74. Logging

74. Logging
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Spring Boot has no mandatory logging dependency, except for the Commons Logging API, of which there are many implementations to choose from. To use Logback you need to include it and jcl-over-slf4j (which implements the Commons Logging API) on the classpath. The simplest 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. For example, using Maven:

<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 then you can do that in application.properties using the "logging.level" prefix, e.g.

logging.level.org.springframework.web=DEBUG
logging.level.org.hibernate=ERROR

You can also set the location of a file to log to (in addition to the console) using "logging.file".

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 (e.g. classpath:logback.xml for Logback), but you can set the location of the config file using the "logging.config" property.

74.1 Configure Logback for logging

If you put a logback.xml in the root of your classpath it will be picked up from there (or logback-spring.xml to take advantage of the templating features provided by Boot). Spring Boot provides a default base configuration that you can include if you just want to set levels, e.g.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <include resource="org/springframework/boot/logging/logback/base.xml"/>
    <logger name="org.springframework.web" level="DEBUG"/>
</configuration>

If you look at that base.xml in the spring-boot jar, you will see that it uses some useful System properties which the LoggingSystem takes care of creating for you. These are:

${PID} the current process ID.
${LOG_FILE} if logging.file was set in Boot’s external configuration.
${LOG_PATH} if logging.path was set (representing a directory for log files to live in).
${LOG_EXCEPTION_CONVERSION_WORD} if logging.exception-conversion-word was set in Boot’s external configuration.

Spring Boot also provides some nice ANSI colour terminal output on a console (but not in a log file) using a custom Logback converter. See the default base.xml configuration for details.

If Groovy is on the classpath you should be able to configure Logback with logback.groovy as well (it will be given preference if present).

74.1.1 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:

<?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 to your application.properties:

logging.file=myapplication.log

74.2 Configure Log4j for logging

Spring Boot supports Log4j 2 for logging configuration if it is on the classpath. If you are using the starters for assembling dependencies that means you have to exclude Logback and then include log4j 2 instead. If you aren’t using the starters then you need to provide jcl-over-slf4j (at least) in addition to Log4j 2.

The simplest path is probably through the starters, even though it requires some jiggling with excludes, .e.g. 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>

	Note
	The use of the Log4j starters gathers together the dependencies for common logging requirements (e.g. including having Tomcat use `java.util.logging` but configuring the output using Log4j 2). See the Actuator Log4j 2 samples for more detail and to see it in action.

74.2.1 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:

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`

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