001// Copyright 2006, 2008, 2009, 2013 The Apache Software Foundation
002//
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
006//
007//     http://www.apache.org/licenses/LICENSE-2.0
008//
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
014
015package org.apache.tapestry5;
016
017import org.apache.tapestry5.ioc.Resource;
018
019/**
020 * An Asset is any kind of resource that can be exposed to the client web browser. Although quite often an Asset is a
021 * resource in a web application's context folder, within Tapestry, Assets may also be resources on the classpath (i.e.,
022 * packaged inside JARs).
023 * <p/>
024 * An Asset's toString() will return the URL for the resource (the same value as {@link #toClientURL()}).
025 * <p/>
026 * Release 5.1.0.0 introduced {@link org.apache.tapestry5.Asset2}, which extends this interface with an additional
027 * method.
028 *
029 * @see org.apache.tapestry5.services.AssetPathConverter
030 */
031public interface Asset
032{
033    /**
034     * Returns a URL that can be passed, unchanged, to the client in order for it to access the resource. The same value
035     * is returned from <code>toString()</code>.
036     * <p/>
037     * Tapestry's built-in asset types (context and classpath) always incorporate a checksum as part of the path,
038     * and alternate implementations are encouraged to do so as well. In addition, Tapestry ensures that context and
039     * classpath assets have a far-future expires header (to ensure aggressive caching by the client).
040     * <p/>Note that starting in Tapestry 5.4, it is expected that Asset instances recognize
041     * when the underlying Resource's content has changed, and update the clientURL to reflect the new content's
042     * checksum. This wasn't an issue in earlier releases where the clientURL incorporated a version number.
043     * <p/>
044     * Finally, starting in 5.4, this value will often be <em>variant</em>: the exact URL returned will depend on
045     * whether the underlying resource content is compressable, whether the current {@link org.apache.tapestry5.services.Request}
046     * supports compression.
047     *
048     * @see org.apache.tapestry5.services.AssetSource
049     * @see org.apache.tapestry5.services.AssetPathConverter
050     */
051    String toClientURL();
052
053    /**
054     * Returns the underlying Resource for the Asset.
055     */
056    Resource getResource();
057
058}