How to make log4j syslog appender write a stack trace in one line?

Question

I am using log4j syslog appender and notice that when an exception occurs the appender writes every entry in the stack trace as a new line.

Is there a way to configure it so that the entire stack trace will go as one line instead of multiple lines?

Solution 1

You can customize the renderer of the stacktraces. There are two available ThrowableRenderer in Log4J ( org.apache.log4j.spi.ThrowableRenderer Interface) One is used by default. So, implements your own ThrowableRenderer with your wished formatting and then set it in your configuration.

In Log4j 1, it works. Never try in Log42. https://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PropertyConfigurator.html

ThrowableRenderer You can customize the way an instance of Throwable is converted to String before being logged. This is done by specifying an ThrowableRenderer. The syntax is: log4j.throwableRenderer=fully.qualified.name.of.rendering.class log4j.throwableRenderer.paramName=paramValue As in, log4j.throwableRenderer=org.apache.log4j.EnhancedThrowableRenderer

Solution 2

I'm using the following log4j2 configuration which is working well sending to syslog and doesn't require code changes to convert all those exceptions to strings. It's just replacing the default linesep with a pipe on exceptions.

    <Syslog name="SYSLOG" host="127.0.0.1" port="515" protocol="TCP" charset="UTF-8"
            immediateFail="false" ignoreExceptions="true" reconnectionDelayMillis="250">
        <PatternLayout pattern="[%d] %-5p %m%n %throwable{separator(|)}"></PatternLayout>
    </Syslog>

Solution 3

Log4j2

See james' answer.

Log4j v1

By default, the syslog appender will send the formatted log as well as each line of the stack trace as separate log entries in syslog. Because syslog uses UDP, the lines may be out of order.

If that's the problem you're trying to solve, then as james mentioned the best way to do this is to use a pattern layout with the %throwable conversion character. He documented how to do it for log4j2. For log4j1, it requires the EnhancedPatternLayout:

<appender name="syslog" class="org.apache.log4j.net.SyslogAppender">
  <param name="SyslogHost" value="127.0.0.1:514"/>
  <layout class="org.apache.log4j.EnhancedPatternLayout">
    <param name="ConversionPattern" value="[%d] %-5p %m%n%throwable"/>
  </layout>
</appender>

Note that the above solution will still use newline characters to separate the individual lines of the stack trace, but it will solve the problem of the stack trace lines being sent as separate log entries in syslog.

If you really want the stack trace on one line, a quick fix is to use the above example with %throwable{short} or %throwable{1}, which will only include the first line of the stack trace.

If you want the entire stack trace in one line, you may prefer to use a custom ThrowableRenderer as proposed by davidxxx. (Note that this will still send the formatted log to syslog separately from the stack trace, so to avoid that you can combine this solution with the one above.)

For example:

import org.apache.log4j.DefaultThrowableRenderer;
import org.apache.log4j.spi.ThrowableRenderer;

import java.util.ArrayList;
import java.util.Arrays;

public class CustomThrowableRenderer implements ThrowableRenderer {
    private final DefaultThrowableRenderer defaultRenderer = new DefaultThrowableRenderer();

    @Override
    public String[] doRender(Throwable throwable) {
        String[] defaultRepresentation = defaultRenderer.doRender(throwable);
        String[] newRepresentation = {String.join("|", Arrays.asList(defaultRepresentation))};

        return newRepresentation;
    }
}

And then in your log4j.xml:

<throwableRenderer class="CustomThrowableRenderer"/>

Comments

[Joly]

Thanks, forgot to mention I'm using this inside JBOSS Fuse so very limited to what I can do code wise, only able to change Log4J properties. Are there any renderers I can use already?

[davidxxx]

I don't see why you could not create your own render class and put in the classpath place of JBoss (for example the lib dir). JBoss has this dir : common/lib. JBoss fuse has not that ? There are two renderers but they does not make what you want. DefaultThrowableRenderer and EnhancedThrowableRenderer.

[davidxxx]

It's not a conventional way to log. You can forget to do log.error(e) and call the classic and common shared way. Keep common shared good practices is important in software development.

[bmaupin]

While this works, it will still send the log message and the stacktrace as two separate syslog messages.

[davidxxx]

@bmaupin Are you sure ? That is pity. Whatever the proposed solutions by you and James are clearly better because it avoids create a class besides the log4j configuration file.

[Andre de Miranda]

Great solution. maybe you should consider replacing the pipeline for \u2028 that happens to be designed for that.

[peter.petrov]

Using EnhancedPatternLayout and %throwable doesn't seem to do anything useful in log4j 1.x. It still prints the stack trace in multiple lines. Why do you say this works?

[peter.petrov]

Is this solution (which you suggest) specific to the appender? Should not be, right? I am using ConsoleAppender.

[peter.petrov]

In the logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/… I don't see anything which suggests to support showing the stack trace in a single line.

[peter.petrov]

@davidxxx Whatever bmaupin suggested sounds good but doesn't work. At least for me it does not work. See please the comments I added to his answer. Do you have any idea? See also: stackoverflow.com/questions/63623141/… Can you help with some ideas?

[bmaupin]

@peter.petrov Great feedback, thanks! I've updated my answer to hopefully make it more clear.

[bmaupin]

Just to be clear, this is how you would use \u2028 in the PatternLayout (since it expects XML escape codes; \u codes won't work): %throwable{separator(
)}