Java The Complete Reference, Seventh Edition

Channels

Channels are defined injava.nio.channels. A channel represents an open connection to an

I/O source or destination. You obtain a channel by callinggetChannel( )on an object that

supports channels. For example,getChannel( )is supported by following I/O classes.

DatagramSocket FileInputStream FileOutputStream

RandomAccessFile Ser verSocket Socket

Thus, to obtain a channel, you first obtain an object of one of these classes and then call

getChannel( )on that object.

The specific type of channel returned depends upon the type of objectgetChannel( )

is called on. For example, when called on aFileInputStream,FileOutputStream,or

RandomAccessFile,getChannel( )returns a channel of typeFileChannel. When called

on aSocket,getChannel( )returns aSocketChannel.

Channels such asFileChannelandSocketChannelsupport variousread( )andwrite( )

methods that enable you to perform I/O operations through the channel. For example,

here are a few of theread( )andwrite( )methods defined forFileChannel. All can throw

anIOException.

Method Description

abstract int read(ByteBufferbb) Reads bytes from the invoking channel intobbuntil the buffer

is full or there is no more input. Returns the number of bytes

actually read.

abstract int read(ByteBufferbb,

longstart)

Beginning at the file location specified bystart,reads bytes

from the invoking channel intobbuntil the buffer is full or

there is no more input. The current position is unchanged.

Returns the number of bytes actually read or –1 ifstartis

beyond the end of the file.

abstract int write(ByteBufferbb) Writes the contents ofbbto the invoking channel, starting

at the current position. Returns the number of bytes written.

abstract int write(ByteBufferbb,

longstart)

Beginning at the file location specified bystart,writes the

contents ofbbto the invoking channel. The current position

is unchanged. Returns the number of bytes written.

All channels support additional methods that give you access to and control over the

channel. For example,FileChannelsupports methods to get or set the current position,

transfer information between file channels, obtain the current size of the channel, and lock

the channel, among others.FileChannelalso provides themap( )method, which lets you

map a file to a buffer.

Charsets and Selectors

Two other entities used by NIO are charsets and selectors. Acharsetdefines the way that

bytes are mapped to characters. You can encode a sequence of characters into bytes using

anencoder.You can decode a sequence of bytes into characters using adecoder.Charsets,

encoders, and decoders are supported by classes defined in thejava.nio.charsetpackage.

818 Part II: The Java Library