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Java I/O history from JSE1.0 – JSE7

I/O in Java 1.1

NIO (JSR-51) in JDK1.4

NIO.2 (JSR-203) in JSE7

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Comparison of I/O features in Releases

File System part of the NIO.2 is platform specific

File tree walk

File Operations (Copy, Delete, Move, etc.)

Symbolic links support

Support for file attributes in NIO.2

Support for handling file permissions

Java 7

All File System were treated the same

No API for walking over the file system

No API for file operations

No support for symbolic links

No support for reading and writing attributes

No support for dealing with permissions

Before Java 7

What NIO.2 providesCurrent lacking IO features

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Comparison of I/O features in Releases

Proper Exception handling

Support for change notification

Provider SPI to add new file system support

Asynchronous I/O

What NIO.2 provides

Java 7

No proper exception handling

No watch API to handle file system changes

No SPI for adding support for new file systems

No Asynchronous I/O support

Current lacking IO features

Before Java 7

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The NIO.2 Architecture

Classes, interfaces reside in java.nioNew set of main classes, interfaces

FileSystems factory, FileSysteminterface: The file system doorway

Path: The object representing an existing or non-existing file or directory in the file system.FileStore: Represents underlying storage devices, partitions and so

on.WatchService: Watching a directory or a file in the file systemAttributes and related classes:

Reading and writing attributes

Permissions and related classes: Reading and writing permissionsetc

A FileSystem

A FileSystemsProvider

WatchService

Attibutes

Path

FileStore

Etc

Through FileSystems

Factory class

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How the NIO.2 works

FileSystem fs = FileSystems.getDefault();

Path p = fs.getPath("/home/abujari/someFile.txt");

try {

Files.deleteIfExists(p);

} catch (IOException ex) {

//handle it

}

Basically everything start with FileSystems and

FileSystem object

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Basic File operations with Path

Copying and moving Files

Attributes state after copying / moving; creating files or directories.

Overwriting the target file

Move can be atomic

FileSystem fs = FileSystems.getDefault();

Path p = fs.getPath("/home/abujari/simple.txt");

Path newCopy = fs.getPath("/home/masoud/copy.txt");

p.copyTo(newCopy,

StandardCopyOption.COPY_ATTRIBUTES,

StandardCopyOption.REPLACE_EXISTING);

newCopy.moveTo(fs.getPath("/home/abujari/moved.txt"),

StandardCopyOption.ATOMIC_MOVE);

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Symbolic links support

• Support for

• Creating and Resolving Symbolic Links

• Creating hard links

FileSystem fs = FileSystems.getDefault();

Path actualFile = fs.getPath("/home/masoud/simple");

//creating a symbolic link

Path symLinkFile = fs.getPath("/home/masoud/simple.sym");

symLinkFile.createSymbolicLink(actualFile);

System.out.println("Target: "+symLinkFile.readSymbolicLink());

//Creating hard link

Path linkFile = fs.getPath("/home/masoud/simple.link");

linkFile.createLink(actualFile);

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Walking The File System Tree

• Files.walkFileTree

* Walks a file tree from a given starting path down to the given depth

* Invoke FileVisitor methods for each file/directory

PreVisitDirectory PreVisitDirectoryFailed VisitFile

VisitFileFailed postVisitDirectory

public class SimpleVisitor {

public static void main(String args[]) throws IOException {

FileSystem fs = FileSystems.getDefault();Path p = fs.getPath("/home/abujari/");MyVisit v = new MyVisit();Files.walkFileTree(p, EnumSet.allOf(FileVisitOption.class), 2, v);

}}

class MyVisit extends SimpleFileVisitor<Path> {

@Overridepublic FileVisitResult visitFile(Path file, BasicFileAttributes attrs)throws IOException {

System.out.println(file.getFileName());return super.visitFile(file, attrs);

}}

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• Reading and Writing File attributes

• Different views for different file systems

– Dos View: DosFileAttributeView

– Posix view: PosixFileAttributeView

– ACL view: AclFileAttributeView

– User defined attributes view: UserDefinedFileAttributeView

– Basic view: BasicFileAttributeView: Common for All FS

• Each view has a corresponding *FileAttributes object holding the attributes for a file system entry.

• Checking for supported attributes using FileSystem.supportedFileAttributeViews()

File attributes and metadata

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File attributes and metadata: basic

attributes

BasicFileAttributes: Common to all FS and contains

basic attributes. Creation Time Last Access Time Modification Time SizeFile Type: isDirectory,IsFile, IsSymbolicLink,IsOtherFile Key

FileSystem fs = FileSystems.getDefault(); Path p = fs.getPath("/home/abujari/simple");

BasicFileAttributeView bav= Files.getFileAttributeView(p, BasicFileAttributeView.class,LinkOption.NOFOLLOW_LINKS);

BasicFileAttributes ba =bav.readAttributes();

System.out.println(p.toString() + " last access: " + ba.lastAccessTime()); System.out.println(p.toString() + " last modified: " + ba.lastModifiedTime());

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File attributes and metadata: Platform

specific Attributes

PosixFileAttributeView Extends BasicFileAttributeView, contain Posix specificattributes.

All Basic attributes Retrieving group Retrieving owner Retrieving permission

FileSystem fs = FileSystems.getDefault();Path p = fs.getPath("/home/masoud/photo.png");

Set<String> supportedViews = fs.supportedFileAttributeViews();

if (supportedViews.contains("unix")) {

PosixFileAttributes pat = Attributes.readPosixFileAttributes(p, LinkOption.NOFOLLOW_LINKS);

System.out.println(pat.group().getName()); System.out.println(pat.owner().

