uk.ac.starlink.table.storage

Class AdaptiveByteStore

java.lang.Object

uk.ac.starlink.table.storage.AdaptiveByteStore

All Implemented Interfaces:

ByteStore

public class AdaptiveByteStore
extends Object
implements ByteStore

ByteStore which adopts a hybrid approach between use of memory and use of disk. Bytes are written into an array in memory up to a given size limit; if the amount written exceeds this limit, it's all put in a temporary file instead.

This class is intended to be a general purpose StoragePolicy implementation that does something sensible most of the time. The details of the implementation may be changed following experience.

The current implementation uses ByteBuffer.allocateDirect(int) for byte arrays in memory apart from rather small ones. On most OSes this corresponds to using malloc(), thus avoiding heavy use of JVM heap memory. Note very large arrays are still stored on disk, not directly allocated.

Since:

5 Nov 2009

Author:

Mark Taylor

Constructor Summary

Constructors

Constructor and Description
AdaptiveByteStore() Constructs a new store with a default memory limit.
AdaptiveByteStore(int memLimit) Constructs a new store with a given maximum memory limit.

Method Summary

Modifier and Type	Method and Description
void	close() Tidies up.
void	copy(OutputStream out) Takes all the data written so far into this store's sink stream and copies it to a destination stream.
static int	getDefaultLimit() Calculates the default memory limit used by this class.
long	getLength() Returns the number of bytes currently stored in this object.
OutputStream	getOutputStream() Returns an output stream which can be used to write to the store.
ByteBuffer[]	toByteBuffers() Returns an array of byte buffers containing the bytes written to this store.

Methods inherited from class java.lang.Object

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

Constructor Detail

AdaptiveByteStore

public AdaptiveByteStore(int memLimit)
                  throws IOException

Constructs a new store with a given maximum memory limit.

Parameters:

memLimit - maximum size of in-memory buffer

Throws:

IOException

AdaptiveByteStore

public AdaptiveByteStore()
                  throws IOException

Constructs a new store with a default memory limit.

Throws:

IOException

Method Detail

getOutputStream

public OutputStream getOutputStream()

Description copied from interface: ByteStore

Returns an output stream which can be used to write to the store. May be called multiple times; always returns the same object. Note that this is not in general buffered - it is the responsibility of the user to take steps like wrapping it in a BufferedOutputStream for efficiency if required.

Specified by:

getOutputStream in interface ByteStore

Returns:

data sink stream

getLength

public long getLength()

Description copied from interface: ByteStore

Returns the number of bytes currently stored in this object.

Specified by:

getLength in interface ByteStore

Returns:

byte count

copy

public void copy(OutputStream out)
          throws IOException

Description copied from interface: ByteStore

Takes all the data written so far into this store's sink stream and copies it to a destination stream. The output stream is not closed.

Specified by:

copy in interface ByteStore

Parameters:

out - data destination stream

Throws:

IOException

toByteBuffers

public ByteBuffer[] toByteBuffers()
                           throws IOException

Description copied from interface: ByteStore

Returns an array of byte buffers containing the bytes written to this store. The stored bytes are all of the bytes from the first buffer in the returned array, followed by all in the second, etc. In many cases the returned array will, and probably should, contain a single buffer, but if the written byte count exceeds Integer.MAX_VALUE, more than one will be required. The limit of each buffer indicates the number of bytes it contains.

Usual usage will be to write all data, then call this method once; this model may affect implementation decisions about efficiency.

Specified by:

toByteBuffers in interface ByteStore

Returns:

byte buffer containing bytes written

Throws:

IOException - if there is an I/O error

close

public void close()

Description copied from interface: ByteStore

Tidies up. Should be called when the data in this object is no longer required. This object may no longer be usable following a call to this method.

Specified by:

close in interface ByteStore

getDefaultLimit

public static int getDefaultLimit()

Calculates the default memory limit used by this class.

Returns:

default memory limit