DatabaseEntry (Sleepycat Software, Inc.

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com.sleepycat.db
Class DatabaseEntry

java.lang.Object
  com.sleepycat.db.DatabaseEntry

Direct Known Subclasses:: MultipleEntry

public class DatabaseEntry
extends Object

Encodes database key and data items as a byte array.

Storage and retrieval for the Database and Cursor methods are based on key/data pairs. Both key and data items are represented by DatabaseEntry objects. Key and data byte arrays may refer to arrays of zero length up to arrays of essentially unlimited length.

The DatabaseEntry class provides simple access to an underlying object whose elements can be examined or changed. DatabaseEntry objects can be subclassed, providing a way to associate with it additional data or references to other structures.

Access to DatabaseEntry objects is not re-entrant. In particular, if multiple threads simultaneously access the same DatabaseEntry object using Database or Cursor methods, the results are undefined.

DatabaseEntry objects may be used in conjunction with the object mapping support provided in the com.sleepycat.bind package.

Input and Output Parameters

DatabaseEntry objects are used for both input data (when writing to a database or specifying a search parameter) and output data (when reading from a database). For certain methods, one parameter may be an input parameter and another may be an output parameter. For example, the Database.get(com.sleepycat.db.Transaction, com.sleepycat.db.DatabaseEntry, com.sleepycat.db.DatabaseEntry, com.sleepycat.db.LockMode) method has an input key parameter and an output data parameter. The documentation for each method describes whether its parameters are input or output parameters.

For DatabaseEntry input parameters, the caller is responsible for initializing the data array of the DatabaseEntry. For DatabaseEntry output parameters, the method called will initialize the data array.

For DatabaseEntry output parameters, by default the method called will reuse the byte array in the DatabaseEntry, if the data returned fits in the byte array. This behavior can be configured with setReuseBuffer(boolean) or setUserBuffer(int, boolean). If an entry is configured to not reuse the byte array, a new array is allocated each time the entry is used as an output parameter, so the application can safely keep a reference to the byte array returned by getData() without danger that the array will be overwritten in a subsequent call.

Offset and Size Properties

By default the Offset property is zero and the Size property is the length of the byte array. However, to allow for optimizations involving the partial use of a byte array, the Offset and Size may be set to non-default values.

For DatabaseEntry output parameters, the Size will always be set to the length of the returned data and the Offset will always be set to zero.

However, for DatabaseEntry input parameters the Offset and Size are set to non-default values by the built-in tuple and serial bindings. For example, with a tuple or serial binding the byte array is grown dynamically as data is output, and the Size is set to the number of bytes actually used. For a serial binding, the Offset is set to a non-zero value in order to implement an optimization having to do with the serialization stream header.

Therefore, for output DatabaseEntry parameters the application can assume that the Offset is zero and the Size is the length of the byte array. However, for input DatabaseEntry parameters the application should not make this assumption. In general, it is safest for the application to always honor the Size and Offset properties, rather than assuming they have default values.

Partial Offset and Length Properties

By default the specified data (byte array, offset and size) corresponds to the full stored key or data item. Optionally, the Partial property can be set to true, and the PartialOffset and PartialLength properties are used to specify the portion of the key or data item to be read or written. For details, see the setPartial(int,int,boolean) method.

Note that the Partial properties are set only by the caller. They will never be set by a Database or Cursor method, nor will they every be set by bindings. Therefore, the application can assume that the Partial properties are not set, unless the application itself sets them explicitly.

Constructor Summary
`DatabaseEntry()` Construct a DatabaseEntry with null data.
`DatabaseEntry(byte[] data)` Construct a DatabaseEntry with a given byte array.
`DatabaseEntry(byte[] data, int offset, int size)` Constructs a DatabaseEntry with a given byte array, offset and size.

