NIO.2 — Path và Files, API hiện đại

File file = new File("data/report.pdf");

// Check exist
boolean e = file.exists();

// Delete
boolean ok = file.delete();   // Return boolean. Fail -> khong biet ly do.

// Rename
boolean ok2 = file.renameTo(new File("archive/report.pdf"));
// Behavior khac nhau giua Windows va Linux
// Co the fail silent

import java.nio.file.*;

Path p1 = Paths.get("data/log.txt");           // Java 7, factory cu
Path p2 = Path.of("data/log.txt");             // Java 11+, recommend
Path p3 = Path.of("/absolute/path/to/file");
Path p4 = Path.of("C:\\Users\\Alice\\doc.pdf");  // Windows
Path p5 = Path.of("data", "subdir", "file.txt"); // Join segments

Path p = Path.of("/var/log");
Path child = p.resolve("app.log");
// p van la "/var/log"
// child la "/var/log/app.log" (path moi)

Path abs = p.toAbsolutePath();   // Path moi, p khong doi
Path norm = Path.of("/a/./b/../c").normalize();   // /a/c - path moi

Path p = Path.of("/var/log/app.log");

p.getFileName();     // "app.log" (Path)
p.getParent();       // "/var/log" (Path)
p.getRoot();         // "/" (Path) hoac null neu relative
p.getNameCount();    // 3
p.getName(0);        // "var"
p.toString();        // "/var/log/app.log"
p.toFile();          // java.io.File (convert qua lai neu can)

Path base = Path.of("/home/user");
Path subdir = base.resolve("docs");              // /home/user/docs
Path file = subdir.resolve("report.pdf");        // /home/user/docs/report.pdf

// resolveSibling: thay filename giu parent
Path other = file.resolveSibling("resume.pdf"); // /home/user/docs/resume.pdf

// relativize: tu A ra B
Path rel = base.relativize(file);                // docs/report.pdf

// normalize: loai . va ..
Path.of("/a/./b/../c").normalize();              // /a/c

// BAD
Path p = Path.of(base + "/" + filename);   // Windows bug

// GOOD
Path p = base.resolve(filename);            // Platform correct

Files.exists(path);
Files.notExists(path);        // Khac !exists (co the khong determinable)
Files.isRegularFile(path);    // La file thuong (khong directory, symlink)
Files.isDirectory(path);
Files.isReadable(path);
Files.isWritable(path);
Files.isExecutable(path);
Files.isHidden(path);
Files.size(path);             // bytes
Files.getLastModifiedTime(path);   // FileTime
Files.isSameFile(p1, p2);     // Chieu sau - kiem tra cung file thuc su (symlink)

Files.createDirectories(path);                    // mkdir -p (tao parent neu can)
Files.createDirectory(path);                       // throw neu parent khong ton tai
Files.createFile(path);                            // throw neu ton tai
Files.createTempFile("prefix-", ".tmp");           // tmp file he thong
Files.createTempFile(dir, "prefix-", ".tmp");      // tmp file trong dir

Files.delete(path);                                // throw neu khong ton tai
Files.deleteIfExists(path);                        // silent neu khong co

Files.copy(src, dst);                                            // copy
Files.copy(src, dst, StandardCopyOption.REPLACE_EXISTING);       // overwrite
Files.copy(src, dst, StandardCopyOption.COPY_ATTRIBUTES);        // metadata

Files.move(src, dst);                                            // move/rename
Files.move(src, dst, StandardCopyOption.ATOMIC_MOVE);            // atomic

// Text
String content = Files.readString(path, StandardCharsets.UTF_8);   // Java 11+
String contentDefault = Files.readString(path);                    // Default UTF-8

List<String> lines = Files.readAllLines(path, StandardCharsets.UTF_8);

// Binary
byte[] bytes = Files.readAllBytes(path);

try (Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8)) {
    long errorCount = lines
        .filter(l -> l.contains("ERROR"))
        .count();
}

try (BufferedReader r = Files.newBufferedReader(path, StandardCharsets.UTF_8)) {
    String line;
    while ((line = r.readLine()) != null) {
        process(line);
    }
}

// Toan file
Files.writeString(path, "Hello\nWorld\n", StandardCharsets.UTF_8);   // Java 11+
Files.write(path, List.of("line1", "line2"), StandardCharsets.UTF_8);

// Append
Files.writeString(path, "more\n", StandardCharsets.UTF_8,
    StandardOpenOption.APPEND);

// Atomic: ghi tmp roi rename (pattern chi tiet o muc 6)

