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}