Logging

Spring Boot has no mandatory logging dependency, except for the Commons Logging API, which is typically provided by Spring Framework’s spring-jcl module. To use Logback, you need to include it and spring-jcl on the classpath. The recommended way to do that is through the starters, which all depend on spring-boot-starter-logging. For a web application, you only need spring-boot-starter-web, since it depends transitively on the logging starter. If you use Maven, the following dependency adds logging for you:

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
</dependency>

Spring Boot has a LoggingSystem abstraction that attempts to configure logging based on the content of the classpath. If Logback is available, it is the first choice.

If the only change you need to make to logging is to set the levels of various loggers, you can do so in application.properties by using the "logging.level" prefix, as shown in the following example:

  • Properties

  • YAML

logging.level.org.springframework.web=debug
logging.level.org.hibernate=error
logging:
  level:
    org.springframework.web: "debug"
    org.hibernate: "error"

You can also set the location of a file to which the log will be written (in addition to the console) by using logging.file.name.

To configure the more fine-grained settings of a logging system, you need to use the native configuration format supported by the LoggingSystem in question. By default, Spring Boot picks up the native configuration from its default location for the system (such as classpath:logback.xml for Logback), but you can set the location of the config file by using the logging.config property.

Configure Logback for Logging

If you need to apply customizations to logback beyond those that can be achieved with application.properties, you will need to add a standard logback configuration file. You can add a logback.xml file to the root of your classpath for logback to find. You can also use logback-spring.xml if you want to use the Spring Boot Logback Extensions.

The Logback documentation has a dedicated section that covers configuration in some detail.

Spring Boot provides a number of logback configurations that can be included in your own configuration. These includes are designed to allow certain common Spring Boot conventions to be re-applied.

The following files are provided under org/springframework/boot/logging/logback/:

  • defaults.xml - Provides conversion rules, pattern properties and common logger configurations.

  • console-appender.xml - Adds a ConsoleAppender using the CONSOLE_LOG_PATTERN.

  • file-appender.xml - Adds a RollingFileAppender using the FILE_LOG_PATTERN and ROLLING_FILE_NAME_PATTERN with appropriate settings.

In addition, a legacy base.xml file is provided for compatibility with earlier versions of Spring Boot.

A typical custom logback.xml file would look something like this:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
	<include resource="org/springframework/boot/logging/logback/defaults.xml"/>
	<include resource="org/springframework/boot/logging/logback/console-appender.xml" />
	<root level="INFO">
		<appender-ref ref="CONSOLE" />
	</root>
	<logger name="org.springframework.web" level="DEBUG"/>
</configuration>

Your logback configuration file can also make use of System properties that the LoggingSystem takes care of creating for you:

  • ${PID}: The current process ID.

  • ${LOG_FILE}: Whether logging.file.name was set in Boot’s external configuration.

  • ${LOG_PATH}: Whether logging.file.path (representing a directory for log files to live in) was set in Boot’s external configuration.

  • ${LOG_EXCEPTION_CONVERSION_WORD}: Whether logging.exception-conversion-word was set in Boot’s external configuration.

  • ${ROLLING_FILE_NAME_PATTERN}: Whether logging.pattern.rolling-file-name was set in Boot’s external configuration.

Spring Boot also provides some nice ANSI color terminal output on a console (but not in a log file) by using a custom Logback converter. See the CONSOLE_LOG_PATTERN in the defaults.xml configuration for an example.

If Groovy is on the classpath, you should be able to configure Logback with logback.groovy as well. If present, this setting is given preference.

Spring extensions are not supported with Groovy configuration. Any logback-spring.groovy files will not be detected.

Configure Logback for File-only Output

If you want to disable console logging and write output only to a file, you need a custom logback-spring.xml that imports file-appender.xml but not console-appender.xml, as shown in the following example:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
	<include resource="org/springframework/boot/logging/logback/defaults.xml" />
	<property name="LOG_FILE" value="${LOG_FILE:-${LOG_PATH:-${LOG_TEMP:-${java.io.tmpdir:-/tmp}}/}spring.log}"/>
	<include resource="org/springframework/boot/logging/logback/file-appender.xml" />
	<root level="INFO">
		<appender-ref ref="FILE" />
	</root>
</configuration>

You also need to add logging.file.name to your application.properties or application.yaml, as shown in the following example:

  • Properties

  • YAML

logging.file.name=myapplication.log
logging:
  file:
    name: "myapplication.log"

Configure Log4j for Logging

Spring Boot supports Log4j 2 for logging configuration if it is on the classpath. If you use the starters for assembling dependencies, you have to exclude Logback and then include Log4j 2 instead. If you do not use the starters, you need to provide (at least) spring-jcl in addition to Log4j 2.

The recommended path is through the starters, even though it requires some jiggling. The following example shows how to set up the starters in Maven:

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter</artifactId>
	<exclusions>
		<exclusion>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-logging</artifactId>
		</exclusion>
	</exclusions>
</dependency>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>

Gradle provides a few different ways to set up the starters. One way is to use a module replacement. To do so, declare a dependency on the Log4j 2 starter and tell Gradle that any occurrences of the default logging starter should be replaced by the Log4j 2 starter, as shown in the following example:

dependencies {
	implementation "org.springframework.boot:spring-boot-starter-log4j2"
	modules {
		module("org.springframework.boot:spring-boot-starter-logging") {
			replacedBy("org.springframework.boot:spring-boot-starter-log4j2", "Use Log4j2 instead of Logback")
		}
	}
}
The Log4j starters gather together the dependencies for common logging requirements (such as having Tomcat use java.util.logging but configuring the output using Log4j 2).
To ensure that debug logging performed using java.util.logging is routed into Log4j 2, configure its JDK logging adapter by setting the java.util.logging.manager system property to org.apache.logging.log4j.jul.LogManager.

Use YAML or JSON to Configure Log4j 2

In addition to its default XML configuration format, Log4j 2 also supports YAML and JSON configuration files. To configure Log4j 2 to use an alternative configuration file format, add the appropriate dependencies to the classpath and name your configuration files to match your chosen file format, as shown in the following example:

Format Dependencies File names

YAML

com.fasterxml.jackson.core:jackson-databind + com.fasterxml.jackson.dataformat:jackson-dataformat-yaml

log4j2.yaml + log4j2.yml

JSON

com.fasterxml.jackson.core:jackson-databind

log4j2.json + log4j2.jsn

Use Composite Configuration to Configure Log4j 2

Log4j 2 has support for combining multiple configuration files into a single composite configuration. To use this support in Spring Boot, configure logging.log4j2.config.override with the locations of one or more secondary configuration files. The secondary configuration files will be merged with the primary configuration, whether the primary’s source is Spring Boot’s defaults, a standard location such as log4j.xml, or the location configured by the logging.config property.