Java Logging Tips/Tricks - 2016

Don't log sensitive information
Include as much dynamic info as possible
Paste logging in PR if applicable
Make sure there is enough logging to help trouble shooting issues in production later

sfl4j
log.warn("hello: {}, {}", "A", "B", new RuntimeException("wtf"));

- will print the exception's stack trace

org.slf4j.helpers.MessageFormatter.arrayFormat(String, Object[])
Throwable throwableCandidate = getThrowableCandidate(argArray);

Logback
-Dlogback.debug=true
<configuration debug="true">

Log method and line number
%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36}.%M\(%line\) - %msg%n

Pattern
Use %M to print method name in debug environment.
%class{1}

MDC - Mapped Diagnostic Context
%X{X-RequestId}, %X{sessionId}
MDC.put("X-RequestId", requestId)
-- Don't forget to MDC.clear() at the beginning and end of the request.

Use MDC in thread pools

@SuppressWarnings("serial")
public class MdcThreadPoolExecutor extends ThreadPoolTaskExecutor {
    public MdcThreadPoolExecutor() {
        super();
    }
    @Override
    public void execute(final Runnable command) {
        super.execute(wrap(command, MDC.getCopyOfContextMap()));
    }
    public static Runnable wrap(final Runnable runnable, final Map<String, String> context) {
        return new Runnable() {
            @Override
            public void run() {
                final Map<String, String> previous = MDC.getCopyOfContextMap();
                if (context == null) {
                    MDC.clear();
                } else {
                    MDC.setContextMap(context);
                }
                try {
                    runnable.run();
                } finally {
                    if (previous == null) {
                        MDC.clear();
                    } else {
                        MDC.setContextMap(previous);
                    }
                }
            }
        };
    }
}

Duplicated log - fix: additivity="false"
Only log stats into app-stats.log

- Don't log to app.log and stdout(in dev)

Async logging

<root level="INFO">
  <appender-ref ref="dailyrolling" />
  <appender-ref ref="STDOUT" />
</root>
<logger name="common.statistic" level="INFO" additivity="false">
  <appender-ref ref="statsRolling" />
</logger>

<appender name="statsRolling-async" class="ch.qos.logback.classic.AsyncAppender">
  <queueSize>1000</queueSize>
  <discardingThreshold>0</discardingThreshold>
  <appender-ref ref="statsRolling" />
</appender>
<if condition='property("env").equals("local") || property("env").equals("docker")'>
  <then>
  </then>
  <else>
    <root level="WARN">
      <appender-ref ref="dailyrolling-async" />
    </root>
    <logger name="com.xyz.services" level="INFO">
      <appender-ref ref="dailyrolling-async" />
    </logger>
    <logger name="com.xyz.statistic" level="INFO">
      <appender-ref ref="statsRolling-async" />
    </logger>
  </else>
</if>

Parameterized Logging
logger.error("xx {} yy {} xx {}", x, y, z);

SiftingAppender

Log4j
-Dlog4j.debug

Log4j2
https://logging.apache.org/log4j/2.0/faq.html#troubleshooting
-DLog4jDefaultStatusLevel=DEBUG
-Dorg.apache.logging.log4j.simplelog.StatusLogger.level=TRACE
Lazy Logging
logger.trace("Some long-running operation returned {}", () -> expensiveOperation());

Logging level configuration
Logging Cassandra Query CQL
Set log level of com.datastax.driver.core.RequestHandler = TRACE
Programmatic configuration of slf4j/logback

splunk - logging best practice
Use clear key-value pairs

To clear log without delete the file
cp /dev/null log
> log