org.archive.queue
Class DiskBackedQueue

java.lang.Object
  org.archive.queue.DiskBackedQueue

All Implemented Interfaces:

java.io.Serializable, Queue

Direct Known Subclasses:

DiskBackedDeque

public class DiskBackedQueue
extends java.lang.Object
implements Queue, java.io.Serializable

Queue which uses a DiskQueue ('tailQ') for spillover entries once a in-memory LinkedList ('headQ') reaches a maximum size.

Author:

Gordon Mohr

See Also:

Serialized Form

Field Summary

protected static float	DISCARD_BACKING_THRESHOLD if all contents would leave head less than this percent full, discard the backing file(s)
protected int	headMax
protected java.util.LinkedList	headQ
protected java.lang.String	name
protected DiskQueue	tailQ

Constructor Summary

DiskBackedQueue(java.io.File dir, java.lang.String name, boolean reuse, int headMax)

Method Summary

protected java.lang.Object	backingDequeue()
protected void	backingUpdate()
protected boolean	canDiscardBacking()
long	deleteMatchedItems(org.apache.commons.collections.Predicate matcher) All objects in the queue where matcher.match(object) returns true will be deleted from the queue.
java.lang.Object	dequeue() remove an entry from the start of the queue
protected void	discardBacking()
void	disconnect() Release any file-handles in arecoverable way.
void	enqueue(java.lang.Object o) Add an entry to the end of queue
protected void	fillHeadQ()
java.util.Iterator	getIterator(boolean inCacheOnly) Returns an iterator for the queue.
protected int	headTargetSize()
boolean	isEmpty() is the queue empty?
long	length() get the number of elements in the queue
java.lang.Object	peek() Give the top object in the queue, leaving it in place to be returned by future peek() or dequeue() invocations.
void	release() release any OS/IO resources associated with Queue
void	setHeadMax(int hm) Set the maximum number of items to keep in memory at the structure's top.
void	unpeek() Releases queue from the obligation to return in the next peek()/dequeue() the same object as returned by any previous peek().

Methods inherited from class java.lang.Object

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

Field Detail

DISCARD_BACKING_THRESHOLD

protected static final float DISCARD_BACKING_THRESHOLD

if all contents would leave head less than this percent full, discard the backing file(s)

See Also:

Constant Field Values

headMax

protected int headMax

headQ

protected java.util.LinkedList headQ

tailQ

protected DiskQueue tailQ

name

protected java.lang.String name

Constructor Detail

DiskBackedQueue

public DiskBackedQueue(java.io.File dir,
                       java.lang.String name,
                       boolean reuse,
                       int headMax)
                throws java.io.IOException

Parameters:

dir -

name -

reuse - whether to reuse any existing backing files

headMax -

Throws:

java.io.IOException

Method Detail

enqueue

public void enqueue(java.lang.Object o)

Description copied from interface: Queue

Add an entry to the end of queue

Specified by:

enqueue in interface Queue

Parameters:

o - the entry to queue

See Also:

Queue.enqueue(java.lang.Object)

isEmpty

public boolean isEmpty()

Description copied from interface: Queue

is the queue empty?

Specified by:

isEmpty in interface Queue

Returns:

true if the queue has no elements

See Also:

Queue.isEmpty()

dequeue

public java.lang.Object dequeue()

Description copied from interface: Queue

remove an entry from the start of the queue

Specified by:

dequeue in interface Queue

Returns:

the object

See Also:

Queue.dequeue()

backingUpdate

protected void backingUpdate()

discardBacking

protected void discardBacking()

disconnect

public void disconnect()

Release any file-handles in arecoverable way.

canDiscardBacking

protected boolean canDiscardBacking()

Returns:

True if can discard backing file.

fillHeadQ

protected void fillHeadQ()

backingDequeue

protected java.lang.Object backingDequeue()

Returns:

backing dequeue queue instance.

headTargetSize

protected int headTargetSize()

Returns:

Maximum size of head queue.

length

public long length()

Description copied from interface: Queue

get the number of elements in the queue

Specified by:

length in interface Queue

Returns:

the number of elements in the queue

See Also:

Queue.length()

release

public void release()

Description copied from interface: Queue

release any OS/IO resources associated with Queue

Specified by:

release in interface Queue

See Also:

Queue.release()

peek

public java.lang.Object peek()

Description copied from interface: Queue

Give the top object in the queue, leaving it in place to be returned by future peek() or dequeue() invocations.

Specified by:

peek in interface Queue

Returns:

top object, without removing it

See Also:

Queue.peek()

unpeek

public void unpeek()

Description copied from interface: Queue

Releases queue from the obligation to return in the next peek()/dequeue() the same object as returned by any previous peek().

Specified by:

unpeek in interface Queue

getIterator

public java.util.Iterator getIterator(boolean inCacheOnly)

Description copied from interface: Queue

Returns an iterator for the queue.

The returned iterator's remove method is considered unsafe.

Editing the queue while using the iterator is not safe.

Specified by:

getIterator in interface Queue

Returns:

an iterator for the queue

See Also:

Queue.getIterator(boolean)

deleteMatchedItems

public long deleteMatchedItems(org.apache.commons.collections.Predicate matcher)

Description copied from interface: Queue

All objects in the queue where matcher.match(object) returns true will be deleted from the queue.

Making other changes to the queue while this method is being processed is not safe.

Specified by:

deleteMatchedItems in interface Queue

Parameters:

matcher - a predicate

Returns:

the number of deleted items

See Also:

Queue.deleteMatchedItems(org.apache.commons.collections.Predicate)

setHeadMax

public void setHeadMax(int hm)

Set the maximum number of items to keep in memory at the structure's top. If more than that number are already in memory, they will remain in memory until dequeued, and thereafter the max will not be exceeded.

Parameters:

hm - Maximum number of items to keep in memory.