Class ObjectDatabaseFile

java.lang.Object
com.coyotegulch.jisp.ObjectDatabaseFile
Direct Known Subclasses:
IndexedObjectDatabase

public class ObjectDatabaseFile extends Object
The 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:
  • Field Details

    • DEL_LIST_END

      protected static final long DEL_LIST_END
      Marks the end of the list of deleted records
      See Also:
    • IS_DELETED

      protected static final byte IS_DELETED
      Marks a given record as deleted
      See Also:
    • IS_ACTIVE

      protected static final byte IS_ACTIVE
      Marks a given record as containing live data
      See Also:
    • m_dataFile

      protected RandomAccessFile m_dataFile
      Physical file containing serialized objects
    • m_fileName

      protected String m_fileName
      Name of the underlying physical file
    • m_firstDeleted

      protected long m_firstDeleted
      File position of the first delete record
    • m_filter

      protected RecordFilter m_filter
      Filter to be applied to each object read or written
  • Constructor Details

    • ObjectDatabaseFile

      public ObjectDatabaseFile(String name, boolean is_new) throws IOException
      Creates or opens a file that stores objects in variable-length records.
      Parameters:
      name - name of the file to be created
      is_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

      public ObjectDatabaseFile(File file, boolean is_new) throws IOException
      Creates or opens a file that stores objects in variable-length records.
      Parameters:
      file - indentity of the file to be created
      is_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
  • Method Details

    • writeObject

      public long writeObject(Serializable obj) throws IOException
      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
    • rewriteObject

      public void rewriteObject(Serializable obj) throws IOException
      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

      public Object readObject() throws IOException, ClassNotFoundException
      Throws:
      IOException
      ClassNotFoundException
    • delete

      public void delete() throws IOException
      Deletes the record in the current file position.
      Throws:
      IOException - when an I/O exception is thrown by an underlying java.io.* class
    • rewind

      public void rewind() throws IOException
      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

      public void skip() throws IOException
      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
    • compact

      public void compact(ObjectDatabaseCallback callback) throws IOException, ClassNotFoundException
      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.* class
      ClassNotFoundException - for a casting error, usually when a persistent object or index does match the expected type
    • assignFilter

      public void assignFilter(RecordFilter filter)
      Assigns a filter object to translated, encrypt, compress, or otherwise manipulate objects as they are written and read.
      Parameters:
      filter - assigned filter
    • getFD

      public final FileDescriptor getFD() throws IOException
      Throws:
      IOException
    • getFilePointer

      public long getFilePointer() throws IOException
      Throws:
      IOException
    • seek

      public void seek(long pos) throws IOException
      Throws:
      IOException
    • length

      public long length() throws IOException
      Throws:
      IOException
    • close

      public void close() throws IOException
      Throws:
      IOException
    • createObjectInputStream

      protected ObjectInputStream createObjectInputStream(InputStream in) throws IOException
      Throws:
      IOException
    • createObjectOutputStream

      protected ObjectOutputStream createObjectOutputStream(OutputStream out) throws IOException
      Throws:
      IOException