Non-blocking I/O (Java)
java.nio (NIO stands for Non-blocking 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 by Sun Microsystems to complement an existing standard I/O. NIO was developed under the Java Community Process as JSR 51. An extension to NIO that offers a new file system API, called NIO.2, was released with Java SE 7 ("Dolphin").
Features and organization
The APIs of NIO were designed to provide access to the low-level I/O operations of modern operating systems. Although the APIs are themselves relatively high-level, the intent is to facilitate an implementation that can directly use the most efficient operations of the underlying platform.
The Java NIO APIs are provided in the
java.nio package and its subpackages. The documentation by Oracle identifies these features.
- Buffers for data of primitive types
- Character set encoders and decoders
- A pattern-matching facility based on Perl-style regular expressions (in package
- Channels, a new primitive I/O abstraction
- A file interface that supports locks and memory mapping of files up to
Integer.MAX_VALUEbytes (2 GiB)
- A multiplexed, non-blocking I/O facility for writing scalable servers
NIO data transfer is based on buffers (
java.nio.Buffer and related classes). These classes represent a contiguous extent of memory, together with a small number of data transfer operations. Although theoretically these are general-purpose data structures, the implementation may select memory for alignment or paging characteristics, which are not otherwise accessible in Java. Typically, this would be used to allow the buffer contents to occupy the same physical memory used by the underlying operating system for its native I/O operations, thus allowing the most direct transfer mechanism, and eliminating the need for any additional copying. In most operating systems, provided the particular area of memory has the right properties, transfer can take place without using the CPU at all. The NIO buffer is intentionally limited in features in order to support these goals.
There are buffer classes for all of Java's primitive types except
boolean, which can share memory with byte buffers and allow arbitrary interpretation of the underlying bytes.
NIO buffers maintain several pointers that dictate the function of their accessor methods. The NIO buffer implementation contains a rich set of methods for modifying these pointers:
flip()method, rather than performing a "flip" or paging function in the canonical sense, moves the position pointer to the origin of the underlying array (if any) and the limit pointer to the former position of the position pointer.
get()methods are supplied for transferring data out of a NIO buffer. The bulk implementation, rather than performing a "get" in the traditional sense, "puts" the data into a specified array. The "offset" argument supplied to this method refers not to the offset from within the buffer from which to read, nor an offset from the position pointer, but rather the offset from 0 within the target array.
- Unless using the absolute
put()is conducted from the position pointer. Should one need to read from a different position within the underlying array, whilst not adjusting the writing position, the
reset()methods have been supplied.
mark()method effectively stores the position of the position pointer by setting the mark pointer to the position of the position pointer. The
reset()method causes the position pointer to move to the mark pointer's position.
- Upon invocation of the
clear()method or the
flip()method the mark pointer is discarded.
clear()method does not ensure zero-ing of the buffer, but does return the limit pointer to the upper boundary of the underlying array, and the position pointer to zero.
get()operations for NIO buffers are not thread safe.
- You can only
Integer.MAX_VALUEin size (2GiB); regions beyond this limit can be accessed using an offset greater than zero.
Channels (classes implementing the interface
java.nio.channels.Channel) are designed to provide for bulk data transfers to and from NIO buffers. This is a low-level data transfer mechanism that exists in parallel with the classes of the higher-level I/O library (packages
java.net). A channel implementation can be obtained from a high-level data transfer class such as
java.net.Socket, and vice versa. Channels are analogous to "file descriptors" found in Unix-like operating systems.
File channels (
java.nio.channels.FileChannel) can use arbitrary buffers but can also establish a buffer directly mapped to file contents using memory-mapped file. They can also interact with file system locks. Similarly, socket channels (
java.nio.channels.ServerSocketChannel) allow for data transfer between sockets and NIO buffers.
FileChannel can be used to do a file copy, which is potentially far more efficient than using old read/write with a byte array. The typical code for this is:
// Getting file channels FileChannel in = new FileInputStream(source).getChannel(); FileChannel out = new FileOutputStream(target).getChannel(); // JavaVM does its best to do this as native I/O operations. in.transferTo(0, in.size(), out); // Closing file channels will close corresponding stream objects as well. out.close(); in.close();
A selector (
java.nio.channels.Selector and subclasses) provides a mechanism for waiting on channels and recognizing when one or more become available for data transfer. When a number of channels are registered with the selector, it enables blocking of the program flow until at least one channel is ready for use, or until an interruption condition occurs.
Although this multiplexing behavior could be implemented with threads, the selector can provide a significantly more efficient implementation using lower-level operating system constructs. A POSIX-compliant operating system, for example, would have direct representations of these concepts, select(). A notable application of this design would be the common paradigm in server software which involves simultaneously waiting for responses on a number of sessions.
In Java, a character set is a mapping between Unicode characters (or a subset of them) and bytes.
java.nio.charset package of NIO provides facilities for identifying character sets and providing encoding and decoding algorithms for new mappings.
It is unexpected that a Channel associated with a Java IO RandomAccess file closes the file descriptor on an interrupt, whereas RandomAccessFiles own read method does not do this. 
JDK 7 and NIO.2
This section needs expansion. You can help by adding to it. (December 2009)
JDK 7 includes a
java.nio.file package which, with the
Path class (also new to JDK 7), among other features, provides extended capabilities for filesystem tasks, e.g. can work with symbolic/hard links and dump big directory listings into buffers more quickly than the old File class does.
java.nio.file package and its related package,
java.nio.file.attribute, provide comprehensive support for file I/O and for accessing the file system. A zip file system provider is also available in JDK 7.
- "Enhancements in Java I/O". Oracle Java Documentation. Retrieved 8 June 2017.
- "JSR 51: New I/O APIs for the JavaTM Platform". The Java Community Process(SM) Program - JSRs: Java Specification Requests. Retrieved 2009-05-23.
- "This JSR will be delivered as part of Java SE 7 "Dolphin"." "JSR 203: More New I/O APIs for the JavaTM Platform ("NIO.2")". The Java Community Process(SM) Program - JSRs: Java Specification Requests. 2006-01-30. Retrieved 2009-05-23.
- Don't Assume You Know What's Best, Shawn Pearce, author of Gerrit (Software), 2010-05-14.
- Java SE 10 API Javadocs
- JDK 5.0 New I/O-related APIs & Developer Guides - description of New I/O by Oracle Corporation
- Learn Java SE 7 | NIO 2 Features
- JSR 51 (NIO)
- JSR 203 (NIO.2)
- Architecture of a Highly Scalable NIO-Based Server - discussion on Java NIO and patterns of usage
- Java NIO tutorial
- The Rox Java NIO Tutorial
- Scalable IO in Java (PDF)