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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.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 Message.getFormattedMessage() first. 

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.

Log4j prevents the above race condition as follows:

If the Message implements ReusableMessage, asynchronous logging components in the Log4j implementation will copy the message content (formatted message, parameters) onto the queue rather than passing the Message instance itself. This ensures that the formatted message will not change when the mutable object is modified. 
If the Message is annotated with AsynchronouslyFormattable, it can be passed to another thread as is.
Otherwise, asynchronous logging components in the Log4j implementation will call 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 LOG4J2-763.)


See Also:
Message
ReusableMessage
LOG4J2-763
Since:
2.8/**
 * 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 {
}