[logback-user] Redirecting JDK 9's JVM logs to Logback?

Martin Grajcar maaartinus at gmail.com
Mon Dec 4 21:55:21 CET 2017

On Mon, Dec 4, 2017 at 4:02 PM, Osvaldo Doederlein <opinali at gmail.com>

> Hi,
> I want to redirect the JVM's internal logs to Logback, is this possible?
> More specifically, I have all my app logs going to a single appender and I
> want JVM logs such as GC to be writing to the same appender, interleaved
> with app logs.
> I have Logback configured with slf4j and jul-to-slf4j, also I wrote a
> System.LoggerFinder

Could you share it?

> service to redirect System.Logger to SLF4J->Logback, which I can see
> working for core lib logs such as java.base / sun.net.www.protocol.http.HttpURLConnection.
> (See http://openjdk.java.net/jeps/158) However, the GC logs are still
> going to stdout. The documentation for JEP 158 seems to imply that the JVM
> logs are something separate from java.util.logging, can only be sent to
> stderr, stdout or a log file, but not to a JUL logger.
> I know that I can hook the System.out/err streams and redirect writes to a
> logger, but this is not ideal, for one thing I cannot configure Logback to
> send logs back to stdout (which I often do in test/interactive runs)
> without risking a stack overflow.  I could write code that walks the stack
> to decide if the call to System.err/out is coming from a logger, but this
> is expensive.
> Anyone knows a magic trick here? :-)

A magic prefix maybe? Something like prefixing every message by "I'm coming
from stdout. If you send me there again, your stack will overflow".

Prefixing every line forwarded from stdout to the logger by "STDOUT "
should do, though you'll miss all lines already starting by the prefix.
Which can be made arbitrarily improbable.

Assuming all lines are unique, you could use a (Concurrent?)HashSet storing
all lines send to stdout by the logger and remove them when you see them
coming back. This is probably much faster than stack walking. Some periodic
cleanup may be necessary in case something gets lost.
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