Implement multi-subject and incremental backup

src/main/java/io/ib67/sfcraft/FileWatcherThread.java

@@ -0,0 +1,69 @@
+package io.ib67.sfcraft;
+
+import lombok.SneakyThrows;
+
+import java.nio.file.*;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.TimeUnit;
+import java.util.function.BiConsumer;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+public class FileWatcherThread extends Thread {
+    protected final Map<Path, String> watchSubject;
+    protected final BiConsumer<String, Path> eventListener;
+
+    public FileWatcherThread(
+            Map<String, Path> watchSubject,
+            BiConsumer<String, Path> eventListener
+    ) {
+        this.watchSubject = watchSubject.entrySet()
+                .stream()
+                .flatMap(it -> walkThrough(it.getValue()).map(r -> Map.entry(r, it.getKey())))
+                .collect(Collectors.toMap(
+                        Map.Entry::getKey,
+                        Map.Entry::getValue
+                ));
+        this.eventListener = eventListener;
+        setName("JustBackup/FileWatcher");
+    }
+
+    @SneakyThrows
+    private static Stream<Path> walkThrough(Path path) {
+        return Files.walk(path);
+    }
+
+    @Override
+    @SneakyThrows
+    public void run() {
+        try (var watcher = FileSystems.getDefault().newWatchService()) {
+            for (Path value : watchSubject.keySet()) {
+                value.register(
+                        watcher,
+                        StandardWatchEventKinds.ENTRY_CREATE,
+                        StandardWatchEventKinds.ENTRY_MODIFY
+                );
+            }
+            for (; ; ) {
+                var key = watcher.poll(10, TimeUnit.SECONDS);
+                if (key == null) continue;
+                if (Thread.interrupted()) return;
+                for (var ev : key.pollEvents()) {
+                    var kind = ev.kind();
+                    if (kind == StandardWatchEventKinds.OVERFLOW)
+                        continue;
+                    var path = ((WatchEvent<Path>) ev).context();
+                    var subject = Objects.requireNonNull(watchSubject.get(path.getParent()));
+                    if (Files.isRegularFile(path)) {
+                        eventListener.accept(subject, path.getParent());
+                    } else if (Files.isDirectory(path) && kind == StandardWatchEventKinds.ENTRY_CREATE) {
+                        path.register(watcher, StandardWatchEventKinds.ENTRY_CREATE, StandardWatchEventKinds.ENTRY_MODIFY);
+                        watchSubject.put(path, subject);
+                    }
+                }
+                key.reset();
+            }
+        }
+    }
+}