Class ObjectDatabaseFile
- Direct Known Subclasses:
IndexedObjectDatabase
ObjectDatabaseFile
provides a random-access file that
serializes objects to variable length records in a random-access file.
An ObjectDatabaseFile
allows the deletion of records and
automatically uses such space for new records.
For the purposes of an ObjectDatabaseFile
, consider record
and object to be synonyms. In traditional database terms, an
ObjectDatabaseFile
is a table, each object is a row,
and each field in an object is a column. Unlike a full-featured
database, an ObjectDatabaseFile
can contain a set of heterogenous
records -- in other words, different objects can be stored together in one
ObjectDatabaseFile
. This class is the foundation of the Jisp
package, providing the fundamental file structures for both indexes and data
files.
You can store any serializable object in an ObjectDatabaseFile
.
Records in an ObjectDatabaseFile
can have varying lengths based
on the serialization requirements of objects. To handle variable-length records,
a ObjectDatabaseFile
records the size of each record. When a record
is deleted, the space it occupied is marked as empty; the files maintains a
linked list of deleted record locations and thier sizes.
Inserting a new record involves a traverse of the deleted list, looking for an empty record that is large enough to contain the new information. If the deleted list is empty, or the new record is too large to fit into any open slots, the new object record is appended to the file.
Reusing deleted record space has a drawback: it leaves dead space in the file when
a newly-inserted record is smaller than the "deleted" space it
overwrites. Deleted records also use space in the file until a new record is
written into their location. Periodically, it makes sense to compact the
file, removing the wasted space and eliminating deleted records. ObjectDatabaseFile
recovers "waste space" by re-writing the file, record by record, using exact record lengths,
ignoring deleted records, and calling a user-provided method so that indexes can be
simultaneously regenerated.
- See Also:
-
IndexedObjectDatabase
PageDatabaseFile
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Field Summary
FieldsModifier and TypeFieldDescriptionprotected static final long
Marks the end of the list of deleted recordsprotected static final byte
Marks a given record as containing live dataprotected static final byte
Marks a given record as deletedprotected RandomAccessFile
Physical file containing serialized objectsprotected String
Name of the underlying physical fileprotected RecordFilter
Filter to be applied to each object read or writtenprotected long
File position of the first delete record -
Constructor Summary
ConstructorsConstructorDescriptionObjectDatabaseFile
(File file, boolean is_new) Creates or opens a file that stores objects in variable-length records.ObjectDatabaseFile
(String name, boolean is_new) Creates or opens a file that stores objects in variable-length records. -
Method Summary
Modifier and TypeMethodDescriptionvoid
assignFilter
(RecordFilter filter) Assigns a filter object to translated, encrypt, compress, or otherwise manipulate objects as they are written and read.void
close()
void
compact
(ObjectDatabaseCallback callback) Compacts a database file by copying it to a new file and deleting the old one.protected ObjectInputStream
protected ObjectOutputStream
void
delete()
Deletes the record in the current file position.final FileDescriptor
getFD()
long
long
length()
void
rewind()
Sets the current file pointer to the first byte beyond the file header.void
Re-writes a serializable object to the file, at the current file position.void
seek
(long pos) void
skip()
Skips the record at the current file position, moving to the next record in sequence.long
writeObject
(Serializable obj) Writes a serializable object to the file.
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Field Details
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DEL_LIST_END
protected static final long DEL_LIST_ENDMarks the end of the list of deleted records- See Also:
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IS_DELETED
protected static final byte IS_DELETEDMarks a given record as deleted- See Also:
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IS_ACTIVE
protected static final byte IS_ACTIVEMarks a given record as containing live data- See Also:
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m_dataFile
Physical file containing serialized objects -
m_fileName
Name of the underlying physical file -
m_firstDeleted
protected long m_firstDeletedFile position of the first delete record -
m_filter
Filter to be applied to each object read or written
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Constructor Details
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ObjectDatabaseFile
Creates or opens a file that stores objects in variable-length records.- Parameters:
name
- name of the file to be createdis_new
-true
when a new file should be created, overwriting any existing file with the same name;false
when opening and using an existing file- Throws:
IOException
- when an I/O exception is thrown by an underlying java.io.* class
-
ObjectDatabaseFile
Creates or opens a file that stores objects in variable-length records.- Parameters:
file
- indentity of the file to be createdis_new
-true
when a new file should be created, overwriting any existing file with the same identity;false
when opening and using an existing file- Throws:
IOException
- when an I/O exception is thrown by an underlying java.io.* class
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Method Details
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writeObject
Writes a serializable object to the file.writeObject
stores the new record in the first "deleted" location of sufficient size, or it appends the new record if suitable deleted space is not available.- Parameters:
obj
- serializable object to be written as a record- Returns:
- file position of the newly-written record
- Throws:
IOException
- when an I/O exception is thrown by an underlying java.io.* class
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rewriteObject
Re-writes a serializable object to the file, at the current file position.- Parameters:
obj
- serializable object to be written as a record- Throws:
IOException
- when an I/O exception is thrown by an underlying java.io.* class
-
readObject
- Throws:
IOException
ClassNotFoundException
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delete
Deletes the record in the current file position.- Throws:
IOException
- when an I/O exception is thrown by an underlying java.io.* class
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rewind
Sets the current file pointer to the first byte beyond the file header.- Throws:
IOException
- when an I/O exception is thrown by an underlying java.io.* class
-
skip
Skips the record at the current file position, moving to the next record in sequence.- Throws:
IOException
- when an I/O exception is thrown by an underlying java.io.* class
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compact
Compacts a database file by copying it to a new file and deleting the old one. The records will be copied in sequence, and the callback function is called for each record as it is rewritten. The callback function is responsible for rebuilding any indexes or other referneces to the records, which will have new file positions after compaction.- Parameters:
callback
- called for each record rewritten to the compacted file- Throws:
IOException
- when an I/O exception is thrown by an underlying java.io.* classClassNotFoundException
- for a casting error, usually when a persistent object or index does match the expected type
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assignFilter
Assigns a filter object to translated, encrypt, compress, or otherwise manipulate objects as they are written and read.- Parameters:
filter
- assigned filter
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getFD
- Throws:
IOException
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getFilePointer
- Throws:
IOException
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seek
- Throws:
IOException
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length
- Throws:
IOException
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close
- Throws:
IOException
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createObjectInputStream
- Throws:
IOException
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createObjectOutputStream
- Throws:
IOException
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