Unravel › Java concepts › I/O and files

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At a glance

1. 1Path dir = Path.of("data", "in"); Path file = dir.resolve("a.txt"); builds paths without worrying about / versus \relativize, normalize, getFileName and getParent take paths apart.
2. 2Files.readString(p), Files.readAllLines(p), Files.writeString(p, text), and Files.lines(p) (lazy stream, must be closed) cover most needs
3. 3Files.walk(dir) visits a whole tree as a stream (close it!), Files.list(dir) one levelFiles.copy(src, dst, REPLACE_EXISTING) and Files.move(src, dst, ATOMIC_MOVE) have options.
4. 4Atomic update pattern: write to a temporary file in the same directory, then Files.move(tmp, target, ATOMIC_MOVE) so readers never see a half-written file

• Files.readAllBytes and readString load everything into memory; stream large files with Files.lines or a BufferedReader
• Files.exists followed by an action is a race: the file can appear or vanish in between
• Path is an abstraction over a FileSystem provider (default, zip file system, in-memory test file systems)

import java.io.IOException;import java.nio.file.*;import java.util.List;import java.util.stream.*;public class Archive {  static List<Path> logs(Path dir) throws IOException {    try (Stream<Path> s = Files.walk(dir)) { return s.filter(p -> p.toString().endsWith(".log")).collect(Collectors.toList()); }  }  static void save(Path target, String text) throws IOException {    Path tmp = Files.createTempFile(target.getParent(), "part", ".tmp");    Files.writeString(tmp, text);    Files.move(tmp, target, StandardCopyOption.ATOMIC_MOVE);  }}

1. line 7 uses walk, the core of NIO.2: Path and Files in practice
2. line 9 uses get, the core of NIO.2: Path and Files in practice
3. line 10 uses createTempFile, the core of NIO.2: Path and Files in practice
4. line 11 uses write, the core of NIO.2: Path and Files in practice
5. line 12 uses move, the core of NIO.2: Path and Files in practice

How Unravel recognises it

When you paste code, Unravel looks for these signs of nio.2: path and files in practice. Spotting them yourself is a good habit when you read unfamiliar code:

• Types: FileChannel ByteBuffer WatchService StandardCopyOption
• Method calls: Files.walk() Files.list() Files.lines() Files.copy() Files.move() Files.createDirectories() Files.createTempFile() Files.exists() Files.walkFileTree() Files.write() Files.readAllBytes() Files.deleteIfExists() Files.size() Path.of() and 4 more

Part of a learning path

• I/O, networking and time · previous: Byte streams, readers and writers · next: Java serialization

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