ImportCandidates Class — spring-boot Architecture

Source Code

core/spring-boot/src/main/java/org/springframework/boot/context/annotation/ImportCandidates.java lines 45–138

public final class ImportCandidates implements Iterable<String> {

	private static final String LOCATION = "META-INF/spring/%s.imports";

	private static final String COMMENT_START = "#";

	private final List<String> candidates;

	private ImportCandidates(List<String> candidates) {
		Assert.notNull(candidates, "'candidates' must not be null");
		this.candidates = Collections.unmodifiableList(candidates);
	}

	@Override
	public Iterator<String> iterator() {
		return this.candidates.iterator();
	}

	/**
	 * Returns the list of loaded import candidates.
	 * @return the list of import candidates
	 */
	public List<String> getCandidates() {
		return this.candidates;
	}

	/**
	 * Loads the names of import candidates from the classpath. The names of the import
	 * candidates are stored in files named
	 * {@code META-INF/spring/full-qualified-annotation-name.imports} on the classpath.
	 * Every line contains the full qualified name of the candidate class. Comments are
	 * supported using the # character.
	 * @param annotation annotation to load
	 * @param classLoader class loader to use for loading
	 * @return list of names of annotated classes
	 */
	public static ImportCandidates load(Class<?> annotation, @Nullable ClassLoader classLoader) {
		Assert.notNull(annotation, "'annotation' must not be null");
		ClassLoader classLoaderToUse = decideClassloader(classLoader);
		String location = String.format(LOCATION, annotation.getName());
		Enumeration<URL> urls = findUrlsInClasspath(classLoaderToUse, location);
		List<String> importCandidates = new ArrayList<>();
		while (urls.hasMoreElements()) {
			URL url = urls.nextElement();
			importCandidates.addAll(readCandidateConfigurations(url));
		}
		return new ImportCandidates(importCandidates);
	}

	private static ClassLoader decideClassloader(@Nullable ClassLoader classLoader) {
		if (classLoader == null) {
			return ImportCandidates.class.getClassLoader();
		}
		return classLoader;
	}

	private static Enumeration<URL> findUrlsInClasspath(ClassLoader classLoader, String location) {
		try {
			return classLoader.getResources(location);
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Failed to load configurations from location [" + location + "]", ex);
		}
	}

	private static List<String> readCandidateConfigurations(URL url) {
		try (BufferedReader reader = new BufferedReader(
				new InputStreamReader(new UrlResource(url).getInputStream(), StandardCharsets.UTF_8))) {
			List<String> candidates = new ArrayList<>();
			String line;
			while ((line = reader.readLine()) != null) {
				line = stripComment(line);
				line = line.trim();
				if (line.isEmpty()) {
					continue;
				}
				candidates.add(line);
			}
			return candidates;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load configurations from location [" + url + "]", ex);
		}
	}

	private static String stripComment(String line) {
		int commentStart = line.indexOf(COMMENT_START);
		if (commentStart == -1) {
			return line;
		}
		return line.substring(0, commentStart);
	}

}