001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements. See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache license, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License. You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the license for the specific language governing permissions and
015     * limitations under the license.
016     */
017    package org.apache.logging.log4j.core;
018    
019    import java.io.Serializable;
020    import java.util.Map;
021    
022    /**
023     * Lays out a {@linkplain LogEvent} in different formats.
024     *
025     * The formats are:
026     * <ul>
027     * <li>
028     * {@code byte[]}</li>
029     * <li>
030     * an implementer of {@linkplain Serializable}, like {@code byte[]}</li>
031     * <li>
032     * {@linkplain String}</li>
033     * <li>
034     * {@linkplain LogEvent}</li>
035     * </ul>
036     *
037     * @param <T>
038     *            The {@link Serializable} type returned by {@link #toSerializable(LogEvent)}
039     *
040     * TODO There is still a need for a character-based layout for character based event sinks (databases, etc). Would
041     * introduce an EventEncoder, EventRenderer or something similar for the logging event to byte encoding. (RG) A layout
042     * can be configured with a Charset and then Strings can be converted to byte arrays. OTOH, it isn't possible to write
043     * byte arrays as character streams.
044     */
045    public interface Layout<T extends Serializable> {
046    
047        /**
048         * Main plugin element type for Layout plugins.
049         *
050         * @since 2.1
051         */
052        String ELEMENT_TYPE = "layout";
053    
054        /**
055         * Returns the format for the layout format.
056         * @return The footer.
057         */
058        byte[] getFooter();
059    
060        /**
061         * Returns the header for the layout format.
062         * @return The header.
063         */
064        byte[] getHeader();
065    
066        /**
067         * Formats the event suitable for display.
068         *
069         * @param event The Logging Event.
070         * @return The formatted event.
071         * TODO Likely better to write to a OutputStream instead of return a byte[]. (RG) That limits how the
072         * Appender can use the Layout. For example, it might concatenate information in front or behind the
073         * data and then write it all to the OutputStream in one call.
074         */
075        byte[] toByteArray(LogEvent event);
076    
077        // TODO: it would be nice to provide ByteBuffers alongside the byte[]s
078    
079        /**
080         * Formats the event as an Object that can be serialized.
081         *
082         * @param event The Logging Event.
083         * @return The formatted event.
084         */
085        T toSerializable(LogEvent event);
086    
087        /**
088         * Returns the content type output by this layout. The base class returns "text/plain".
089         *
090         * @return the content type.
091         */
092        String getContentType();
093    
094        /**
095         * Returns a description of the content format.
096         *
097         * @return a Map of key/value pairs describing the Layout-specific content format, or an empty Map if no content
098         * format descriptors are specified.
099         */
100        Map<String, String> getContentFormat();
101    }