Java NIO Overview

Java NIO consist of the following core components:

Channels
Buffers
Selectors

Java NIO has more classes and components than these, but the

Channel

,

Buffer

and

Selector

forms the core of the API, in my opinion. The rest of the components, like

Pipe

and

FileLock

are merely
utility classes to be used in conjunction with the three core components. Therefore, I'll focus on these three components in this NIO overview. The other components are explained in their own texts elsewhere in this tutorial. See the menu at the top corner
of this page.

Channels and Buffers

Typically, all IO in NIO starts with a

Channel

. A

Channel

is a bit like a stream. From the

Channel

data can be read into a

Buffer

. Data can also be written from a

Buffer

into a

Channel

.
Here is an illustration of that:

Java NIO: Channels read data into Buffers, and Buffers write data into Channels

There are several

Channel

and

Buffer

types. Here is a list of the primary

Channel

implementations in Java NIO:

FileChannel
DatagramChannel
SocketChannel
ServerSocketChannel

As you can see, these channels cover UDP + TCP network IO, and file IO.

There are a few interesting interfaces accompanying these classes too, but I'll keep them out of this Java NIO overview for simplicity's sake. They'll be explained where relevant, in other texts of this Java NIO tutorial.

Here is a list of the core

Buffer

implementations in Java NIO:

ByteBuffer
CharBuffer
DoubleBuffer
FloatBuffer
IntBuffer
LongBuffer
ShortBuffer

These

Buffer

's cover the basic data types that you can send via IO: byte, short, int, long, float, double and characters.

Java NIO also has a

MappedByteBuffer

which is used in conjunction with memory mapped files. I'll leave this

Buffer

out of this overview though.

Selectors

A

Selector

allows a single thread to handle multiple

Channel

's. This is handy if your application has many connections (Channels) open, but only has low traffic on each connection. For instance, in a chat server.

Here is an illustration of a thread using a

Selector

to handle 3

Channel

's:

Java NIO: A Thread uses a Selector to handle 3 Channel's

To use a

Selector

you register the

Channel

's with it. Then you call it's

select()

method. This method will block until there is an event ready for one of the registered channels. Once the method returns, the thread can
then process these events. Examples of events are incoming connection, data received etc.