Override Logback error output

Question

In my custom exception class I've overridden toString():

@Override
public String toString() {
    final String msg = getLocalizedMessage();

    // base
    String str = getClass().getName() + ": [" + code + "]";

    // message
    if (msg != null)
        str += " " + msg;

    // extra
    if (extra != null) {
        str += '\n' + extra.toString();
    }

    return str;
}

(yes, I'm aware I should use StringBuilder there)

However, when I log such an exception (via org.slf4j.Logger.warn(String msg, Throwable err)) the output is as for vanilla exceptions:

webersg.util.service.ServiceError: null
    at webersg.util.service.ServiceTools.decodeException(ServiceTools.java:39) ~[bin/:na]
    at tr.silvercar.rummikub.robot.LobbyConnection.sendRequestTo(LobbyConnection.java:143) ~[bin/:na]
    at tr.silvercar.rummikub.robot.LobbyConnection.sendRequest(LobbyConnection.java:98) ~[bin/:na]
    at tr.silvercar.rummikub.robot.Robot.<init>(Robot.java:32) ~[bin/:na]
    at tr.silvercar.rummikub.robot.RobotController.start(RobotController.java:81) ~[bin/:na]
    at tr.silvercar.rummikub.robot.runners.LocalAltinRobot.main(LocalSilverRobot.java:132) [bin/:na]

How can I get my extra data to appear? My log implementation is Logback.

Answer 1

A quick and dirty way is this: logger.warn("your_message_here \n{}", err.toString());

A better way would be to write your own custom conversion specifier (see logback manual so that the custom error handling is abstracted from your application code, and you can configure it's use through a custom conversion word in your logback.xml.

Here's a simple example that will do what you're after:

package com.example.logging;

import ch.qos.logback.classic.pattern.ClassicConverter;
import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.classic.spi.IThrowableProxy;
import ch.qos.logback.classic.spi.ThrowableProxy;

/**
 * Simple exception converter.
 *
 */
public class MyExceptionConverter extends ClassicConverter {
    /* (non-Javadoc)
     * @see ch.qos.logback.core.pattern.Converter#convert(java.lang.Object)
     */
    @Override
    public String convert(ILoggingEvent event) {
        IThrowableProxy itp = event.getThrowableProxy();
        if (itp instanceof ThrowableProxy) {
            ThrowableProxy tp = (ThrowableProxy)itp;
            return tp.getThrowable().toString();
        }

        return "";
    }
}

Then in your logback.xml you'll need to reference it like this:

  <conversionRule conversionWord="myCon"
              converterClass="com.example.logging.MyExceptionConverter" />

And then use %myCon in an encoder pattern like this:

<encoder>
    <pattern>%d{dd MMM yyyy HH:mm:ss.SSS} %logger{0}: %myCon %nopex%n</pattern>
</encoder>

Note that adding %nopex stops the usual exception handling