* [Adam Langley's Weblog][langley]. Pins are either base64 SHA-256 hashes as in
 * [HTTP Public Key Pinning (HPKP)][rfc_7469] or SHA-1 base64 hashes as in Chromium's
 * [static certificates][static_certificates].
 *
 * ## Setting up Certificate Pinning
 *
 * The easiest way to pin a host is turn on pinning with a broken configuration and read the
 * expected configuration when the connection fails. Be sure to do this on a trusted network, and

	if m.handlers.hasSubhandler(subID) && !id.isTestHandler() {
		return fmt.Errorf("handler %v, subroute:%v: %w", id.String(), s, ErrHandlerAlreadyExists)
	}
	m.handlers.subSingle[subID] = h
	// Copy so clients can also pick it up for other subpaths.
	m.handlers.subSingle[makeZeroSubHandlerID(id)] = h
	return nil
}

/*
// RegisterStateless will register a stateless handler that serves
// []byte -> stream of ([]byte, error) requests.

        mimetypeMap.put(mimetype, filetype);
    }

    /**
     * Retrieves the file type for a given MIME type.
     *
     * @param mimetype the MIME type to look up
     * @return the corresponding file type, or the default value if not found
     */
    public String get(final String mimetype) {
        final String filetype = mimetypeMap.get(mimetype);

              + " map in that class.");
    }
    Object result;
    try {
      result = supplier.get();
    } catch (Throwable t) {
      if (predicate.apply(t)) {
        // We are careful to set up INSTANCE_OF to match each Predicate to its target Class.
        @SuppressWarnings("unchecked")
        T caught = (T) t;
        return caught;
      }
      throw new AssertionError(

              + " map in that class.");
    }
    Object result;
    try {
      result = supplier.get();
    } catch (Throwable t) {
      if (predicate.apply(t)) {
        // We are careful to set up INSTANCE_OF to match each Predicate to its target Class.
        @SuppressWarnings("unchecked")
        T caught = (T) t;
        return caught;
      }
      throw new AssertionError(

              + " map in that class.");
    }
    Object result;
    try {
      result = supplier.get();
    } catch (Throwable t) {
      if (predicate.apply(t)) {
        // We are careful to set up INSTANCE_OF to match each Predicate to its target Class.
        @SuppressWarnings("unchecked")
        T caught = (T) t;
        return caught;
      }
      throw new AssertionError(

              + " map in that class.");
    }
    Object result;
    try {
      result = supplier.get();
    } catch (Throwable t) {
      if (predicate.apply(t)) {
        // We are careful to set up INSTANCE_OF to match each Predicate to its target Class.
        @SuppressWarnings("unchecked")
        T caught = (T) t;
        return caught;
      }
      throw new AssertionError(

<img src="/img/tutorial/path-operation-configuration/image01.png">

### Tags with Enums { #tags-with-enums }

If you have a big application, you might end up accumulating **several tags**, and you would want to make sure you always use the **same tag** for related *path operations*.

In these cases, it could make sense to store the tags in an `Enum`.

    protected boolean favoriteSupport;

    /** Flag indicating whether thumbnail generation is enabled. */
    protected boolean thumbnailSupport;

    /**
     * Hook method called before action execution. Sets up search-related flags and
     * registers popular words if enabled.
     *
     * @param runtime the action runtime context
     * @return the action response, or null to continue with normal processing
     */

    //                                                                               Hook
    //                                                                              ======
    /**
     * Sets up HTML data for rendering, including help link.
     *
     * @param runtime the action runtime
     */
    @Override
    protected void setupHtmlData(final ActionRuntime runtime) {
        super.setupHtmlData(runtime);