org.eclipse.jgit.internal.storage.dfs

Interface ReadableChannel

All Superinterfaces:

AutoCloseable, Channel, Closeable, ReadableByteChannel

public interface ReadableChannel
extends ReadableByteChannel

Readable random access byte channel from a file.

Method Summary

Methods

Modifier and Type	Method and Description
int	blockSize() Get the recommended alignment for reads.
long	position() Get the current position of the channel.
void	position(long newPosition) Seek the current position of the channel to a new offset.
void	setReadAheadBytes(int bufferSize) Recommend the channel maintain a read-ahead buffer.
long	size() Get the total size of the channel.

Methods inherited from interface java.nio.channels.ReadableByteChannel

read

Methods inherited from interface java.nio.channels.Channel

close, isOpen

Method Detail

position

long position()
              throws IOException

Get the current position of the channel.

Returns:

r current offset.

Throws:

IOException - the channel's current position cannot be obtained.

position

void position(long newPosition)
              throws IOException

Seek the current position of the channel to a new offset.

Parameters:

newPosition - position to move the channel to. The next read will start from here. This should be a multiple of the blockSize().

Throws:

IOException - the position cannot be updated. This may be because the channel only supports block aligned IO and the current position is not block aligned.

size

long size()
          throws IOException

Get the total size of the channel.

Prior to reading from a channel the size might not yet be known. Implementors may return -1 until after the first read method call. Once a read has been completed, the underlying file size should be available.

Returns:

r total size of the channel; -1 if not yet available.

Throws:

IOException - the size cannot be determined.

blockSize

int blockSize()

Get the recommended alignment for reads.

Starting a read at multiples of the blockSize is more efficient than starting a read at any other position. If 0 or -1 the channel does not have any specific block size recommendation.

Channels should not recommend large block sizes. Sizes up to 1-4 MiB may be reasonable, but sizes above that may be horribly inefficient. The DfsBlockCache favors the alignment suggested by the channel rather than the configured size under the assumption that reads are very expensive and the channel knows what size is best to access it with.

Returns:

recommended alignment size for randomly positioned reads. Does not need to be a power of 2.

setReadAheadBytes

void setReadAheadBytes(int bufferSize)
                       throws IOException

Recommend the channel maintain a read-ahead buffer.

A read-ahead buffer of approximately bufferSize in bytes may be allocated and used by the channel to smooth out latency for read.

Callers can continue to read in smaller than bufferSize chunks. With read-ahead buffering enabled read latency may fluctuate in a pattern of one slower read followed by (bufferSize / readSize) - 1 fast reads satisfied by the read-ahead buffer. When summed up overall time to read the same contiguous range should be lower than if read-ahead was not enabled, as the implementation can combine reads to increase throughput.

To avoid unnecessary IO callers should only enable read-ahead if the majority of the channel will be accessed in order.

Implementations may chose to read-ahead using asynchronous APIs or background threads, or may simply aggregate reads using a buffer.

This read ahead stays in effect until the channel is closed or the buffer size is set to 0.

Parameters:

bufferSize - requested size of the read ahead buffer, in bytes.

Throws:

IOException - if the read ahead cannot be adjusted.