}

basic unix dos acl owner

user etc.

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• Using Attributes utility class methods:

• Attributes.readBasicFileAttributes(...);

• Attributes.readDosFileAttributes(...);

• Attributes.readPosixFileAttributes(...)

• Bulk read using Path.readAttributes(...) method:

Map<String, ?> att = (Map<String, String>) p.readAttributes("posix:owner,basic:*");

•Using Path and AttributeViews:

Path.getFileAttributeView(Class<V> type, LinkOption...

options)

Different ways for retrieving file attributes

Comma separated list of [View name: Attribute name] Default view is Basic View.For example:basic:lastModifiedTime

basic:size

posix:group

dos:hidden

posix:*

fileKey → with no view name

BasicFileAttributeView DosFileAttributeView

PosixFileAttributeView

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File attributes and metadata:

•

•

•

Managing POSIX files permissions

PosixFilePermission: A posix permission

PosixFilePermissions: Utility class to convert permissions

from between OS notation and a set of PosixFilePermission

Attributes: To read file attributes

PosixFileAttributes: includes posix file permissions

FileSystem fs = FileSystems.getDefault();

Path p = fs.getPath("/home/abujari/script.sh");

PosixFileAttributes patts = Attributes.readPosixFileAttributes(p, LinkOption.NOFOLLOW_LINKS);Set<PosixFilePermission> st = patts.permissions();

System.out.println(PosixFilePermissions.toString(st));

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FileStore features

•The FileStore: Represent the file system

underlying storage devices, pools, partitions, and

so on.

FileSystem fs = FileSystems.getFileSystem(new URI("File:///"));

Path p = fs.getPath("/home/abujari");

FileStore fstore = p.getFileStore();

FileStoreSpaceAttributes attrs = Attributes.readFileStoreSpaceAttributes(fstore); long total = attrs.totalSpace() / (1024*1024);long used = (attrs.totalSpace() - attrs.unallocatedSpace()) / (1024*1024); long avail = attrs.usableSpace() /(1024*1024);System.out.println(" device size(MB)used(MB) available(MB) "); System.out.format("%-20s %12d %12d %12d%n", fstore, total, used, avail);

We can use FileSystem.getFileStores() to get all underlying file stores.

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File system change notification

• How does it works:

• Use underlying FS facilities when available.

• Retreat to rudimentary polling if underlying FS does not provide change notification.

• API consists of four elements:

• Watchable: Path implements Watchable interface so the watch service works

over it.

• A set of event types: whether we want to receive creation, deletion, … events. In our case we will use StandardWatchEventKind which implements the WatchEvent.Kind<T>.

• The Watcher: This is the entity watching watchable. In our case the watcher (

java.nio.file.WatchService) watches the File System for changes.

• Watch Key:

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Change notification: Sample Code

public static void main(String args[]) {FileSystem fs = FileSystems.getDefault(); WatchService ws = fs.newWatchService();

Path path = fs.getPath("/home/abujari/Pictures"); path.register(ws, StandardWatchEventKinds.ENTRY_CREATE,

StandardWatchEventKinds.ENTRY_DELETE);

WatchKey key = ws.take(); //wait!List<WatchEvent<?>> events = key.pollEvents();

for (WatchEvent object : events) {if (object.kind() == StandardWatchEventKinds.ENTRY_DELETE)

System.out.println("Delete: " +path.getFileName()+"/"+ object.context().toString());

if (object.kind() == StandardWatchEventKinds.ENTRY_CREATE) System.out.println("Created: " +path.getFileName()+"/"+

object.context().toString());}

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(Non-)Blocking and Asynchronous I/O

Prior to Java SE 1.4 : Everything was blocking and and use of threads was inevitable. ( which causes scalability issues and performance overhead)

Java SE 1.4: Brings the JSR-51 or NIO with non-blocking IO; Introduction of channels and selectors. Non-Blocking not Asynchronous.

Java SE 7: Addition of asynchronous IO to the pack; included inside java.nio.channels

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Asynchronous I/O for sockets and Files

•What is Asynchronous I/O

• No blocking, no quick pass over the buffer, simple manual

check backs or call back handlers (completion handlers).

•Why Asynchronous I/O

• More scalability, benefiting from underlying OS IO features.

•How Asynchronous I/O works

• Using java.util.concurrent.Future: Manual check

backs.

• Using java.nio.channels.CompletionHandler:

Callback handler for completion or failure of IO operation

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AsynchronousChannel

AsynchronousByteChannel

AsynchronousFileChannel

Asynchronous read/writeNo global file position/offset

Each read/write specifies position in file Access different parts of file concurrently

Open method specifies options: READ, WRITE, APPEND...

AsynchronousSocketChannel

Asynchronous connectAsynchronous read/writeAsynchronous scatter/gather (multiple buffers)Read/write operations support timeout: failed method invoked with timeout exceptionImplements NetworkChannel for binding, setting socket options, etc.

AsynchronousServerSocketChannel

Asynchronous accept handler invoked when connection accepted Result is AsynchronousSocketConnection

Implements NetworkChannel for binding, setting socket options, etc.

AsynchronousDatagramChannel

Asynchronous read/write (connected)•Asynchronous receive/send (unconnected)

Implements NetworkChannel for binding, setting socket options, etc.

Implements MulticastChannel to join multicast groups

Different types of Async. Channels