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Getting started with new I/O (NIO) Page 5 of 25
In this section, we'll learn a little bit about reading and writing data in Java programs. We'll go
over the main components of NIO (buffers, channels, and some related methods) and see how
they interact for reading and writing. In the sections that follow, we will look at each of these
components and interactions in more detail.
Reading from a file
For our first exercise, we'll read some data from a file. If we were using original I/O, we would
simply create a FileInputStream and read from that. In NIO, however, things work a little
differently: we first get a Channel object from the FileInputStream, and then use that channel to
read the data.
Any time you perform a read operation in an NIO system, you are reading from a channel, but you
don't read directly from a channel. Since all data ultimately resides in the buffer, you read from a
channel into a buffer.
So reading from a file involves three steps: (1) getting the Channel from FileInputStream; (2)
creating the Buffer; and (3) reading from the Channel into the Buffer.
Now, let's see how this works.
Three easy steps
Our first step is to get a channel. We get the channel from the FileInputStream:
FileInputStream fin = new FileInputStream( "readandshow.txt" );
FileChannel fc = fin.getChannel();
The next step is to create a buffer:
ByteBuffer buffer = ByteBuffer.allocate( 1024 );
And, finally, we need to read from the channel into the buffer, as shown here:
fc.read( buffer );
You'll notice that we didn't need to tell the channel how much to read into the buffer. Each buffer
has a sophisticated internal accounting system that keeps track of how much data has been read
and how much room there is for more data. We'll talk more about the buffer accounting system in
the Buffer internals overview.
Writing to a file
Writing to a file in NIO is similar to reading from one. We start by getting a channel from a
FileOutputStream:
FileOutputStream fout = new FileOutputStream( "writesomebytes.txt" );
FileChannel fc = fout.getChannel();
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