This document discusses the NIO.2 file handling API in Java. It provides an overview of some key limitations of the existing java.io.File API and then describes improvements made in NIO.2, including support for file attributes, directory streams, file walking visits, and watch services. The presentation also outlines how the new API is designed for consistency across platforms and allows for custom file system implementations via a provider interface.

1. JavaOne 2013
Clean Sweep
The File System
Manish Maheshwari
mmaheshwari@verisign.com
Anirban Mukherjee
amukherjee@verisign.com

2. Speakers
Manish
Software Architect, Verisign
Anirban
Software Architect, Verisign
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3. Why another API for File Handling
What's wrong with java.io.File?
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4. Why another API for File Handling
• Lack of Information on what’s going on
• File operations are not atomic
• No bulk access to file attributes
• Doesn’t scale for Directory Listing
• Missing basic file operations
• Copying File vs Copying File Attributes
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5. Why another API for File Handling
• Limited support for file metadata
• Limited support for Symbolic Links
• No way to extend the API for custom File System
implementations
• No support for File Notifications
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6. The NIO-2 API Design
• Informative Exceptions
• DirectoryNotEmptyException
• FileSystemLoopException
• Varargs
• Files.newBufferedWriter(path, UTF_8,
CREATE, WRITE, APPEND, DELETE_ON_CLOSE);
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7. NIO-2 API
• Packages
• java.nio.file
• java.nio.file.attribute
• java.nio.file.spi
• Path
• File System Path
• Files
• Factory of most common static
operations
• FileSystem
• Factory for objects to access
elements of File System
• FileStore
• Underlying storage, device, etc.
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8. Path
• Represents an absolute or relative path
• Iterable over Path elements
• Immutable
• Operations to:
• Access Path Elements
• Combine Paths
• Compare Paths
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9. Creating Path
Path path =
FileSystems.getDefault().getPath("/
foo");
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10. Creating Path
Path path = Paths.get("/foo");
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11. Creating Path
URI uri = URI.create("file:///foo");
Path path = Paths.get(uri);
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12. Creating Path
File file = new File("foo");
Path path = file.getPath();
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13. Access Path Elements
Path path = Paths.get("/","foo","bar","x");
Path file = path.getFileName();
Path parent = path.getParent();
Path root = path.getRoot();
Path subpath = path.subpath(1,3);
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14. Access Path Elements
Path path = Paths.get("top","foo","bar","x");
for (Path element : path ) {
print("%s%n", element);
}
top
foo
bar
x
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15. Combine Paths
Path top = Paths.get("/", "foo");
Path bar = top.resolve("bar");
Path baz = top.resolveSibling("baz");
Path qux = Paths.get("top","baz","qux");
Path rel = top.relativize(qux);
../baz/quz
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16. Test Paths
Path path = Paths.get("top", "foo");
boolean isTop = path.startsWith("to");
boolean isPot = path.endsWith("pot");
boolean isAbs = path.isAbsolute();
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17. Other Operations
Path path = …
Path normal = path.normalize();
Path abs = path.toAbsolutePath();
Path real = path.toRealPath();
URI uri = path.toURI();
File file = path.toFile();
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18. Files
• Static operations that operate on Files
• Most operations expect Path as an argument
• SymLink aware
• Accept Standard Options
• Operations for
• Reading and Writing files
• Streams, Readers, Writers
• Symbolic Links
• Working on Directory
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19. Files
• Mostly delegates to the FileSystemProvider SPI
• Throws meaningful exceptions on failure
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20. Files: Basic Operations
Path file = Files.createFile(path);
Path dir = Files.createDirectory(pathDir);
Path dir = Files.createDirectories(pathDir);
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21. Files: Basic Operations
import static java.nio.charset.StandardCharsets.*;
List<String> lines = Files.readAllLines(file,
UTF_8);
byte[] bytes = Files.readAllBytes(file);
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22. Reader and Writer
import java.nio.file.StandardOpenOption.*;
Path file = …
BufferedReader reader =
Files.newBufferedReader(file, UTF_8);
BufferedWriter writer =
Files.newBufferedWriter(file, UTF_16, CREATE_NEW);
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23. Copy and Open Options
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24. Streams
import java.nio.file.StandardOpenOption.*;
Path file = …
InputStream is = Files.newInputStream(file);
OutputStream os = Files.newOutputStream(file);
OutputStream osAppend =
Files.newOutputStream(file, CREATE, APPEND);
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25. Symbolic Links
Path link = Paths.get("top", "link");
Path target = Paths.get("top", "foo", "tar");
Files.createSymbolicLink(link, target);
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26. Symbolic Links
Path link = Paths.get("top", "link");
boolean isSymLink = Files.isSymbolicLink(link);
Path tar = Files.readSymbolicLink(link);
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27. Symbolic Links
Path p1 = …
