/*
 * $HeadURL: http://svn.apache.org/repos/asf/httpcomponents/httpcore/trunk/module-main/src/main/java/org/apache/http/HttpEntity.java $
 * $Revision: 645824 $
 * $Date: 2008-04-08 03:12:41 -0700 (Tue, 08 Apr 2008) $
 *
 * ====================================================================
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 *
 */

package org.apache.http;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

/**
 * An entity that can be sent or received with an HTTP message.
 * Entities can be found in some
 * {@link HttpEntityEnclosingRequest requests} and in
 * {@link HttpResponse responses}, where they are optional.
 * <p>
 * In some places, the JavaDoc distinguishes three kinds of entities,
 * depending on where their {@link #getContent content} originates:
 * <ul>
 * <li><b>streamed</b>: The content is received from a stream, or
 *     generated on the fly. In particular, this category includes
 *     entities being received from a {@link HttpConnection connection}.
 *     {@link #isStreaming Streamed} entities are generally not
 *      {@link #isRepeatable repeatable}.
 *     </li>
 * <li><b>self-contained</b>: The content is in memory or obtained by
 *     means that are independent from a connection or other entity.
 *     Self-contained entities are generally {@link #isRepeatable repeatable}.
 *     </li>
 * <li><b>wrapping</b>: The content is obtained from another entity.
 *     </li>
 * </ul>
 * This distinction is important for connection management with incoming
 * entities. For entities that are created by an application and only sent
 * using the HTTP components framework, the difference between streamed
 * and self-contained is of little importance. In that case, it is suggested
 * to consider non-repeatable entities as streamed, and those that are
 * repeatable (without a huge effort) as self-contained.
 *
 * @author <a href="mailto:oleg at ural.ru">Oleg Kalnichevski</a>
 *
 * @version $Revision: 645824 $
 * 
 * @since 4.0
 */
public interface HttpEntity {

    /**
     * Tells if the entity is capable to produce its data more than once.
     * A repeatable entity's getContent() and writeTo(OutputStream) methods
     * can be called more than once whereas a non-repeatable entity's can not.
     * @return true if the entity is repeatable, false otherwise.
     */
    boolean isRepeatable();

    /**
     * Tells about chunked encoding for this entity.
     * The primary purpose of this method is to indicate whether
     * chunked encoding should be used when the entity is sent.
     * For entities that are received, it can also indicate whether
     * the entity was received with chunked encoding.
     * <br/>
     * The behavior of wrapping entities is implementation dependent,
     * but should respect the primary purpose.
     *
     * @return  <code>true</code> if chunked encoding is preferred for this
     *          entity, or <code>false</code> if it is not
     */
    boolean isChunked();

    /**
     * Tells the length of the content, if known.
     *
     * @return  the number of bytes of the content, or
     *          a negative number if unknown. If the content length is known
     *          but exceeds {@link java.lang.Long#MAX_VALUE Long.MAX_VALUE},
     *          a negative number is returned.
     */
    long getContentLength();

    /**
     * Obtains the Content-Type header, if known.
     * This is the header that should be used when sending the entity,
     * or the one that was received with the entity. It can include a
     * charset attribute.
     *
     * @return  the Content-Type header for this entity, or
     *          <code>null</code> if the content type is unknown
     */
    Header getContentType();

    /**
     * Obtains the Content-Encoding header, if known.
     * This is the header that should be used when sending the entity,
     * or the one that was received with the entity.
     * Wrapping entities that modify the content encoding should
     * adjust this header accordingly.
     *
     * @return  the Content-Encoding header for this entity, or
     *          <code>null</code> if the content encoding is unknown
     */
    Header getContentEncoding();

    /**
     * Creates a new InputStream object of the entity.
     * It is a programming error
     * to return the same InputStream object more than once.
     * Entities that are not {@link #isRepeatable repeatable}
     * will throw an exception if this method is called multiple times.
     *
     * @return a new input stream that returns the entity data.
     *
     * @throws IOException if the stream could not be created
     * @throws IllegalStateException
     *  if this entity is not repeatable and the stream
     *  has already been obtained previously
     */
    InputStream getContent() throws IOException, IllegalStateException;

    /**
     * Writes the entity content to the output stream.  
     * 
     * @param outstream the output stream to write entity content to
     * 
     * @throws IOException if an I/O error occurs
     */
    void writeTo(OutputStream outstream) throws IOException;

    /**
     * Tells whether this entity depends on an underlying stream.
     * Streamed entities should return <code>true</code> until the
     * content has been consumed, <code>false</code> afterwards.
     * Self-contained entities should return <code>false</code>.
     * Wrapping entities should delegate this call to the wrapped entity.
     * <br/>
     * The content of a streamed entity is consumed when the stream
     * returned by {@link #getContent getContent} has been read to EOF,
     * or after {@link #consumeContent consumeContent} has been called.
     * If a streamed entity can not detect whether the stream has been
     * read to EOF, it should return <code>true</code> until
     * {@link #consumeContent consumeContent} is called.
     *
     * @return  <code>true</code> if the entity content is streamed and
     *          not yet consumed, <code>false</code> otherwise
     */
    boolean isStreaming(); // don't expect an exception here

    /**
     * TODO: The name of this method is misnomer. It will be renamed to
     * #finish() in the next major release.
     * <br/>
     * This method is called to indicate that the content of this entity
     * is no longer required. All entity implementations are expected to
     * release all allocated resources as a result of this method 
     * invocation. Content streaming entities are also expected to 
     * dispose of the remaining content, if any. Wrapping entities should 
     * delegate this call to the wrapped entity.
     * <br/>
     * This method is of particular importance for entities being
     * received from a {@link HttpConnection connection}. The entity
     * needs to be consumed completely in order to re-use the connection
     * with keep-alive.
     *
     * @throws IOException if an I/O error occurs.
     *          This indicates that connection keep-alive is not possible.
     */
    void consumeContent() throws IOException;

} // interface HttpEntity