Logging with Java

overview

• loggers define which package and which log level you want to use
• appenders define how messages are logged (File, Console, RollingFile etc.)
• you can connect one or more appenders to one logger
• if no appender 'catches' the log it will be handled by the root logger (which you need to connect to a logger as well as set a log level)

scafffolding

• use slf4j with simple-logger to print to console
• if you want to configure loggers and appenders use log4j2 or logback
• if you have no strong opinion on that use logback because it rolling filling appender config is way more convenient
• if you use spring boot
• just use slf4j loggers in code (Spring boot uses logback under the hood)
• if you want to configuration
  • configure with logging.level.* in properties.yaml
    • e.g. ogging.level.org=DEBUG
  • or add a logback-spring.xml to resources folder
• if you use logback-spring.xml you can use properties from appliyation.yaml with <springProperty name="targetname" source="spring.property.name"/>

slf4j + simple logger

This setting does not need a configuration to work.

slf4j + logback + spring

• remember it is called logback not lockback
• Paste listing content into src/main/resources/logback-spring.xml
• add app.name Property to src/main/resources/application.properties

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

    <!-- to load default spring layout -->
    <include resource="org/springframework/boot/logging/logback/defaults.xml"/>

    <!-- to load properties from application.properties -->

    <property resource="application.properties" />

    <!-- app.name needs to be defined in application.properties (otherwise it will use '${app.name}' as folder name)-->

    <property name="APPLICATION_NAME" value="${app.name}"/>
    <property name="PATH_LOGS" value="/tmp/${APPLICATION_NAME}/logs" />
    <property name="LAYOUT" value="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    <appender name="Console" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <!-- default spring layout -->
            <Pattern>${CONSOLE_LOG_PATTERN}</Pattern>
        </encoder>
    </appender>

    <appender name="RollingFile" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>${PATH_LOGS}/${APPLICATION_NAME}.log</file>
        <encoder>
            <Pattern>${LAYOUT}</Pattern>
        </encoder>
        <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
            <fileNamePattern>${PATH_LOGS}/archived/${APPLICATION_NAME}-%d{yyyy-MM-dd}.%i.log</fileNamePattern>
            <maxFileSize>10MB</maxFileSize>
            <!-- deletes oldest files when totalSizeCap or maxHostory is reached -->
            <totalSizeCap>100MB</totalSizeCap>
            <maxHistory>30</maxHistory> 
        </rollingPolicy>
    </appender>

    <logger name="de.weyrich" level="debug" additivity="false">
        <appender-ref ref="RollingFile" />
        <appender-ref ref="Console" />
    </logger>

    <root level="info">
        <appender-ref ref="RollingFile" />
        <appender-ref ref="Console" />
    </root>

</configuration>    

slf4j + log4j2

• does not need a configuration to funciton
• add log4j2.xml to resources folder (not log4j.xml!)
• the name of the logger defines which packages are targeted. You do not need wildcards, you just end with the package part you want to include (e.g. de.weyrich)
• you cann access system properties with ${sys:propertyName} Maven:

    <properties>
        <slf4j.version>1.7.5</slf4j.version>
        <log4j.version>2.13.3</log4j.version>
    </properties>
    ...
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-api</artifactId>
        <version>${slf4j.version}</version>
    </dependency>
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-slf4j-impl</artifactId>
        <version>${log4j.version}</version>
    </dependency>
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-api</artifactId>
        <version>${log4j.version}</version>
    </dependency>
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-core</artifactId>
        <version>${log4j.version}</version>
    </dependency>

log4j2.xml

<Configuration status="info">
    <Appenders>
        <File name="FILE" fileName="application.log">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
        </File>
        <Console name="CONSOLE">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
        </Console>
    </Appenders>
    <Loggers>
        <Logger name="de" level="DEBUG">
            <AppenderRef ref="CONSOLE"/>
            <AppenderRef ref="FILE"/>
        </Logger>
         <!-- addtional loggers e.g. for used libraries-->

        <Root level="WARN">
            <AppenderRef ref="CONSOLE"/>
        </Root>
    </Loggers>
</Configuration>