org.mapdb

Class Caches.HashTable

java.lang.Object

org.mapdb.EngineWrapper

org.mapdb.Caches.HashTable

All Implemented Interfaces:

Engine

Enclosing class:

Caches

public static class Caches.HashTable
extends EngineWrapper
implements Engine

Fixed size cache which uses hash table. Is thread-safe and requires only minimal locking. Items are randomly removed and replaced by hash collisions.

This is simple, concurrent, small-overhead, random cache.

Author:

Jan Kotek

Nested Class Summary

Nested classes/interfaces inherited from class org.mapdb.EngineWrapper

EngineWrapper.CloseOnJVMShutdown, EngineWrapper.ImmutabilityCheckEngine, EngineWrapper.ReadOnlyEngine, EngineWrapper.SerializerCheckEngineWrapper, EngineWrapper.SynchronizedEngineWrapper

Field Summary

Fields

Modifier and Type	Field and Description
protected int	cacheMaxSize
protected int	cacheMaxSizeMask
protected long	hashSalt Salt added to keys before hashing, so it is harder to trigger hash collision attack.
protected org.mapdb.Caches.HashTable.HashItem[]	items
protected ReentrantLock[]	locks

Fields inherited from class org.mapdb.EngineWrapper

CLOSED

Fields inherited from interface org.mapdb.Engine

CATALOG_RECID, CHECK_RECORD, CLASS_INFO_RECID, LAST_RESERVED_RECID

Constructor Summary

Constructors

Constructor and Description
HashTable(Engine engine, int cacheMaxSize, boolean disableLocks)

Method Summary

Modifier and Type	Method and Description
void	clearCache() clears any underlying cache
void	close() Close store/cache.
<A> boolean	compareAndSwap(long recid, A expectedOldValue, A newValue, Serializer<A> serializer) Updates existing record in atomic (Compare And Swap) manner.
<A> void	delete(long recid, Serializer<A> serializer) Remove existing record from store/cache Recid must be a number returned by 'put' method.
<A> A	get(long recid, Serializer<A> serializer) Get existing record.
<A> long	put(A value, Serializer<A> serializer) Insert new record.
void	rollback() Undoes all changes made in the current transaction.
<A> void	update(long recid, A value, Serializer<A> serializer) Update existing record with new value.

Methods inherited from class org.mapdb.EngineWrapper

canRollback, canSnapshot, checkClosed, closeListenerRegister, closeListenerUnregister, commit, compact, getSerializerPojo, getWrappedEngine, isClosed, isReadOnly, preallocate, preallocate, snapshot

Methods inherited from class java.lang.Object

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

Methods inherited from interface org.mapdb.Engine

canRollback, canSnapshot, closeListenerRegister, closeListenerUnregister, commit, compact, getSerializerPojo, isClosed, isReadOnly, preallocate, preallocate, snapshot

Field Detail

locks

protected final ReentrantLock[] locks

items

protected org.mapdb.Caches.HashTable.HashItem[] items

cacheMaxSize

protected final int cacheMaxSize

cacheMaxSizeMask

protected final int cacheMaxSizeMask

hashSalt

protected final long hashSalt

Salt added to keys before hashing, so it is harder to trigger hash collision attack.

Constructor Detail

HashTable

public HashTable(Engine engine,
                 int cacheMaxSize,
                 boolean disableLocks)

Method Detail

put

public <A> long put(A value,
                    Serializer<A> serializer)

Description copied from interface: Engine

Insert new record.

Specified by:

put in interface Engine

Overrides:

put in class EngineWrapper

Parameters:

value - records to be added

serializer - used to convert record into/from binary form

Returns:

recid (record identifier) under which record is stored.

get

public <A> A get(long recid,
                 Serializer<A> serializer)

Description copied from interface: Engine

Get existing record.

Recid must be a number returned by 'put' method. Behaviour for invalid recid (random number or already deleted record) is not defined, typically it returns null or throws 'EndOfFileException'

Specified by:

get in interface Engine

Overrides:

get in class EngineWrapper

Parameters:

recid - (record identifier) under which record was persisted

serializer - used to deserialize record from binary form

Returns:

record matching given recid, or null if record is not found under given recid.

update

public <A> void update(long recid,
                       A value,
                       Serializer<A> serializer)

Description copied from interface: Engine

Update existing record with new value.

Recid must be a number returned by 'put' method. Behaviour for invalid recid (random number or already deleted record) is not defined, typically it throws 'EndOfFileException', but it may also corrupt store.

Specified by:

update in interface Engine

Overrides:

update in class EngineWrapper

Parameters:

recid - (record identifier) under which record was persisted.

value - new record value to be stored

serializer - used to serialize record into binary form

compareAndSwap

public <A> boolean compareAndSwap(long recid,
                                  A expectedOldValue,
                                  A newValue,
                                  Serializer<A> serializer)

Description copied from interface: Engine

Updates existing record in atomic (Compare And Swap) manner. Value is modified only if old value matches expected value. There are three ways to match values, MapDB may use any of them:

1. Equality check oldValue==expectedOldValue when old value is found in instance cache
2. Deserializing oldValue using serializer and checking oldValue.equals(expectedOldValue)
3. Serializing expectedOldValue using serializer and comparing binary array with already serialized oldValue

Recid must be a number returned by 'put' method. Behaviour for invalid recid (random number or already deleted record) is not defined, typically it throws 'EndOfFileException', but it may also corrupt store.

Specified by:

compareAndSwap in interface Engine

Overrides:

compareAndSwap in class EngineWrapper

Parameters:

recid - (record identifier) under which record was persisted.

expectedOldValue - old value to be compared with existing record

newValue - to be written if values are matching

serializer - used to serialize record into binary form

Returns:

true if values matched and newValue was written

delete

public <A> void delete(long recid,
                       Serializer<A> serializer)

Description copied from interface: Engine

Remove existing record from store/cache

Recid must be a number returned by 'put' method. Behaviour for invalid recid (random number or already deleted record) is not defined, typically it throws 'EndOfFileException', but it may also corrupt store.

Specified by:

delete in interface Engine

Overrides:

delete in class EngineWrapper

Parameters:

recid - (record identifier) under which was record persisted

serializer - which may be used in some circumstances to deserialize and store old object

close

public void close()

Description copied from interface: Engine

Close store/cache. This method must be called before JVM exits to flush all caches and prevent store corruption. Also it releases resources used by MapDB (disk, memory..).

Engine can no longer be used after this method was called. If Engine is used after closing, it may throw any exception including NullPointerException

There is an configuration option DBMaker.closeOnJvmShutdown() which uses shutdown hook to automatically close Engine when JVM shutdowns.

Specified by:

close in interface Engine

Overrides:

close in class EngineWrapper

rollback

public void rollback()

Description copied from interface: Engine

Undoes all changes made in the current transaction. If transactions are disabled it throws UnsupportedOperationException.

Specified by:

rollback in interface Engine

Overrides:

rollback in class EngineWrapper

clearCache

public void clearCache()

Description copied from interface: Engine

clears any underlying cache

Specified by:

clearCache in interface Engine

Overrides:

clearCache in class EngineWrapper