    001// Licensed under the Apache License, Version 2.0 (the "License");
    002// you may not use this file except in compliance with the License.
    003// You may obtain a copy of the License at
    004//
    005//     http://www.apache.org/licenses/LICENSE-2.0
    006//
    007// Unless required by applicable law or agreed to in writing, software
    008// distributed under the License is distributed on an "AS IS" BASIS,
    009// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    010// See the License for the specific language governing permissions and
    011// limitations under the License.
    012
    013package org.apache.tapestry5;
    014
    015import org.apache.tapestry5.ioc.Resource;
    016
    017/**
    018 * An Asset is any kind of resource that can be exposed to the client web browser. Although quite often an Asset is a
    019 * resource in a web application's context folder, within Tapestry, Assets may also be resources on the classpath (i.e.,
    020 * packaged inside JARs).
    021 *
    022 * An Asset's toString() will return the URL for the resource (the same value as {@link #toClientURL()}).
    023 *
    024 * Release 5.1.0.0 introduced {@link org.apache.tapestry5.Asset2}, which extends this interface with an additional
    025 * method.
    026 *
    027 * @see org.apache.tapestry5.services.AssetPathConverter
    028 */
    029public interface Asset
    030{
    031    /**
    032     * Returns a URL that can be passed, unchanged, to the client in order for it to access the resource. The same value
    033     * is returned from <code>toString()</code>.
    034     *
    035     * Tapestry's built-in asset types (context and classpath) always incorporate a checksum as part of the path,
    036     * and alternate implementations are encouraged to do so as well. In addition, Tapestry ensures that context and
    037     * classpath assets have a far-future expires header (to ensure aggressive caching by the client).
    038     * Note that starting in Tapestry 5.4, it is expected that Asset instances recognize
    039     * when the underlying Resource's content has changed, and update the clientURL to reflect the new content's
    040     * checksum. This wasn't an issue in earlier releases where the clientURL incorporated a version number.
    041     *
    042     * Finally, starting in 5.4, this value will often be <em>variant</em>: the exact URL returned will depend on
    043     * whether the underlying resource content is compressable, whether the current {@link org.apache.tapestry5.services.Request}
    044     * supports compression.
    045     *
    046     * @see org.apache.tapestry5.services.AssetSource
    047     * @see org.apache.tapestry5.services.AssetPathConverter
    048     */
    049    String toClientURL();
    050
    051    /**
    052     * Returns the underlying Resource for the Asset.
    053     */
    054    Resource getResource();
    055
    056}