I/O - Files, Paths, NIO.2

Why this step matters

Backend systems constantly read and write files: imports, logs, exports, and config. NIO.2 (java.nio.file) is the modern, safe API for this.

Path and Files

Use Path to represent file system locations and Files for operations.

Path report = Path.of("data", "report.txt");
Files.createDirectories(report.getParent());
Files.writeString(report, "hello\n");
String content = Files.readString(report);

Read and write patterns

For small files:

• Files.readString(...)
• Files.writeString(...)

For larger content, use buffered streams/readers:

try (BufferedReader reader = Files.newBufferedReader(report)) {
    String line;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }
}

Directory traversal

Path root = Path.of("logs");
try (Stream<Path> stream = Files.walk(root)) {
    stream
        .filter(Files::isRegularFile)
        .forEach(System.out::println);
}

Use Files.list(...) for one level and Files.walk(...) for recursive traversal.

Useful options

Files.writeString(
    report,
    "new line\n",
    StandardOpenOption.CREATE,
    StandardOpenOption.APPEND
);

Common options:

• CREATE
• TRUNCATE_EXISTING
• APPEND

Error handling and safety

I/O throws checked exceptions (IOException). Handle failures explicitly.

Also:

• validate paths from user input
• avoid path traversal vulnerabilities
• prefer UTF-8 explicitly when needed

Common mistakes

• forgetting try-with-resources
• loading very large files entirely in memory
• using string concatenation for paths instead of Path.of(...)
• writing files without creating parent directories

Takeaway

1. Use Path + Files as default file API
2. Choose buffered APIs for larger files
3. Traverse directories with Files.list/Files.walk
4. Handle IOException and validate file paths