New I/O

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New I/O, usually called NIO, 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. As of 2006, an extension to NIO, called NIO2, is being developed under JSR 203; JSR 203 is scheduled to be included in Java SE 7 ("Dolphin").

1 Features and organization
2 External links

[edit] 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 Sun Microsystems 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 java.util.regex)
Channels, a new primitive I/O abstraction
A file interface that supports locks and memory mapping
A multiplexed, non-blocking I/O facility for writing scalable servers

[edit] NIO buffers

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.

[edit] Channels

Channels (classes related to 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.io and java.net). A channel implementation can be obtained from a high-level data transfer class such as java.io.File, java.net.ServerSocket, or java.net.Socket, and vice versa.

File channels (java.nio.channels.FileChannel) can use arbitrary buffers but can also establish a buffer directly mapped to file contents using memory-mapped I/O. They can also interact with file system locks. Similarly, socket channels (java.nio.channels.SocketChannel and 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();

[edit] Selectors

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 behaviour could be implemented with Java threads, the selector can provide a significantly more efficient implementation using native platform threads or, more likely, even lower-level operating system constructs. A POSIX-compliant operating system, for example, would have direct representations of these concepts. 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.

[edit] Character sets

In Java, a character set is a mapping between Unicode characters (or a subset of them) and bytes. The java.nio.charset package of NIO provides facilities for identifying character sets and providing encoding and decoding algorithms for new mappings.

[edit] Regular expressions

The regular expression library in the java.util.regex package provides a powerful search facility for character data based on regular expression matching.

The following example was adopted from the NIO API guide examples, where there are more examples.

import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.charset.*;
import java.util.regex.*;

public class Grep {

    // Charset and decoder for ISO-8859-15
    private static Charset charset = Charset.forName("ISO-8859-15");
    private static CharsetDecoder decoder = charset.newDecoder();

    // Pattern used to parse lines
    private static Pattern linePattern = Pattern.compile(".*\r?\n");

    // The input pattern that we're looking for
    private static Pattern pattern;

    // Compile the pattern from the command line
    private static void compile(String pat) {
        try {
            pattern = Pattern.compile(pat);
        } catch (PatternSyntaxException x) {
            System.err.println(x.getMessage());
            System.exit(1);
        }
    }

    // Use the linePattern to break the given CharBuffer into lines, applying
    // the input pattern to each line to see if we have a match
    private static void grep(File f, CharBuffer cb) {
        Matcher lm = linePattern.matcher(cb);  // Line matcher
        Matcher pm = null;                     // Pattern matcher
        int lines = 0;
        while (lm.find()) {
            lines++;
            CharSequence cs = lm.group();      // The current line
            if (pm == null)
                pm = pattern.matcher(cs);
            else
                pm.reset(cs);
            if (pm.find())
                System.out.print(f + ":" + lines + ":" + cs);
            if (lm.end() == cb.limit())
                break;
        }
    }

    // Search for occurrences of the input pattern in the given file
    private static void grep(File f) throws IOException {

        // Open the file and then get a channel from the stream
        FileInputStream fis = new FileInputStream(f);
        FileChannel fc = fis.getChannel();

        // Get the file's size and then map it into memory
        int sz = (int)fc.size();
        MappedByteBuffer bb = fc.map(FileChannel.MapMode.READ_ONLY, 0, sz);

        // Decode the file into a char buffer
        CharBuffer cb = decoder.decode(bb);

        // Perform the search
        grep(f, cb);

        // Close the channel and the stream
        fc.close();
    }

    public static void main(String[] args) {
        if (args.length < 2) {
            System.err.println("Usage: java Grep pattern file...");
            return;
        }
        compile(args[0]);
        for (int i = 1; i < args.length; i++) {
            File f = new File(args[i]);
            try {
                grep(f);
            } catch (IOException x) {
                System.err.println(f + ": " + x);
            }
        }
    }
}