Files.write(path, byteArray);

try (BufferedWriter w = Files.newBufferedWriter(path, StandardCharsets.UTF_8)) {
    for (int i = 0; i < 1_000_000; i++) {
        w.write("line " + i + "\n");
    }
}

Files.writeString(path, data, UTF_8,
    StandardOpenOption.CREATE_NEW,
    StandardOpenOption.SYNC);
// Tao moi, throw neu ton tai, fsync sau write

try (Stream<Path> paths = Files.walk(root)) {
    paths.filter(Files::isRegularFile)
         .filter(p -> p.toString().endsWith(".java"))
         .forEach(System.out::println);
}

Files.walk(root, 3);   // Toi da depth 3

try (Stream<Path> java = Files.find(root, 10,
        (p, attrs) -> p.toString().endsWith(".java") && attrs.isRegularFile())) {
    java.forEach(System.out::println);
}

try (Stream<Path> children = Files.list(dir)) {
    children.forEach(System.out::println);
}

Files.walkFileTree(root, new SimpleFileVisitor<Path>() {
    @Override
    public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
        if (dir.getFileName().toString().startsWith(".")) {
            return FileVisitResult.SKIP_SUBTREE;   // skip hidden directory
        }
        return FileVisitResult.CONTINUE;
    }

    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
        System.out.println(file);
        return FileVisitResult.CONTINUE;
    }

    @Override
    public FileVisitResult visitFileFailed(Path file, IOException exc) {
        System.err.println("Fail: " + file + " - " + exc);
        return FileVisitResult.CONTINUE;   // Skip file loi, tiep tuc
    }
});

Path config = Path.of("/etc/myapp/config.json");

// 1. Ghi tmp trong cung thu muc
Path tmp = Files.createTempFile(config.getParent(), "cfg-", ".tmp");
try {
    Files.writeString(tmp, newConfigJson, StandardCharsets.UTF_8);

    // 2. Force xuong disk (optional, cho durability)
    try (FileChannel ch = FileChannel.open(tmp, StandardOpenOption.WRITE)) {
        ch.force(true);
    }

    // 3. Atomic rename
    Files.move(tmp, config, StandardCopyOption.ATOMIC_MOVE,
                            StandardCopyOption.REPLACE_EXISTING);
} catch (IOException e) {
    Files.deleteIfExists(tmp);   // Clean up khi fail
    throw e;
}

try (WatchService watcher = FileSystems.getDefault().newWatchService()) {
    Path dir = Path.of("data");
    dir.register(watcher,
        StandardWatchEventKinds.ENTRY_CREATE,
        StandardWatchEventKinds.ENTRY_MODIFY,
        StandardWatchEventKinds.ENTRY_DELETE);

    while (true) {
        WatchKey key = watcher.take();   // Block cho event
        for (WatchEvent<?> event : key.pollEvents()) {
            WatchEvent.Kind<?> kind = event.kind();
            Path filename = (Path) event.context();
            System.out.println(kind + ": " + filename);
        }
        if (!key.reset()) break;   // key invalid -> dung
    }
}

// Basic - platform independent
BasicFileAttributes attrs = Files.readAttributes(path, BasicFileAttributes.class);
attrs.creationTime();
attrs.lastModifiedTime();
attrs.size();
attrs.isRegularFile();

// POSIX (Linux, macOS)
PosixFileAttributes posix = Files.readAttributes(path, PosixFileAttributes.class);
posix.owner();                                    // UserPrincipal
posix.group();
Set<PosixFilePermission> perms = posix.permissions();
PosixFilePermissions.toString(perms);             // "rwxr-x---"

// Set permission
Files.setPosixFilePermissions(path,
    PosixFilePermissions.fromString("rw-r--r--"));

// DOS (Windows)
DosFileAttributes dos = Files.readAttributes(path, DosFileAttributes.class);
dos.isReadOnly();
dos.isHidden();
dos.isArchive();

Files.lines(path).filter(...).count();   // Leak FD

try (Stream<String> lines = Files.lines(path)) {
    lines.filter(...).count();
}

byte[] data = Files.readAllBytes(path);   // OOM neu 10GB

Files.walk(root).forEach(...);   // Stream giu file handle

Path p = Path.of(base + "/" + filename);   // Bug separator Windows

Files.copy(src, dst);   // FileAlreadyExistsException

Files.move(/tmp/x, /home/user/x, ATOMIC_MOVE);   // AtomicMoveNotSupportedException neu khac FS

Path tmp = Files.createTempFile(target.getParent(), "tmp-", ".tmp");

file.delete();   // Return boolean - fail thi sao?

Files.delete(path);   // Throw NoSuchFileException / AccessDeniedException / ...