Is it possible to prefix every line of a stacktrace in log4j?

Question

when you write

logger.error("message", exception);

log4j produces the message and the complete stack trace:

Aug  9 06:26:13 10.175.60.14 myPrefix: [error] [TP-Processor114] [my.class.Name] message : exception
at fatherOfException
at fatherof_fatherOfException
at fatherof_fatherof_fatherOfException
...

my conversion pattern is

log4j.appender.syslog.layout.ConversionPattern=myPrefix: [%p] [%t] [%c] [%x] - %m%n

So, is it possible to prefix every line with myPrefix, as:

    Aug  9 06:26:13 10.175.60.14 myPrefix: [error] [TP-Processor114] [my.class.Name] message : exception
myPrefix    at fatherOfException
myPrefix    at fatherof_fatherOfException
myPrefix    at fatherof_fatherof_fatherOfException
    ...

When I grep my logs on myPrefix, i don't see the stack trace. We have many different prefixes (one per module)

Thanks in advance.

Answer 1

Refer to Alex's answer as it's a cleaner one.

You can write your own implementation of org.apache.log4j.spi.ThrowableRenderer :

http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/spi/ThrowableRenderer.html

Then, edit your log4j configuration:

log4j.throwableRenderer= your-custom-class-name

The ThrowableRenderer returns an array of String s. Here's your code:

String prefix = "myPrefix"; // Or some constant

List<String> l = new LinkedList<String>();
l.add(String.format("%s %s: %s", prefix, t.getClass().getName(), t.getMessage()));

for (StackTraceElement ste: t.getStackTrace()){
    l.add(String.format("%s %s", prefix, ste.toString()));
}

return (String[]) l.toArray();

Another idea would be to print the Throwable into a PrintWriter that is wrapping some Writer that writes into memory, and then re-iterate over strings delimited by line.separator , adding each line to a list:

StringWriter sw = new StringWriter();
t.printStackTrace(new PrintWriter(sw));

List<String> l = new LinkedList<String>();
for (String s: sw.toString().split(System.lineSeparator())) {
    l.add(String.format("%s %s", prefix, s));
}

return (String[]) l.toArray();

Answer 2

Subclass ThrowableRenderer , for example:

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

public class LogThrowableRenderer implements ThrowableRenderer {

    DefaultThrowableRenderer def = new DefaultThrowableRenderer();

    @Override
    public String[] doRender(Throwable t) {
        String[] temp = def.doRender(t);
        for (int i = 0; i < temp.length; i++) {
            temp[i] = "myPrefix "+temp[i];
        }
        return temp;
    }

}

Add to your log4j.properties :

log4j.throwableRenderer=whatever.package.LogThrowableRenderer

This uses the existing DefaultThrowableRenderer to render the stacktrace in the familiar way before adding the prefix, so it will include the Throwable class, message, and cause.

Answer 3

With log4j2 you can add a prefix to each new line of your stacktraces by specifying the separator as a newline followed by your prefix:

%xThrowable{separator(\nmyPrefix)}%n

You can similarly add prefixes to each new line of each multi-line message:

%replace{%m}{[\r\n]+}{\nmyPrefix}

For example, to add a whitespace prefix to each multi-line log and stacktrace line:

appender.rolling.layout.pattern = %d [%t] %-5p %c %L - %replace{%m}{[\r\n]+}{\n }%xThrowable{separator(\n )}%n

Note that you want place the %xThrowable bit after your %m .

Answer 4

Write a wrapper function to do it for you.

private void writeToLogs(String message, Exception excp) {
    logger.error("myPrefix\t" + message, excp);
}

Answer 5

Here is one solution that I found for logback and I used. The API for now does not let you prefix every line of the stack trace. For the other lines I already had a Pattern in place with a tracking ID. So I had to use that. So what I did is

1. I created a CustomLayout extends LayoutBase
2. In the doLayout method you can check if you have on the event an TrowableProxy and from that you can parse every line and add a prefix. I just checked if I have on the line my tracking ID, if there is not, I just appended it.

Here is the code

public class MyLayout extends LayoutBase<ILoggingEvent> {

    private PatternLayout pattern = null;
    private String thePattern;

   @Override
    public String doLayout(ILoggingEvent event) {
        if (!pattern.isStarted()) {
            pattern.setPattern(thePattern);
            pattern.start();
        }

        String patternLayoutResult = pattern.doLayout(event);
        StringBuilder sb = new StringBuilder();

        String trackingId = MDC.get(TRACKING_ID);
        if (trackingId != null) {
            for (String line : patternLayoutResult.split(LINE_SEPARATOR)) {
                if (!line.contains(trackingId)) {
                    sb.append("Exception [");
                    sb.append(TRACKING_ID);
                    sb.append(":");
                    sb.append(trackingId);
                    sb.append("] ");

                }
                sb.append(line);
                sb.append(LINE_SEPARATOR);
            }

            return sb.toString();
        }

        return patternLayoutResult;
    }

And in the logback.xml

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="ch.qos.logback.core.encoder.LayoutWrappingEncoder">
            <layout class="mypackage.CustomLoggingLayout">
                <thePattern>%d{dd.MM.yyyy HH:mm:ss} %-5p [trackingId:%X{trackingId}] %c{1}:%L - %m%n</thePattern>
                <Pattern></Pattern>
            </layout>
            <charset>UTF-8</charset>
        </encoder>
    </appender>

And now the stackTrace looks like:

Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:1154)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at org.glassfish.jersey.jdkhttp.JdkHttpHandlerContainer.handle(JdkHttpHandlerContainer.java:161)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:79)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at sun.net.httpserver.AuthFilter.doFilter(AuthFilter.java:83)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:82)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at sun.net.httpserver.ServerImpl$Exchange$LinkHandler.handle(ServerImpl.java:700)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:79)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at sun.net.httpserver.ServerImpl$Exchange.run(ServerImpl.java:672)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
Exception [trackingId:d3746a12a86e46f991a338b4317045f5] at java.lang.Thread.run(Thread.java:748)