feat: File watching service

src/main/java/eu/ztsh/lfr/files/FileModifiedEvent.java

@@ -0,0 +1,5 @@
+package eu.ztsh.lfr.files;
+
+public record FileModifiedEvent(long timestamp) {
+
+}

src/main/java/eu/ztsh/lfr/files/WatcherService.java

@@ -0,0 +1,76 @@
+package eu.ztsh.lfr.files;
+
+import eu.ztsh.lfr.config.DataProperties;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.context.event.ApplicationReadyEvent;
+import org.springframework.context.ApplicationEventPublisher;
+import org.springframework.context.event.EventListener;
+import org.springframework.stereotype.Service;
+
+import java.io.IOException;
+import java.nio.file.FileSystems;
+import java.nio.file.Paths;
+import java.nio.file.StandardWatchEventKinds;
+import java.nio.file.WatchKey;
+import java.nio.file.WatchService;
+
+@Service
+public class WatcherService {
+
+    private static final Logger log = LoggerFactory.getLogger(WatcherService.class);
+
+    private final WatchService watchService;
+    private final ApplicationEventPublisher eventPublisher;
+
+    private final String fileName;
+
+    @Autowired
+    public WatcherService(DataProperties dataProperties, ApplicationEventPublisher eventPublisher) throws IOException {
+        this.eventPublisher = eventPublisher;
+        this.watchService = FileSystems.getDefault().newWatchService();
+
+        var filePath = Paths.get(dataProperties.fileUrl());
+        fileName = filePath.getFileName().toString();
+        filePath.getParent().register(
+            watchService,
+            // register file creation as some editors modify files in temp, and then replaces original file.
+            StandardWatchEventKinds.ENTRY_CREATE,
+            StandardWatchEventKinds.ENTRY_MODIFY
+        );
+    }
+
+    @SuppressWarnings("BusyWait")
+    @EventListener(ApplicationReadyEvent.class)
+    public void watch() throws InterruptedException {
+        WatchKey key;
+        while ((key = watchService.take()) != null) {
+            /*
+            When modifying in IntelliJ, one line edit ends with:
+            ENTRY_CREATE: temperatures.csv~
+            ENTRY_MODIFY: temperatures.csv~
+            ENTRY_MODIFY: temperatures.csv~
+            ENTRY_MODIFY: temperatures.csv
+            ENTRY_MODIFY: temperatures.csv
+            ENTRY_MODIFY: temperatures.csv
+            Thread.sleep is used to filter out duplicated events.
+            origin: https://stackoverflow.com/a/25221600
+             */
+            Thread.sleep(50);
+            var maybeLastEvent = key.pollEvents().stream()
+                .filter(event -> event.context() != null)
+                .filter(event -> event.context().toString().endsWith(fileName))
+                .reduce((o1, o2) -> o2);
+            if (maybeLastEvent.isPresent()) {
+                var lastEvent = maybeLastEvent.get();
+                log.info("Got event of kind:{}", lastEvent.kind());
+                eventPublisher.publishEvent(new FileModifiedEvent(System.currentTimeMillis()));
+            } else {
+                log.trace("Got event for not watched file");
+            }
+            key.reset();
+        }
+    }
+
+}