Java Reference
In-Depth Information
Channels
Channels
were introduced in the 1.4 release of Java to provide a faster capability for input and output
operations with files, network sockets, and piped I/O operations between programs than the methods
provided by the stream classes. We will only be discussing channels in the context of files. The channel
mechanism can take advantage of buffering and other capabilities of the underlying operating system
and therefore is considerably more efficient than using the operations provided directly within the file
stream classes. As we said earlier, a channel transfers data between a file and one or more buffers. We
will take a quick look at the overall relationship between the various classes that define channels and
buffers, and then look into the details of how you use channels with file streams.
There are a considerable number of classes and interfaces defining both channels and buffers. They also
have similar names such as
ByteBuffer
and
ByteChannel
. Of course,
File
and file stream objects
are also involved in file I/O operations so you will be using at least four different types of objects
working together when you read from or write to files. Just to clarify what they all do, here's a summary
of the essential role of each of them in file operations:
A
File
object encapsulates a path to a file or a directory, and such an object encapsulating a
file path can be used to construct a file stream object.
❑
A
FileInputStream
object encapsulates a file that can be read by a channel. A
FileOutputstream
object encapsulates a file that can be written by a channel. As we will
see in the next chapter, a
RandomAccessFile
object can encapsulate a file that can be both
read and written by a channel.
❑
A buffer just holds data in memory. You load into a buffer what is to be written to a file using
the buffer's
put()
methods. You use a buffer's
get()
methods to retrieve data that has been
read from a file.
❑
You obtain a
FileChannel
object from a file stream object or a
RandomAccessFile
object. A
FileChannel
object can read and write a file using
read()
and
write()
methods with a buffer or buffers as the source or destination of the data.
❑
The channel interfaces and classes that we will be using are in the
java.nio.channels
package. The
classes that define buffers are defined in the
java.nio
package. In a program that reads or writes files
we will therefore need
import
statements for class names from at least three packages, the two
packages we have just introduced plus the
java.io
package.
Channel Operations
There are a series of channel interfaces, each of which declares a set of one or more related operations that a
channel may perform. They all extend a common interface,
Channel
, which declares two methods:
The
close()
method that closes a channel
❑
The
isOpen()
method that tests the state of the channel, returning
true
if it is open and
false
otherwise
❑
Note that closing a channel does not necessarily close the file that the channel is attached to but closing a file
also closes its channel. The channel interfaces are related as illustrated in the hierarchy shown below:
