/*
* 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.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* Annotation that signals to asynchronous logging components that messages of this type can safely be passed to
* a background thread without calling {@link Message#getFormattedMessage()} first.
* <p>
* Generally, logging mutable objects asynchronously always has the risk that the object is modified between the time
* the logger is called and the time the log message is formatted and written to disk. Strictly speaking it is the
* responsibility of the application to ensure that mutable objects are not modified after they have been logged,
* but this is not always possible.
* </p><p>
* Log4j prevents the above race condition as follows:
* </p><ol>
* <li>If the Message implements {@link ReusableMessage}, asynchronous logging components in the Log4j implementation
* will copy the message content (formatted message, parameters) onto the queue rather than passing the
* {@code Message} instance itself. This ensures that the formatted message will not change
* when the mutable object is modified.
* </li>
* <li>If the Message is annotated with {@link AsynchronouslyFormattable}, it can be passed to another thread as is.</li>
* <li>Otherwise, asynchronous logging components in the Log4j implementation will call
* {@link Message#getFormattedMessage()} before passing the Message object to another thread.
* This gives the Message implementation class a chance to create a formatted message String with the current value
* of the mutable object. The intention is that the Message implementation caches this formatted message and returns
* it on subsequent calls.
* (See <a href="https://issues.apache.org/jira/browse/LOG4J2-763">LOG4J2-763</a>.)
* </li>
* </ol>
*
* @see Message
* @see ReusableMessage
* @see <a href="https://issues.apache.org/jira/browse/LOG4J2-763">LOG4J2-763</a>
* @since 2.8
*/
@Documented // This annotation is part of the public API of annotated elements.
@Target(ElementType.TYPE) // Only applies to types.
@Retention(RetentionPolicy.RUNTIME) //Needs to be reflectively discoverable runtime.
public @interface AsynchronouslyFormattable {
}