Make file detection more resilient across restarts

Retain file snapshot state across devtools restarts to help prevent
detection failures.

Closes gh-19543

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/LocalDevToolsAutoConfiguration.java

@@ -31,6 +31,7 @@ import org.springframework.boot.devtools.classpath.ClassPathRestartStrategy;
 import org.springframework.boot.devtools.classpath.PatternClassPathRestartStrategy;
 import org.springframework.boot.devtools.filewatch.FileSystemWatcher;
 import org.springframework.boot.devtools.filewatch.FileSystemWatcherFactory;
+import org.springframework.boot.devtools.filewatch.SnapshotStateRepository;
 import org.springframework.boot.devtools.livereload.LiveReloadServer;
 import org.springframework.boot.devtools.restart.ConditionalOnInitializedRestarter;
 import org.springframework.boot.devtools.restart.RestartScope;
@@ -141,7 +142,7 @@ public class LocalDevToolsAutoConfiguration {
 		private FileSystemWatcher newFileSystemWatcher() {
 			Restart restartProperties = this.properties.getRestart();
 			FileSystemWatcher watcher = new FileSystemWatcher(true, restartProperties.getPollInterval(),
-					restartProperties.getQuietPeriod());
+					restartProperties.getQuietPeriod(), SnapshotStateRepository.STATIC);
 			String triggerFile = restartProperties.getTriggerFile();
 			if (StringUtils.hasLength(triggerFile)) {
 				watcher.setTriggerFilter(new TriggerFileFilter(triggerFile));

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.java

@@ -54,6 +54,8 @@ public class FileSystemWatcher {
 
 	private final long quietPeriod;
 
+	private final SnapshotStateRepository snapshotStateRepository;
+
 	private final AtomicInteger remainingScans = new AtomicInteger(-1);
 
 	private final Map<File, DirectorySnapshot> directories = new HashMap<>();
@@ -79,6 +81,20 @@ public class FileSystemWatcher {
 	 * ensure that updates have completed
 	 */
 	public FileSystemWatcher(boolean daemon, Duration pollInterval, Duration quietPeriod) {
+		this(daemon, pollInterval, quietPeriod, null);
+	}
+
+	/**
+	 * Create a new {@link FileSystemWatcher} instance.
+	 * @param daemon if a daemon thread used to monitor changes
+	 * @param pollInterval the amount of time to wait between checking for changes
+	 * @param quietPeriod the amount of time required after a change has been detected to
+	 * ensure that updates have completed
+	 * @param snapshotStateRepository the snapshot state repository
+	 * @since 2.4.0
+	 */
+	public FileSystemWatcher(boolean daemon, Duration pollInterval, Duration quietPeriod,
+			SnapshotStateRepository snapshotStateRepository) {
 		Assert.notNull(pollInterval, "PollInterval must not be null");
 		Assert.notNull(quietPeriod, "QuietPeriod must not be null");
 		Assert.isTrue(pollInterval.toMillis() > 0, "PollInterval must be positive");
@@ -88,6 +104,8 @@ public class FileSystemWatcher {
 		this.daemon = daemon;
 		this.pollInterval = pollInterval.toMillis();
 		this.quietPeriod = quietPeriod.toMillis();
+		this.snapshotStateRepository = (snapshotStateRepository != null) ? snapshotStateRepository
+				: SnapshotStateRepository.NONE;
 	}
 
 	/**
@@ -150,11 +168,12 @@ public class FileSystemWatcher {
 	 */
 	public void start() {
 		synchronized (this.monitor) {
-			saveInitialSnapshots();
+			createOrRestoreInitialSnapshots();
 			if (this.watchThread == null) {
 				Map<File, DirectorySnapshot> localDirectories = new HashMap<>(this.directories);
-				this.watchThread = new Thread(new Watcher(this.remainingScans, new ArrayList<>(this.listeners),
-						this.triggerFilter, this.pollInterval, this.quietPeriod, localDirectories));
+				Watcher watcher = new Watcher(this.remainingScans, new ArrayList<>(this.listeners), this.triggerFilter,
+						this.pollInterval, this.quietPeriod, localDirectories, this.snapshotStateRepository);
+				this.watchThread = new Thread(watcher);
 				this.watchThread.setName("File Watcher");
 				this.watchThread.setDaemon(this.daemon);
 				this.watchThread.start();
@@ -162,8 +181,13 @@ public class FileSystemWatcher {
 		}
 	}
 
-	private void saveInitialSnapshots() {
-		this.directories.replaceAll((f, v) -> new DirectorySnapshot(f));
+	@SuppressWarnings("unchecked")
+	private void createOrRestoreInitialSnapshots() {
+		Map<File, DirectorySnapshot> restored = (Map<File, DirectorySnapshot>) this.snapshotStateRepository.restore();
+		this.directories.replaceAll((f, v) -> {
+			DirectorySnapshot restoredSnapshot = (restored != null) ? restored.get(f) : null;
+			return (restoredSnapshot != null) ? restoredSnapshot : new DirectorySnapshot(f);
+		});
 	}
 
 	/**
@@ -213,14 +237,19 @@ public class FileSystemWatcher {
 
 		private Map<File, DirectorySnapshot> directories;
 
