1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache license, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the license for the specific language governing permissions and 15 * limitations under the license. 16 */ 17 package org.apache.logging.log4j.message; 18 19 import java.lang.annotation.Documented; 20 import java.lang.annotation.ElementType; 21 import java.lang.annotation.Retention; 22 import java.lang.annotation.RetentionPolicy; 23 import java.lang.annotation.Target; 24 25 /** 26 * Annotation that signals to asynchronous logging components that messages of this type can safely be passed to 27 * a background thread without calling {@link Message#getFormattedMessage()} first. 28 * <p> 29 * Generally, logging mutable objects asynchronously always has the risk that the object is modified between the time 30 * the logger is called and the time the log message is formatted and written to disk. Strictly speaking it is the 31 * responsibility of the application to ensure that mutable objects are not modified after they have been logged, 32 * but this is not always possible. 33 * </p><p> 34 * Log4j prevents the above race condition as follows: 35 * </p><ol> 36 * <li>If the Message implements {@link ReusableMessage}, asynchronous logging components in the Log4j implementation 37 * will copy the message content (formatted message, parameters) onto the queue rather than passing the 38 * {@code Message} instance itself. This ensures that the formatted message will not change 39 * when the mutable object is modified. 40 * </li> 41 * <li>If the Message is annotated with {@link AsynchronouslyFormattable}, it can be passed to another thread as is.</li> 42 * <li>Otherwise, asynchronous logging components in the Log4j implementation will call 43 * {@link Message#getFormattedMessage()} before passing the Message object to another thread. 44 * This gives the Message implementation class a chance to create a formatted message String with the current value 45 * of the mutable object. The intention is that the Message implementation caches this formatted message and returns 46 * it on subsequent calls. 47 * (See <a href="https://issues.apache.org/jira/browse/LOG4J2-763">LOG4J2-763</a>.) 48 * </li> 49 * </ol> 50 * 51 * @see Message 52 * @see ReusableMessage 53 * @see <a href="https://issues.apache.org/jira/browse/LOG4J2-763">LOG4J2-763</a> 54 * @since 2.8 55 */ 56 @Documented // This annotation is part of the public API of annotated elements. 57 @Target(ElementType.TYPE) // Only applies to types. 58 @Retention(RetentionPolicy.RUNTIME) //Needs to be reflectively discoverable runtime. 59 public @interface AsynchronouslyFormattable { 60 }