Thursday, October 23, 2014

NIO (New Input/Output) vs IO (Input/Output) and NIO.2 in Java

Non-blocking I/O (usually called NIO, and sometimes called "New I/O") is a collection of Java programming language APIs that offer features for intensive I/O operations. It was introduced with the J2SE 1.4 release of Java to complement an existing standard I/O. An extension to NIO that offers a new file system API, called NIO2, was released with Java SE 7. In this article, we understand the concept of NIO and how is it useful as compare to IO (java.io.*). 




What Is Input/Output
Input/output (I/O) deals with reading data from a source and writing data to a destination. Data is read from the input source (or simply input) and written to the output destination (or simply output). For example, your keyboard works as a standard input, letting you read data entered using the keyboard into your program. You have been using the System.out.println() method to print text on the standard output from the very first Java program without your knowledge that you have been performing I/O.
The java.io and java.nio (nio stands for New I/O) packages contain Java classes that deal with I/O. The java.io package has an overwhelming number of classes to perform I/O. It makes learning Java I/O a little complex. The situation where the number of classes increases to an unmanageable extent is called a class explosion and the java.io package is a good example of that.

Input/Output Streams
The literal meaning of the word stream is "an unbroken flow of something." In Java I/O, a stream means an unbroken flow (or sequential flow) of data. The data in the stream could be bytes, characters, objects, etc.

A river is a stream of water where the water flows from a source to its destination in an unbroken sequence. Similarly, in Java I/O, the data flows from a source known as a data source to a destination known as a data sink
The data is read from a data source to a Java program. A Java program writes data to a data sink. The stream that connects a data source and a Java program is called an input stream. The stream that connects a Java program and a data sink is called an output stream
In a natural stream, such as a river, the source and the destination are connected through the continuous flow of water. However, in Java I/O, a Java program comes between an input stream and an output stream. Data flows from a data source through an input stream to a Java program. The data flows from the Java program through an output stream to a data sink. In other words, a Java program reads data from the input stream and writes data to the output stream. Following diagram the flow of data from an input stream to a Java program and from a Java program to an output stream.
How we work with input/output stream

To read data from a data source into a Java program, you need to perform the following steps:

  • Identify the data source. It may be a file, a string, an array, a network connection, etc.
  • Construct an input stream using the data source that you have identified.
  • Read the data from the input stream. Typically, you read the data in a loop until you have read all the data from the input stream. The methods of an input stream return a special value to indicate the end of the input stream.
  • Close the input stream. Note that constructing an input stream itself opens it for reading. There is no explicit step to open an input stream. However, you must close the input stream when you are done reading data from it
To write data to a data sink from a Java program, you need to perform the following steps:

  • Identify the data sink. That is, identify the destination where data will be written. It may be a file, a string, an array, a network connection, etc.
  • Construct an output stream using the data sink that you have identified.
  • Write the data to the output stream.
  • Close the output stream. Note that constructing an output stream itself opens it for writing.There is no explicit step to open an output stream. However, you must close the output stream when you are done writing data to it.

Input/output stream classes in Java are based on the decorator pattern.


What Is NIO?
The stream-based I/O uses streams to transfer data between a data source/sink and a Java program. The Java program reads from or writes to a stream a byte at a time. This approach to performing I/O operations is slow. The New Input/Ouput (NIO) solves the slow speed problem in the older stream-based I/O.
In NIO, you deal with channels and buffers for I/O operations. A channel is like a stream. It represents a connection between a data source/sink and a Java program for data transfer. 
There is one difference between a channel and a stream

  • A stream can be used for one-way data transfer. That is, an input stream can only transfer data from a data source to a Java program; an output stream can only transfer data from a Java program to a data sink. 
  • However, a channel provides a two-way data transfer facility. 

You can use a channel to read data as well as to write data. You can obtain a read-only channel, a write-only channel, or a read-write channel depending on your needs.

In stream-based I/O, the basic unit of data transfer is a byte. In channel-based NIO, the basic unit of data transfer is a buffer.
A buffer is a bounded data container. That is, a buffer has a fixed capacity that determines the upper limit of the data it may contain. In stream-based I/O, you write data directly to the stream. In channel-based I/O, you write data into a buffer; you pass that buffer to the channel, which writes the data to the data sink. Similarly, when you want to read data from a data source, you pass a buffer to a channel. The channel reads data from the data source into a buffer. You read data from the buffer. Above diagram depicts the interaction between a channel, a buffer, a data source, a data sink, and a Java program. It is evident that the most important parts in this interaction are reading from a buffer and writing into a buffer.

New Input/Output 2 (NIO.2)
Java 7 introduced New Input/Output 2 (NIO.2) API, which provides a new I/O API. It provides many features that were lacking in the original File I/O API. The features provided in NIO.2 are essential for working with a file system efficiently. It adds three packages to the Java class library: java.nio.file, java.nio.file.attribute, and java.nio.file.spi. The following are some of the new features of NIO.2:

  • It lets you deal with all file systems in a uniform way. The file system support provided by NIO.2 is extensible. You can use the default implementation for a file system or you can choose to implement your own file system.
  • It supports basic file operations (copy, move, and delete) on all file systems. It supports an atomic file move operation. It has improved exception handling support.
  • It has support for symbolic links. Whenever applicable, operations on a symbolic link are redirected to the target file.
  • One of the most important additions to NIO.2 is the support for accessing the attributes of file systems and files.
  • It lets you create a watch service to watch for any events on a directory such as adding a new file or a subdirectory, deleting a file, etc. When such an event occurs on the directory, your program receives a notification through the watch service.
  • It added an API that lets you walk through a file tree. You can perform a file operation on a node as you walk through the file tree.
  • It supports asynchronous I/O on network sockets and files.
  • It supports multicasting using a DatagramChannel.


I hope this article has covered the basic introduction to NIO2 and how it differ from IO.


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