Method Summary
`byte[]`	`getData()` Return the byte array.
`int`	`getOffset()` Return the byte offset into the data array.
`boolean`	`getPartial()` Return whether this DatabaseEntry is configured to read or write partial records.
`int`	`getPartialLength()` Return the byte length of the partial record being read or written by the application, in bytes.
`int`	`getPartialOffset()` Return the offset of the partial record being read or written by the application, in bytes.
`int`	`getRecordNumber()` Return the record number encoded in this entry's buffer.
`boolean`	`getReuseBuffer()` Return true if the whether the entry is configured to reuse the buffer.
`int`	`getSize()` Return the byte size of the data array.
`boolean`	`getUserBuffer()` Return true if the whether the buffer in this entry is owned by the application.
`int`	`getUserBufferLength()` Return the length of the application's buffer.
`void`	`setData(byte[] data)` Sets the byte array.
`void`	`setData(byte[] data, int offset, int size)` Sets the byte array, offset and size.
`void`	`setOffset(int offset)` Set the byte offset into the data array.
`void`	`setPartial(boolean partial)` Configure this DatabaseEntry to read or write partial records.
`void`	`setPartial(int doff, int dlen, boolean partial)` Configures this DatabaseEntry to read or write partial records.
`void`	`setPartialLength(int dlen)` Set the byte length of the partial record being read or written by the application, in bytes.
`void`	`setPartialOffset(int doff)` Set the offset of the partial record being read or written by the application, in bytes.
`void`	`setRecordNumber(int recno)` Initialize the entry from a logical record number.
`void`	`setReuseBuffer(boolean reuse)` Configures the entry to try to reuse the buffer before allocating a new one.
`void`	`setSize(int size)` Set the byte size of the data array.
`void`	`setUserBuffer(int length, boolean usermem)` Configures the entry with an application-owned buffer.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

DatabaseEntry

public DatabaseEntry()

Construct a DatabaseEntry with null data. The offset and size are set to zero.

DatabaseEntry

public DatabaseEntry(byte[] data)

Construct a DatabaseEntry with a given byte array. The offset is set to zero; the size is set to the length of the array, or to zero if null is passed.

Parameters:

data - Byte array wrapped by the DatabaseEntry.

DatabaseEntry

public DatabaseEntry(byte[] data,
                     int offset,
                     int size)

Constructs a DatabaseEntry with a given byte array, offset and size.

Parameters:

data - Byte array wrapped by the DatabaseEntry.

offset - Offset in the first byte in the byte array to be included.

size - Number of bytes in the byte array to be included.

Method Detail

getData

public byte[] getData()

Return the byte array.

For a DatabaseEntry that is used as an output parameter, the byte array will always be a newly allocated array. The byte array specified by the caller will not be used and may be null.

Returns:: The byte array.

setData

public void setData(byte[] data)

Sets the byte array. The offset is set to zero; the size is set to the length of the array, or to zero if null is passed.

Parameters:: data - Byte array wrapped by the DatabaseEntry.

setData

public void setData(byte[] data,
                    int offset,
                    int size)

Sets the byte array, offset and size.

Parameters:: data - Byte array wrapped by the DatabaseEntry.; offset - Offset in the first byte in the byte array to be included.; size - Number of bytes in the byte array to be included.

setPartial

public void setPartial(int doff,
                       int dlen,
                       boolean partial)

Configures this DatabaseEntry to read or write partial records.

Do partial retrieval or storage of an item. If the calling application is doing a retrieval, length bytes specified by dlen, starting at the offset set by doff bytes from the beginning of the retrieved data record are returned as if they comprised the entire record. If any or all of the specified bytes do not exist in the record, the get is successful, and any existing bytes are returned.

For example, if the data portion of a retrieved record was 100 bytes, and a partial retrieval was done using a DatabaseEntry having a partial length of 20 and a partial offset of 85, the retrieval would succeed and the retrieved data would be the last 15 bytes of the record.

If the calling application is storing an item, length bytes specified by dlen, starting at the offset set by doff bytes from the beginning of the specified key's data item are replaced by the data specified by the DatabaseEntry. If the partial length is smaller than the data, the record will grow; if the partial length is larger than the data, the record will shrink. If the specified bytes do not exist, the record will be extended using nul bytes as necessary, and the store will succeed.

It is an error to specify a partial key when performing a put operation of any kind.

It is an error to attempt a partial store using the Database.put method in a database that supports duplicate records. Partial stores in databases supporting duplicate records must be done using a cursor method.