logback: newline before exception stack trace but not otherwise?

Question

If I put a newline before the exception stack trace in the logging pattern:

<Pattern>%d{HH:mm:ss.SSS} %-5level [%thread] %logger{36} %msg %n %rEx{short} %n</Pattern>

then an extra newline is printed, so normal logging statements end up with blank lines in between them.

If I remove the extra newline:

<Pattern>%d{HH:mm:ss.SSS} %-5level [%thread] %logger{36} %msg %rEx{short} %n</Pattern>

then stack traces do not start on a new line, making them hard to read.

Can anyone think of a way to get logback to only print the extra newline when there is a stack trace, and not otherwise?

Answer 1

I have a working solution, but I dislike it because it's overengineered and should be possible more elegantly.

It's a specific case of a more generic problem: output a pattern only if a certain evaluator (one that checks the presence of an exception) matches. I got the implementation from somewhere here on SO long ago.

Requires janino. If anyone more logback-experienced than me can trim this down to a more reasonable amount of code, please do.

IfPresentConverter.java

public class IfPresentConverter extends CompositeConverter<ILoggingEvent> {
    private final List<EventEvaluator<ILoggingEvent>> evaluatorList = new ArrayList<>();
    private int errorCount = 0;

    @Override
    @SuppressWarnings("unchecked")
    public void start() {
        final List<String> optionList = getOptionList();
        final Map<?, ?> evaluatorMap = (Map<?, ?>) getContext().getObject(CoreConstants.EVALUATOR_MAP);

        for (final String evaluatorStr : optionList) {
            final EventEvaluator<ILoggingEvent> ee = (EventEvaluator<ILoggingEvent>) evaluatorMap.get(evaluatorStr);
            if (ee != null) {
                evaluatorList.add(ee);
            }
        }

        if (evaluatorList.isEmpty()) {
            addError("At least one evaluator is expected, but you have declared none.");
            return;
        }

        super.start();
    }

    @Override
    public String convert(final ILoggingEvent event) {
        boolean evalResult = true;
        for (final EventEvaluator<ILoggingEvent> ee : evaluatorList) {
            try {
                if (!ee.evaluate(event)) {
                    evalResult = false;
                    break;
                }
            } catch (final EvaluationException eex) {
                evalResult = false;

                errorCount++;
                if (errorCount < CoreConstants.MAX_ERROR_COUNT) {
                    addError("Exception thrown for evaluator named [" + ee.getName() + "].", eex);
                } else {
                    final ErrorStatus errorStatus = new ErrorStatus("Exception thrown for evaluator named [" + ee.getName() + "].", this, eex);
                    errorStatus.add(new ErrorStatus("This was the last warning about this evaluator's errors. " + "We don't want the StatusManager to get flooded.", this));
                    addStatus(errorStatus);
                }
            }
        }

        if (evalResult) {
            return super.convert(event);
        } else {
            return CoreConstants.EMPTY_STRING;
        }
    }

    @Override
    protected String transform(final ILoggingEvent event, final String in) {
        return in;
    }
}

logback.xml

<configuration>
    <conversionRule conversionWord="ifPresent" converterClass="org.example.IfPresentConverter"/>

    <evaluator name="has_ex">
        <expression>return throwableProxy != null;</expression>
    </evaluator>

    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>lorem ipsum %msg{}%ifPresent(\n%ex{full}){has_ex}%n</pattern>
        </encoder>
    </appender>

    <root>
        <appender-ref ref="CONSOLE"/>
    </root>
</configuration>