{"id":5413,"date":"2023-09-26T07:56:53","date_gmt":"2023-09-26T06:56:53","guid":{"rendered":"https:\/\/www.baeldung.com\/java-log4j-properties-guide"},"modified":"2023-09-26T07:56:53","modified_gmt":"2023-09-26T06:56:53","slug":"a-guide-to-log4j-and-the-log4j-properties-file-in-java","status":"publish","type":"post","link":"https:\/\/gamefootballmobileanimeiphone.com\/index.php\/2023\/09\/26\/a-guide-to-log4j-and-the-log4j-properties-file-in-java\/","title":{"rendered":"A Guide to Log4j and the log4j.properties File in Java"},"content":{"rendered":"

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1. Introduction<\/h2>\n
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Log4J is a popular, open-source logging framework<\/a> written in Java. Various Java-based applications widely use Log4j. Moreover, it’s thread-safe, fast, and provides a named Logger<\/em> hierarchy. Log4j is distributed under the open-source Apache Software License.<\/p>\n

Log4j 1.x reached the end of life on August 5, 2015. Therefore, as of today, Log4j2<\/a> is the latest upgrade to Log4j.<\/strong><\/p>\n

In this tutorial, we’ll learn about Log4j and how to configure the core Log4j components using the log4j.properties<\/em> file in Java.<\/p>\n

2. Maven Setup<\/h2>\n
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We’ll need the log4j-core<\/em>\u00a0dependency in our\u00a0pom.xml<\/em> to start with:<\/p>\n

<dependency>\r\n    <groupId>org.apache.logging.log4j<\/groupId>\r\n    <artifactId>log4j-api<\/artifactId>\r\n    <version>1.2.17<\/version>\r\n<\/dependency><\/code><\/pre>\n
The latest version of log4j-core\u00a0<\/em>can be found\u00a0here<\/a>.<\/p>\n
3. The Log4j API<\/h2>\n
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The Log4j API provides the mechanism to pass on the logging information based on various levels of priorities and direct it to various destinations such as files, consoles, databases, etc. It also supports filtering log events before passing them to logger<\/em>s or appender<\/em>s<\/a>.<\/p>\n
The Log4j API has a layered architecture that provides two types of objects in the Log4j framework – core objects and support objects.<\/p>\n
4. Log4j Components<\/h2>\n
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There are three main components of Log4j – logger<\/em>s, appender<\/em>s, and layout<\/em>s – that could be used together to print the customized log statements at the desired destinations. Let’s look at them in brief.<\/p>\n
4.1. Logger<\/em><\/h3>\n
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The Logger<\/em> object is responsible for representing the logging information. It’s the first mandatory layer in Log4j architecture. The Logger<\/em> class is defined in package org.apache.log4j<\/em>.<\/p>\n
Generally, we create one Logger<\/em> instance per application class to log important events belonging to that class<\/strong>. Also, we generally create this instance at the beginning of the class using a static factory method that accepts the class name as a parameter:<\/p>\n
private static final Logger logger = Logger.getLogger(JavaClass.class.getName());<\/code><\/pre>\n
Subsequently, we can use various methods of the Logger<\/em> class to log or print important events depending on their categories. These methods are trace()<\/em>, debug()<\/em>, info()<\/em>, warn()<\/em>, error()<\/em>, fatal()<\/em>. These methods determine the level of a logging request.<\/p>\n
The priority order of the Logger<\/em> methods is: TRACE<\/em> < DEBUG<\/em> < INFO<\/em> < WARN<\/em> < ERROR<\/em> < FATAL<\/em>. Therefore, these methods print the log messages depending on the logger<\/em> level set in the log4j.properties<\/em> file. This means if we set the logger<\/em> level as INFO<\/em>, then all the INFO<\/em>, WARN<\/em>, ERROR<\/em>, and FATAL<\/em> events will be logged.<\/p>\n
4.2. Appender<\/em><\/h3>\n
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Appender<\/em> denotes the destination of log output. We can print the log out to multiple preferred destinations using Log4j like console, files, remote socket server, database, etc. We refer to these output destinations as Appender<\/em>s. Moreover, we can attach multiple appender<\/em>s to a Logger<\/em>.<\/p>\n
Appender<\/em>s work according to the appender<\/em> additivity rule<\/a>. This rule states that the output of a log statement of any Logger<\/em> will go to all of its appender<\/em>s and its ancestors –<\/strong> the appender<\/em>s that are higher in the hierarchy<\/b>.<\/p>\n
Log4j has multiple appender<\/em>s defined for files, consoles, GUI components, remote socket servers, JMS, etc.<\/p>\n
4.3. Layout<\/em><\/h3>\n
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We use layout<\/em>s for customizing the format of the log statements. We can do this by associating a layout<\/em> with the already defined appender<\/em>. Thus, a combination of layout<\/em> and appender<\/em>s helps us send the formatted log statements to the desired destinations.<\/p>\n
We can specify the format of log statements using conversion patterns. The class PatternLayout<\/em><\/a> explains more about conversion characters that we can use based on our needs.<\/strong><\/p>\n
We’ll also understand about few of the conversion characters through examples in the following sections.<\/p>\n
5. The log4j.properties<\/em> File<\/h2>\n
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We can configure Log4j using XML or the properties file. The log4j.properties<\/em> file stores the configurations in key-value pairs.<\/p>\n
The default name of the log4j properties configuration file is log4j.properties<\/em>. The Logger<\/em> looks for this file name in the CLASSPATH<\/a><\/em>. However, if we need to use a different configuration file name, we can set it using the system property log4j.configuration<\/em>.<\/strong><\/p>\n
The log4j.properties<\/em> file contains the specifications of appender<\/em>s, their names and types, and layout<\/em> patterns. It also contains specifications about the default root Logger<\/em> and its log levels.<\/p>\n
6. Syntax of the log4j.properties<\/em> File<\/h2>\n
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In a general log4j.properties<\/em> file, we define the following configurations:<\/p>\n