Java NIO (1) Java NIO Overview

Java NIO is grouped by the following core parts:

• Channels

• Buffers

• Selectors

Although there are many classes and components in Java NIO, in my opinion, Channel, Buffer and Selector constitute the core The API. Other components, such as Pipe and FileLock, are just tool classes used with the three core components. Therefore, in this overview I will focus on these three components. Other components are covered in separate chapters.

Channel and Buffer

Basically, all IO starts from a Channel in NIO. Channels are a bit like streams. Data can be read from the Channel to the Buffer, or written from the Buffer to the Channel. Here is an illustration:

There are several types of Channel and Buffer. The following are the implementations of some major Channels in JAVA NIO:

• FileChannel

• DatagramChannel

• SocketChannel

• ServerSocketChannel

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

Along with these classes are some interesting interfaces, but for the sake of simplicity, I tried not to mention them in the overview. I will explain them in other chapters of this tutorial where they are relevant.

The following is the key Buffer implementation in Java NIO:

• ByteBuffer

• CharBuffer

• DoubleBuffer

• FloatBuffer

• IntBuffer

• LongBuffer

• ShortBuffer

These Buffers cover the basic data types that you can send via IO: byte, short, int, long, float, double and char.

Java NIO also has a MappedByteBuffer, which is used to represent memory mapped files. I am not going to explain it in the overview.

Selector

Selector allows a single thread to process multiple Channels. If your application opens multiple connections (channels), but the traffic of each connection is very low, using Selector can be convenient. For example, in a chat server.

This is an illustration of using a Selector to process three Channels in a single thread:

To use the Selector, you must register the Channel with the Selector, and then Call its select() method. This method will block until a registered channel has an event ready. Once this method returns, the thread can handle these events. Examples of events include new connections coming in, data receiving, etc.

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