Perfect World

How to find out how the system works?

• Debugging
• Logging
• (Exploratory) testing
• and others

Debugging

Logging

Log levels

• FATAL
• ERROR
• WARN(ING)
• INFO
• DEBUG
• TRACE
• OFF

Setup

/path/to/bin/gradle wrapper --gradle-version 4.3.1
                

./gradlew init --type java-application
                

Let's log something

https://en.wikipedia.org/wiki/Scribe#/media/File:Escribano.jpg

Dependencies

dependencies {
    compile "org.slf4j:slf4j-api:1.7.25"
    runtimeOnly "ch.qos.logback:logback-classic:1.2.3"
}
            

Parameters

logger.debug("Fancy log statement with param1 {} and param2 {}",
             param1, getParam2());
            

catch(Exception e) {
    logger.debug("Aw, snap, an exception...", e);
}
            

Status

// assume SLF4J is bound to logback in the current environment
LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();
// print logback's internal status
StatusPrinter.print(lc);
                

Appenders

• Console
• (Rolling)File
• many others

Bugs

JMX Configurator

<contextName>logging-example-simple</contextName>
<jmxConfigurator />
            

jconsole

Some life in configuration

Janino

"Inheritance"

https://en.wikipedia.org/wiki/Charles,_Prince_of_Wales#/media/File:Charles,_Prince_of_Wales_at_COP21.jpg

Filtering

Mapped Diagnostic Context

[user: %mdc{username}] - %msg %n
            

Next week: Logging, part 2

Graylog: one tool to log 'em all.