only be specified with the
Return multiple data items in the buffer to which the data parameter refers.
In the case of Btree or Hash databases, all of the data items associated with the specified key are entered into the buffer. In the case of Queue or Recno databases, all of the data items in the database, starting at, and subsequent to, the specified key, are entered into the buffer.
The buffer to which the data parameter refers must be provided from user memory (see
Db.DB_DBT_USERMEM ). The buffer must be at least as large as the page size of the underlying database, aligned for unsigned integer access, and be a multiple of 1024 bytes in size. If the buffer size is insufficient, then upon return from the call the size field of the data parameter will have been set to an estimated buffer size, and a DbMemoryException is thrown. (The size is an estimate as the exact size needed may not be known until all entries are read. It is best to initially provide a relatively large buffer, but applications should be prepared to resize the buffer as necessary and repeatedly call the method.)
Db.DB_MULTIPLE flag may only be used alone, or with the
Db.DB_SET_RECNO options. The
Db.DB_MULTIPLE flag may not be used when accessing databases made into secondary indices using the Db.associate method.
See DbMultipleDataIterator for more information.
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.
Because the Db.get method will not hold locks across Berkeley DB calls in non-transactional operations, the
Db.DB_RMW flag to the Db.get call is meaningful only in the presence of transactions.
|key ||The key Dbt operated on. |
|pkey ||the return key from the primary database. |
|txnid ||If the operation is to be transaction-protected, the txnid parameter is a transaction handle returned from DbEnv.txnBegin; otherwise, null. |
Unless otherwise specified, the Db.get
method throws an exception that encapsulates a non-zero error value on failure.
Definition at line 4761 of file Db.java.
return db_javaJNI.Db_get__SWIG_1(swigCPtr, DbTxn.getCPtr(txnid), key, pkey, data, flags);