Path p2 = …
boolean isSame = Files.isSameFile(p1, p2);
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28. File Operations
Files.delete(path);
Files.deleteIfExists(path);
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29. File Operations
Path source = …
Path target = …
Files.copy(source, target);
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30. File Operations
Path source = …
Path target = …
Files.move(source, target);
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31. File Operations
import static java.nio.file.StandardCopyOption.*;
Path source = …
Path target = …
Files.copy(source, target,
ATOMIC_MOVE, REPLACE_EXISTING, COPY_ATTRIBUTES);
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32. File Attributes
• Meta data of files
• Example
• Created, Updated, Access timestamps
• rwxr--r--
• Defined by Platform and File Systems
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33. File Attributes
• BasicFileAttributes
• Supported by all platforms
• TimeStamps, FileTypes, size, filekey
• DosFileAttributes
• Hidden, archive, readonly, system
• PosixFileAttributes
• Owner, group, other permission sets
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34. File Attributes
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35. File Attributes
BasicFileAttributes basicAttr =
Files.readAttributes(path, BasicFileAttributes.class);
String key = basicAttr.fileKey();
long size = basicAttr.size();
boolean isDirectory = basicAttr.isDirectory();
FileTime lastModified = basicAttr.lastModifiedTime();
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36. File Attributes
PosixFileAttributes posixAttr =
Files.readAttributes(path, PosixFileAttributes.class);
UserPrincipal owner = posixAttr.owner();
GroupPrincipal group = posixAttr.group();
String perms = PosixFilePermissions.toString(posixAttr.permissions()));
Files.setPosixFilePermissions(path,
PosixFilePermissions.fromString("rwxrwxrwx"));
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37. Directory Listing
• Efficient iteration of directory listings
• Scales for large directories, remote file systems, etc.
• In-built filter support
• glob
• regex
• Custom Filter
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38. Directory Listing
try
(DirectoryStream<Path> dirStream =
Files.newDirectoryStream(dir,"*c*"))
{
for (Path path : dirStream) {
print("%s%n", path.toAbsolutePath());
}
}
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39. Directory Listing
DirectoryStream.Filter<Path> filter =
new DirectoryStream.Filter<Path>() {
public boolean accept(Path entry) throws IOException {
return Files.size(entry) > 1024;
}
};
try
(DirectoryStream<Path> dirStream = Files.newDirectoryStream(dir,filter))
{
…
}
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40. Walk the File Tree
• Visitor pattern that walks the file tree
• Files.walkFileTree(start, fileVisitor)
• start: node to begin from
• fileVisitor: defines operation to perform on nodes
• Depth first traversal
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41. Walk the File Tree
• FileVisitor callbacks:
• preVisitDirectory
• visitFile
• postVisitDirectory
• visitFileFailed
• Enum FileVisitResult
• CONTINUE
• TERMINATE
• SKIP_SUBTREE
• SKIP_SIBLINGS
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42. Walk the File Tree
Path start = Paths.get("/foo/bar");
Files.walkFileTree(start, new SimpleFileVisitor<Path>(){
public FileVisitResult preVisitDirectory(Path dir,
BasicFileAttributes attrs) throws IOException {
print("preVisitDirectory: %s%n", dir);
return FileVisitResult.CONTINUE;
}
public FileVisitResult visitFile(Path file,
BasicFileAttributes attrs) throws IOException {
print("visitFile: %s%n", file);
return FileVisitResult.CONTINUE;
}
});
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43. Watch Service
• Watch a file system for changes
• Creates, update, delete
• Uses native notifications
• Falls back to polling
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44. Watch Service
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45. Watch Service Registration
WatchService watcher =
FileSystems.getDefault().newWatchService();
Path dir = …
WatchKey watchKey = dir.register(watcher, ENTRY_MODIFY);
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46. Watch Service Polling
boolean keepWatching = true;
while(keepWatching){
WatchKey key = watcher.poll(10, TimeUnit.SECONDS);
for (WatchEvent<?> event: key.pollEvents()) {
if (event.kind() == ENTRY_MODIFY) {
Path name = (Path) event.context();
…
}
}
key.reset();
}
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47. Watch Service … Things to Watch in Real World
• Sub-directories are not watched
• Use Files.walkFileTree to register subdirectories upon
visitFile()
• Some editors create a new file when updating
• Expecting an Update, but received Delete + Create events
• Some file systems create a zero byte file initially
• Expecting a Create, but observed Create + Update events
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48. Custom File System
• Allows creation of custom file systems
• FileSystemProvider
• Zip File System Provider is shipped as a reference
implementation
• Open Source implementations
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49. Conclusion …. Final Thoughts
• NIO-2 works consistently across platforms
• Supports new operations
• Supports bulk access to File Attributes
• Better exceptions
• SPI for custom provider interface
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50. Thank You
© 2013 VeriSign, Inc. All rights reserved. VERISIGN and other trademarks, service marks, and
designs are registered or unregistered trademarks of VeriSign, Inc. and its subsidiaries in the United
States and in foreign countries. All other trademarks are property of their respective owners.