Log4j logging
Logging Docs
See https://docs.spring.io/spring-boot/docs/2.3.6.RELEASE/reference/html/howto.html#howto-configure-log4j-for-logging for details.
Goals¶
- Convert
println
s to use logging via the SLF4J logging facade - Configure and use the Log4J 2 logging framework instead of the default Logback logger
a. Replace System.out.println with Logger¶
-
Add the following imports:
import org.slf4j.Logger; import org.slf4j.LoggerFactory;
-
Create the
LOGGER
private final static Logger LOGGER = LoggerFactory.getLogger(ClassName.class);
Replace
ClassName
with the class containing the logger. -
Find all of the
System.out.println
statements and replace them with a mix ofLOGGER.info
andLOGGER.debug
statements. Be sure to use the correct logging syntax, e.g.:LOGGER.info("{} has been instantiated.", this.getClass().getName());
b. Switch to Log4J 2¶
Since Logback is the default logger for Spring Boot, to use Log4J 2 instead, the Maven POM file needs to exclude Logback and add in Log4J 2.
Modify the pom.xml
file by adding the following two sections to the <dependencies>
section.
The first one excludes the Logback logger, the second one adds the Log4J 2 logger:
<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>
Try running the application again to see how things change.
(The changes are slight, e.g., the class name for Log4J 2 displays as c.t.c.CoffeeOrderComponent
instead of c.t.coffeekiosk.CoffeeOrderComponent
, though this can be configured by the log4j2.xml file mentioned later.)
c. Use Log4J 2 Directly¶
If there are features from Log4J 2 that you need that aren't supported by the SLF4J facade, you can replace the slf4j
imports with:
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager;
Then create the logger this way:
private static final Logger LOGGER = LogManager.getLogger(ClassName.class);
Try changing one of the classes to use the log4j-specific logger.
d. Change Logging Level¶
With Spring Boot, one way to change the logging level is to specify it in the application.properties
file, e.g.:
logging.level.org.springframework.web=DEBUG
logging.level.com.welltestedlearning=WARN
Now when you run the application, you won't see any logging output from our classes as its level was set to WARN
.
You can comment out (add a #
at the beginning of a line) or remove the lines to restore the default level.
Logging Levels
Custom Log4J 2 Configuration File
You can also use a log4j2.xml
configuration file for more complex needs as shown below.
This file would be placed in the /src/main/resources
directory.
<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
<Appenders>
<Console name="Console" target="SYSTEM_OUT">
<PatternLayout
pattern="%style{%d{ISO8601}}{black} %highlight{%-5level }[%style{%t}{bright,blue}] %style{%C{1.}}{bright,yellow}: %msg%n%throwable" />
</Console>
<RollingFile name="RollingFile"
fileName="./logs/spring-boot-logger-log4j2.log"
filePattern="./logs/$${date:yyyy-MM}/spring-boot-logger-log4j2-%d{-dd-MMMM-yyyy}-%i.log.gz">
<PatternLayout>
<pattern>%d %p %C{1.} [%t] %m%n</pattern>
</PatternLayout>
<Policies>
<!-- rollover policies -->
<OnStartupTriggeringPolicy /> <!-- rollover upon startup -->
<SizeBasedTriggeringPolicy size="10 MB" /> <!-- rollover at 100MB file size -->
<TimeBasedTriggeringPolicy /> <!-- rollover on a daily basis -->
</Policies>
</RollingFile>
</Appenders>
<Loggers>
<!-- Default logging at INFO level -->
<Root level="info">
<AppenderRef ref="Console" />
<AppenderRef ref="RollingFile" />
</Root>
<!-- LOG "com.welltestedlearning.**" at WARN level -->
<Logger name="com.welltestedlearning" level="warn"></Logger>
</Loggers>
</Configuration>