Java NIO - Channels

Channels are the other most important part of the NIO API.

A channel is a communication channel, that is a pipe of data.

The Channel interface has two methods: isOpen() and close() ( the latter may throws an IOException). Most Channels are specified using Interfaces. For example, there is the InterruptibleChannel interface which is a marker interface specifying how the channel behaves when a thread accessing it is interrupted.

There are different types of Channels.

• ReadableByteChannel and WritableByteChannel (Interfaces which define how to read from or write in a channel using a ByteBuffer)
• GatheringByteChannel and ScatteringByteChannel (interfaces defining methods to read from or write to a channel using an array of ByteBuffers)
• InterruptibleChannel ( interface defining the behaviour of the channel when the thread using it is interrupted)
• FileChannel (class for the interaction with a Channel based on a File)
• SocketChannel and ServerSocketChannel, and DatagramChannel ( classes for the communication based on sockets)

Channels cannot be opened directly. The method used to create a Channel object depends on the type of Channels. For example FileChannel are opened using Streams or RandomAccessFiles.

ChannelsUtilityClass There is a Utility Class called java.nio.channels.Channels, which has a number of utility methods to convert between the old and the new API. There are two Channel creating methods: newChannel(InputStream) and newChannel(OutputStream) which return a ReadableByteChannel and a WritableByteChannel respectively, and there are the newInputStream(ReadableByteChannel ch) and newOutputStream(WriteableByteChannel ch) which return InputStream and OutputStream respectively. The same kind of methods exist also for Readers and Writers: newReader and newWriter methods return the appropriate classes.

FileChannel FileChannel have different facets. Using the utility class Channels we can open FileChannel instances. Note that Channels opened from InputStreams are only readable while Channels from OutputStreams are only writable.

A number of methods are important for FileChannels. The force() methods writes to the disk the data which is actually cached ( for performance reasons). The truncate() methods gives a way to cut off all the data which is further than a given data. The methods position() and positions(long positions) are methods to decide at which part of the file the Channel should go.

Pipe Another important class is the Pipe class in general it is a conduit through which data can only go in one direction. Each pipe has a SourceChannel and a SinkChannel. The information written in the SinkChannel (a WritableChannel) is then transmitted to the SourceChannel (a ReadableChannel).

MemoryMappedFiles This type of Buffer and Channels are a far reaching topic so we treat them separately. SocketChannels SocketChannels are also a separate topic so we treat them also separately. Locks The topics of locks will also be covered in its own entry.