LTTng » LTTng-UST

There is an agent protocol message to list events, however I realize now that all it does on the Java side is list the event names that are enabled in the session. I'm pretty sure it was always like that. But it doesn't seem very useful, this information is already present in the session daemon, isn't it?

Java domains don't have the notion of event names like C applications do, they use logger names instead of event names. Loggers are usually organized in a hierarchy, such as "com", "com.mycompany", "com.mycomponent.mycomponent", etc. and typically correspond to package or class names.

It would be possible to iterate through all loggers currently active in the JVM, and for each of them go through all their attached handlers, and report the names of those that have a least one LTTng-JUL (or -logj4) handler attached. Is this what we would want to do?

Is this what we would want to do?

Yes, this is what the Python agent does, and IIRC this is what JUL/log4j did in 2.6.

See https://github.com/lttng/lttng-ust/blob/stable-2.6/liblttng-ust-java-agent/java/org/lttng/ust/agent/jul/LTTngJUL.java, for example:

    public Iterator<String> listLoggers() {
        Vector<String> logs = new Vector<String>();
        for (Enumeration<String> loggers = LogManager.getLogManager().getLoggerNames(); loggers.hasMoreElements(); ) {
            String name = loggers.nextElement();
            /* Skip the root logger */
            if (name.equals("")) {
                continue;
            }

            logs.add(name);
        }

        return logs.iterator();
    }