+		private SnapshotStateRepository snapshotStateRepository;
+
 		private Watcher(AtomicInteger remainingScans, List<FileChangeListener> listeners, FileFilter triggerFilter,
-				long pollInterval, long quietPeriod, Map<File, DirectorySnapshot> directories) {
+				long pollInterval, long quietPeriod, Map<File, DirectorySnapshot> directories,
+				SnapshotStateRepository snapshotStateRepository) {
 			this.remainingScans = remainingScans;
 			this.listeners = listeners;
 			this.triggerFilter = triggerFilter;
 			this.pollInterval = pollInterval;
 			this.quietPeriod = quietPeriod;
 			this.directories = directories;
+			this.snapshotStateRepository = snapshotStateRepository;
+
 		}
 
 		@Override
@@ -288,10 +317,11 @@ public class FileSystemWatcher {
 					changeSet.add(changedFiles);
 				}
 			}
+			this.directories = updated;
+			this.snapshotStateRepository.save(updated);
 			if (!changeSet.isEmpty()) {
 				fireListeners(Collections.unmodifiableSet(changeSet));
 			}
-			this.directories = updated;
 		}
 
 		private void fireListeners(Set<ChangedFiles> changeSet) {

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/SnapshotStateRepository.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.devtools.filewatch;
+
+/**
+ * Repository used by {@link FileSystemWatcher} to save file/directory snapshots across
+ * restarts.
+ *
+ * @author Phillip Webb
+ * @since 2.4.0
+ */
+public interface SnapshotStateRepository {
+
+	/**
+	 * A No-op {@link SnapshotStateRepository} that does not save state.
+	 */
+	SnapshotStateRepository NONE = new SnapshotStateRepository() {
+
+		@Override
+		public void save(Object state) {
+		}
+
+		@Override
+		public Object restore() {
+			return null;
+		}
+
+	};
+
+	/**
+	 * A {@link SnapshotStateRepository} that uses a static instance to keep state across
+	 * restarts.
+	 */
+	SnapshotStateRepository STATIC = StaticSnapshotStateRepository.INSTANCE;
+
+	/**
+	 * Save the given state in the repository.
+	 * @param state the state to save
+	 */
+	void save(Object state);
+
+	/**
+	 * Restore any previously saved state.
+	 * @return the previously saved state or {@code null}
+	 */
+	Object restore();
+
+}

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/StaticSnapshotStateRepository.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.devtools.filewatch;
+
+/**
+ * {@link SnapshotStateRepository} that uses a single static instance.
+ *
+ * @author Phillip Webb
+ */
+class StaticSnapshotStateRepository implements SnapshotStateRepository {
+
+	static final StaticSnapshotStateRepository INSTANCE = new StaticSnapshotStateRepository();
+
+	private volatile Object state;
+
+	@Override
+	public void save(Object state) {
+		this.state = state;
+	}
+
+	@Override
+	public Object restore() {
+		return this.state;
+	}
+
+}

spring-boot-project/spring-boot-devtools/src/test/java/org/springframework/boot/devtools/filewatch/FileSystemWatcherTests.java

@@ -273,8 +273,37 @@ class FileSystemWatcherTests {
 		assertThat(actual).isEqualTo(expected);
 	}
 
+	@Test
+	void withSnapshotRepository() throws Exception {
+		SnapshotStateRepository repository = new TestSnapshotStateRepository();
+		setupWatcher(20, 10, repository);
+		File directory = new File(this.tempDir, UUID.randomUUID().toString());
+		directory.mkdir();
+		File file = touch(new File(directory, "file.txt"));
+		this.watcher.addSourceDirectory(directory);
+		this.watcher.start();
+		file.delete();
+		this.watcher.stopAfter(1);
+		this.changes.clear();
+		File recreate = touch(new File(directory, "file.txt"));
+		setupWatcher(20, 10, repository);
+		this.watcher.addSourceDirectory(directory);
+		this.watcher.start();
+		this.watcher.stopAfter(1);
+		ChangedFiles changedFiles = getSingleChangedFiles();
+		Set<ChangedFile> actual = changedFiles.getFiles();
+		Set<ChangedFile> expected = new HashSet<>();
+		expected.add(new ChangedFile(directory, recreate, Type.ADD));
+		assertThat(actual).isEqualTo(expected);
+	}
+
 	private void setupWatcher(long pollingInterval, long quietPeriod) {
-		this.watcher = new FileSystemWatcher(false, Duration.ofMillis(pollingInterval), Duration.ofMillis(quietPeriod));
+		setupWatcher(pollingInterval, quietPeriod, null);
+	}
+
+	private void setupWatcher(long pollingInterval, long quietPeriod, SnapshotStateRepository snapshotStateRepository) {
+		this.watcher = new FileSystemWatcher(false, Duration.ofMillis(pollingInterval), Duration.ofMillis(quietPeriod),
+				snapshotStateRepository);
 		this.watcher.addListener((changeSet) -> FileSystemWatcherTests.this.changes.add(changeSet));
 	}
 
@@ -304,4 +333,20 @@ class FileSystemWatcherTests {
 		return file;
 	}
 
+	private static class TestSnapshotStateRepository implements SnapshotStateRepository {
+
+		private Object state;
+
+		@Override
+		public void save(Object state) {
+			this.state = state;
+		}
+
+		@Override
+		public Object restore() {
+			return this.state;
+		}
+
+	}
+
 }