WatchDir.java

package restx.shell;

import java.io.IOException;
import java.nio.file.*;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.HashMap;
import java.util.Map;

import static java.nio.file.LinkOption.NOFOLLOW_LINKS;
import static java.nio.file.StandardWatchEventKinds.*;
 
public class WatchDir {
    private final WatchService watcher;
    private final Map<WatchKey,Path> keys;
    private final boolean recursive;
    private boolean trace = false;
 
    @SuppressWarnings("unchecked")
    static <T> WatchEvent<T> cast(WatchEvent<?> event) {
        return (WatchEvent<T>)event;
    }
 
    /**
     * Register the given directory with the WatchService
     */
    private void register(Path dir) throws IOException {
        WatchKey key = dir.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);
        if (trace) {
            Path prev = keys.get(key);
            if (prev == null) {
                System.out.format("register: %s\n", dir);
            } else {
                if (!dir.equals(prev)) {
                    System.out.format("update: %s -> %s\n", prev, dir);
                }
            }
        }
        keys.put(key, dir);
    }
 
    /**
     * Register the given directory, and all its sub-directories, with the
     * WatchService.
     */
    private void registerAll(final Path start) throws IOException {
        // register directory and sub-directories
        Files.walkFileTree(start, new SimpleFileVisitor<Path>() {
            @Override
            public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs)
                throws IOException
            {
                register(dir);
                return FileVisitResult.CONTINUE;
            }
        });
    }
 
    /**
     * Creates a WatchService and registers the given directory
     */
    WatchDir(Path dir, boolean recursive) throws IOException {
        this.watcher = FileSystems.getDefault().newWatchService();
        this.keys = new HashMap<>();
        this.recursive = recursive;
 
        if (recursive) {
            registerAll(dir);
        } else {
            register(dir);
        }
    }
 
    /**
     * Process all events for keys queued to the watcher
     */
    void processEvents() {
        for (;;) {
 
            // wait for key to be signalled
            WatchKey key;
            try {
                key = watcher.take();
            } catch (InterruptedException x) {
                return;
            }
 
            Path dir = keys.get(key);
            if (dir == null) {
                System.err.println("WatchKey not recognized!!");
                continue;
            }
 
            for (WatchEvent<?> event: key.pollEvents()) {
                WatchEvent.Kind kind = event.kind();
 
                // TBD - provide example of how OVERFLOW event is handled
                if (kind == OVERFLOW) {
                    continue;
                }
 
                // Context for directory entry event is the file name of entry
                WatchEvent<Path> ev = cast(event);
                Path name = ev.context();
                Path child = dir.resolve(name);
 
                onEvent(event.kind(), child);
 
                // if directory is created, and watching recursively, then
                // register it and its sub-directories
                if (recursive && (kind == ENTRY_CREATE)) {
                    try {
                        if (Files.isDirectory(child, NOFOLLOW_LINKS)) {
                            registerAll(child);
                        }
                    } catch (IOException x) {
                        // ignore to keep sample readbale
                    }
                }
            }
 
            // reset key and remove from set if directory no longer accessible
            boolean valid = key.reset();
            if (!valid) {
                keys.remove(key);
 
                // all directories are inaccessible
                if (keys.isEmpty()) {
                    break;
                }
            }
        }
    }

    protected void onEvent(WatchEvent.Kind<?> kind, Path path) {
        System.out.format("%s: %s\n", kind.name(), path);
    }
}