Tolerate non-existent source folders in DevTools

Closes gh-13620

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

@@ -125,8 +125,7 @@ public class FileSystemWatcher {
 	 */
 	public void addSourceFolder(File folder) {
 		Assert.notNull(folder, "Folder must not be null");
-		Assert.isTrue(folder.isDirectory(),
-				"Folder '" + folder + "' must exist and must" + " be a directory");
+		Assert.isTrue(!folder.isFile(), "Folder '" + folder + "' must not be a file");
 		synchronized (this.monitor) {
 			checkNotStarted();
 			this.folders.put(folder, null);

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

@@ -52,7 +52,7 @@ class FolderSnapshot {
 	 */
 	FolderSnapshot(File folder) {
 		Assert.notNull(folder, "Folder must not be null");
-		Assert.isTrue(folder.isDirectory(), "Folder must not be a file");
+		Assert.isTrue(!folder.isFile(), "Folder '" + folder + "' must not be a file");
 		this.folder = folder;
 		this.time = new Date();
 		Set<FileSnapshot> files = new LinkedHashSet<>();

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

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -106,21 +106,11 @@ public class FileSystemWatcherTests {
 	}
 
 	@Test
-	public void sourceFolderMustExist() {
-		File folder = new File("does/not/exist");
-		assertThat(folder.exists()).isFalse();
-		this.thrown.expect(IllegalArgumentException.class);
-		this.thrown.expectMessage(
-				"Folder '" + folder + "' must exist and must be a directory");
-		this.watcher.addSourceFolder(folder);
-	}
-
-	@Test
-	public void sourceFolderMustBeADirectory() {
+	public void sourceFolderMustNotBeAFile() {
 		File folder = new File("pom.xml");
 		assertThat(folder.isFile()).isTrue();
 		this.thrown.expect(IllegalArgumentException.class);
-		this.thrown.expectMessage("Folder 'pom.xml' must exist and must be a directory");
+		this.thrown.expectMessage("Folder 'pom.xml' must not be a file");
 		this.watcher.addSourceFolder(new File("pom.xml"));
 	}
 
@@ -152,6 +142,19 @@ public class FileSystemWatcherTests {
 		assertThat(changedFiles.getFiles()).contains(expected);
 	}
 
+	@Test
+	public void createSourceFolderAndAddFile() throws IOException {
+		File folder = new File(this.temp.getRoot(), "does/not/exist");
+		assertThat(folder.exists()).isFalse();
+		this.watcher.addSourceFolder(folder);
+		this.watcher.start();
+		folder.mkdirs();
+		touch(new File(folder, "text.txt"));
+		this.watcher.stopAfter(1);
+		ChangedFiles changedFiles = getSingleChangedFiles();
+		System.out.println(changedFiles);
+	}
+
 	@Test
 	public void waitsForPollingInterval() throws Exception {
 		setupWatcher(10, 1);

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

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -62,9 +62,17 @@ public class FolderSnapshotTests {
 
 	@Test
 	public void folderMustNotBeFile() throws Exception {
+		File file = this.temporaryFolder.newFile();
 		this.thrown.expect(IllegalArgumentException.class);
-		this.thrown.expectMessage("Folder must not be a file");
-		new FolderSnapshot(this.temporaryFolder.newFile());
+		this.thrown.expectMessage("Folder '" + file + "' must not be a file");
+		new FolderSnapshot(file);
+	}
+
+	@Test
+	public void folderDoesNotHaveToExist() throws Exception {
+		File file = new File(this.temporaryFolder.getRoot(), "does/not/exist");
+		FolderSnapshot snapshot = new FolderSnapshot(file);
+		assertThat(snapshot).isEqualTo(new FolderSnapshot(file));
 	}
 
 	@Test