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The Pax Logging API Library is to allow for the Pax Logging Service to be reloaded without stopping the many dependent bundles. It also contains the OSGi Log Service API and the Knopflerfish Log API.

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/*
 * 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.
 */
package org.apache.logging.log4j.message;

import java.io.Serializable;

/**
 * An interface for various Message implementations that can be logged. Messages can act as wrappers
 * around Objects so that user can have control over converting Objects to Strings when necessary without
 * requiring complicated formatters and as a way to manipulate the message based on information available
 * at runtime such as the locale of the system.
 *
 * Note: Message objects should not be considered to be thread safe nor should they be assumed to be
 * safely reusable even on the same thread. The logging system may provide information to the Message
 * objects and the Messages might be queued for asynchronous delivery. Thus, any modifications to a
 * Message object by an application should by avoided after the Message has been passed as a parameter on
 * a Logger method.
 * 
 * TODO Interfaces should rarely extend Serializable according to Effective Java 2nd Ed pg 291.
 * (RG) That section also says "If a class or interface exists primarily to participate in a framework that
 * requires all participants to implement Serializable, then it makes perfect sense for the class or
 * interface to implement or extend Serializable". Such is the case here as the LogEvent must be Serializable.
 */
public interface Message extends Serializable {

    /**
     * Gets the Message formatted as a String. Each Message implementation determines the
     * appropriate way to format the data encapsulated in the Message. Messages that provide
     * more than one way of formatting the Message will implement MultiformatMessage.
     *
     * @return The message String.
     */
    String getFormattedMessage();

    /**
     * Gets the format portion of the Message.
     *
     * @return The message format. Some implementations, such as ParameterizedMessage, will use this as
     * the message "pattern". Other Messages may simply return an empty String.
     * TODO Do all messages have a format?  What syntax?  Using a Formatter object could be cleaner.
     * (RG) In SimpleMessage the format is identical to the formatted message. In ParameterizedMessage and
     * StructuredDataMessage it is not. It is up to the Message implementer to determine what this
     * method will return. A Formatter is inappropriate as this is very specific to the Message
     * implementation so it isn't clear to me how having a Formatter separate from the Message would be cleaner.
     */
    String getFormat();

    /**
     * Gets parameter values, if any.
     *
     * @return An array of parameter values or null.
     */
    Object[] getParameters();

    /**
     * Gets the throwable, if any.
     *
     * @return the throwable or null.
     */
    Throwable getThrowable();
}