Class WebappServiceLoader

java/org/apache/catalina/startup/WebappServiceLoader.java:59–239

A variation of Java's JAR ServiceLoader that respects exclusion rules for web applications. Primarily intended for use loading ServletContainerInitializers as defined by Servlet 8.2.4. This implementation does not attempt lazy loading as the container is required to introspect all implementation

 * @see java.util.ServiceLoader
 */
public class WebappServiceLoader<T> {
    private static final String CLASSES = "/WEB-INF/classes/";
    private static final String LIB = "/WEB-INF/lib/";
    private static final String SERVICES = "META-INF/services/";

    private final Context context;
    private final ServletContext servletContext;
    private final Pattern containerSciFilterPattern;


    /**
     * Construct a loader to load services from a ServletContext.
     *
     * @param context the context to use
     */
    public WebappServiceLoader(Context context) {
        this.context = context;
        this.servletContext = context.getServletContext();
        String containerSciFilter = context.getContainerSciFilter();
        if (containerSciFilter != null && !containerSciFilter.isEmpty()) {
            containerSciFilterPattern = Pattern.compile(containerSciFilter);
        } else {
            containerSciFilterPattern = null;
        }
    }


    /**
     * Load the providers for a service type. Container defined services will be loaded before application defined
     * services in case the application depends on a Container provided service. Note that services are always loaded
     * via the Context (web application) class loader so it is possible for an application to provide an alternative
     * implementation of what would normally be a Container provided service.
     *
     * @param serviceType the type of service to load
     *
     * @return an unmodifiable collection of service providers
     *
     * @throws IOException if there was a problem loading any service
     */
    public List<T> load(Class<T> serviceType) throws IOException {
        String configFile = SERVICES + serviceType.getName();

        // Obtain the Container provided service configuration files.
        ClassLoader loader = context.getParentClassLoader();
        Enumeration<URL> containerResources;
        if (loader == null) {
            containerResources = ClassLoader.getSystemResources(configFile);
        } else {
            containerResources = loader.getResources(configFile);
        }

        // Extract the Container provided service class names. Each
        // configuration file may list more than one service class name. This
        // uses a LinkedHashSet so if a service class name appears more than
        // once in the configuration files, only the first one found is used.
        LinkedHashSet<String> containerServiceClassNames = new LinkedHashSet<>();
        Set<URL> containerServiceConfigFiles = new HashSet<>();
        while (containerResources.hasMoreElements()) {