Java IO

Jun 13, 2020 | - views

The package contains many classes that your programs can use to read and write data. Most of the classes implement sequential access streams. The sequential access streams can be divided into two groups: those that read and write bytes and those that read and write Unicode characters. Each sequential access stream has a speciality, such as reading from or writing to a file, filtering data as its read or written, or serializing an object.

The java.nio.file package provides extensive support for file and file system I/O. This is a very comprehensive API, but the key entry points are as follows:

Path file = ...;
byte[] fileArray = Files.readAllBytes(file);
Charset charset = Charset.forName("US-ASCII");
try (BufferedReader reader = Files.newBufferedReader(file, charset)) {
    String line = null;
    while ((line = reader.readLine()) != null) {
} catch (IOException x) {
    System.err.format("IOException: %s%n", x);
Path file = ...;
try (InputStream in = Files.newInputStream(file);
    BufferedReader reader =
      new BufferedReader(new InputStreamReader(in))) {
    String line = null;
    while ((line = reader.readLine()) != null) {
} catch (IOException x) {
// Defaults to READ
try (SeekableByteChannel sbc = Files.newByteChannel(file)) {
    ByteBuffer buf = ByteBuffer.allocate(10);

    // Read the bytes with the proper encoding for this platform.  If
    // you skip this step, you might see something that looks like
    // Chinese characters when you expect Latin-style characters.
    String encoding = System.getProperty("file.encoding");
    while ( > 0) {
} catch (IOException x) {
    System.out.println("caught exception: " + x);

MIME Content Type

Path path = Path.of("data/io-fileiomethods.gif");

Walking the File Tree

To walk a file tree, you first need to implement a FileVisitor. A FileVisitor specifies the required behavior at key points in the traversal process: when a file is visited, before a directory is accessed, after a directory is accessed, or when a failure occurs. If you don't need to implement all four of the FileVisitor methods, instead of implementing the FileVisitor interface, you can extend the SimpleFileVisitor class. This class, which implements the FileVisitor interface, visits all files in a tree and throws an IOError when an error is encountered. You can extend this class and override only the methods that you require.

Files.walkFileTree(Path.of("src/"), new SimpleFileVisitor<>(){
    public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
        System.out.println(String.format("Found dir = %s", dir));
        return super.preVisitDirectory(dir, attrs);

    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
        System.out.println(String.format("Found file = %s", file));
        return super.visitFile(file, attrs);

Watching a Directory for Changes

The WatchService API is fairly low level, allowing you to customize it. You can use it as is, or you can choose to create a high-level API on top of this mechanism so that it is suited to your particular needs.

try (WatchService watcher = FileSystems.getDefault().newWatchService()) {
    Path path = Path.of("data/");
    WatchKey watchKey = path.register(watcher,

    for (; ; ) {
        for (WatchEvent<?> event : watchKey.pollEvents()) {
            WatchEvent.Kind<?> kind = event.kind();
            WatchEvent<Path> ev = (WatchEvent<Path>)event;


The Watch Service API is designed for applications that need to be notified about file change events. It is well suited for any application, like an editor or IDE, that potentially has many open files and needs to ensure that the files are synchronized with the file system. It is also well suited for an application server that watches a directory, perhaps waiting for .jsp or .jar files to drop, in order to deploy them.

This API is not designed for indexing a hard drive. Most file system implementations have native support for file change notification. The Watch Service API takes advantage of this support where available. However, when a file system does not support this mechanism, the Watch Service will poll the file system, waiting for events.

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