[logback-dev] removing context selectors will break our libraries and applications

Ceki ceki at qos.ch
Sun Apr 28 21:15:06 CEST 2019

Hi Garret,

Off the top of my head, the migration to ServiceLoader provider 
mechanism is intended to be transparent for users of the API. (On the 
other hand, logging implementations , i.e. backends,  need to make some 

Static binding and ServiceLoader accomplish the overall same task, with 
the ServiceLoader provide mechanism being a little more flexible. The 
move was mandatory in order to support JPMS/Java 9 modules.

Regarding ContextSelector, it would be possible to reintroduce it, 
provided there is interest by the community. I guess that's the bottom line.

More inline...


On 28.04.2019 16:00, Garret Wilson wrote:> Hi, everyone. We're really 
wanting to release a new version of our Clogr
> library to work with the latest Logback, but there's so much unclear 
> about the current status. First, if you could be so kind as to read my 
> previous email blow, which I sent almost a year ago. Now here are the 
> areas I'm unclear on:
>   * I had indicated that our library relies on ContextSelector. This
>     class appears to be back in 1.3.0-alpha4, yet I see that
>     https://logback.qos.ch/news.html still says that, "Due to lack of
>     user interest, logback-classic no longer supports logging separation
>     by way of ContextSelector." Is it the case that the news page is
>     simply out of date and invalid?

The class ContextSelector might still be around but the use of 
ContextSelector was commented out in LogbackServiceProvider. It would be 
possible to uncomment said code without too much trouble I believe.

>   * As I explain in depth at https://stackoverflow.com/a/38317149/421049
>     , SLF4J previous relied on StaticLoggerBinder to determine which
>     SLF4J implementation would be used. The Logback implementation
>     ContextSelectorStaticBinder would then determine which
>     ContextSelector to use. Now that there is no StaticLoggerBinder, how
>     does Logback call ContextSelectorStaticBinder.init() so that the
>     default context selector will be set up?

As I tried to explain in the beginning of this message, the move to 
ServiceLoader does not change the general mechanism.

>   * In previous versions our library would call
>     StaticLoggerBinder.getSingleton() to ensure that SLF4J would
>     initialize the binding and set up the Logback default logger
>     context. Since that is now gone, what can we call to make sure that
>     SLF4J is bound to the correct implementation and the default logger
>     context is available in Logback?

Loggerfactory.getProvider() ?

>   * Is
>     ContextSelectorStaticBinder.getSingleton().getContextSelector().getDefaultLoggerContext()
>     still guaranteed to be the default logger context that was
>     initialized by Logback?

ContextSelectorStaticBinder has been defanged in LogbackServiceProvider.

>   * If we switch to Logback 1.3.0-alpha4 and SLF4J 1.8.0-beta4, will all
>     the other libraries that were compiled against SLF4J 1.7.25 still work?\


>   * Is it recommended that applications switch to Logback 1.3.0-alpha4
>     and SLF4J 1.8.0-beta4, or is this just test code that is going to be
>     experimented with for several years, and production code should stay
>     with Logback 1.2.3 and SLFJ 1.7.25?

No, production should stay with SLF4J 1.7.25 and logback 1.2.3.

> These are important, essential questions. Thanks in advance for 
> explanations.
> Garret
> On 7/6/2018 10:49 AM, Garret Wilson wrote:
>> Hi,
>> I've been pretty deep inside the Logback code. In fact I wrote a 
>> comprehensive explanation on the bootstrap procedure here: 
>> https://stackoverflow.com/a/38317149/421049
>> We've created a library called Csar that allows one to configure 
>> various configuration "contexts" within the same JVM, for a program 
>> that has different locales at the same time, or different logging 
>> configurations for different users, etc. I explain this all at 
>> https://dzone.com/articles/creating-local-globals-with-csar .
>> Our Clogr library used Csar to allow different logging configurations 
>> (or even different logging libraries!) to be set up for different 
>> parts of the application. (Imagine a server with different user 
>> sessions, each session being logged to a different file.) We got this 
>> working great for Logback. In fact we integrated it into our software 
>> development course; read about it here: https://clogr.io/learn (See 
>> the section on Clogr and compartmentalized configurations with Logback 
>> at the bottom.)
>> The problem is that the initial bootstrap sequence of Logback was 
>> brittle, relying on a static class that could appear across modules. 
>> The appearance of the Java 9+ module system prohibits this altogether. 
>> But I had already created a ticket 
>> https://jira.qos.ch/browse/LOGBACK-1196 for how to fix this, long 
>> before the Java system was released, using the standard service 
>> provider mechanism.
>> But now I see at https://logback.qos.ch/news.html that Logback will 
>> remove contexts altogether. This will break our ability to configure 
>> loggers separately in our application, and to use Clogr to manage them.
>> Why remove this? There was nothing wrong with the simple context 
>> selection. The problem was how it was configured, using a static 
>> classes that had to be placed in a module outside its package. Just 
>> implement https://jira.qos.ch/browse/LOGBACK-1196, please. Don't break 
>> our application. I even provide much of the code to do this at 
>> LOGBACK-1196.
>> What can we do to keep this from being removed and breaking our 
>> library? Why do we need to remove it in the first place? Why not fix it?
>> (Ironically I was just told that Log4j 2 supports contexts, so maybe 
>> our application will have to switch back to Log4j. But I would prefer 
>> to have Clogr support both Logback and Log4j 2. There is no reason to 
>> remove this capability from Logback.)
>> As I mentioned two years ago in LOGBACK-1196, I'm willing to 
>> contribute to fixing this. Let me know how to go forward.
>> Thanks,
>> Garret Wilson
>> President
>> GlobalMentor, Inc.
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Ceki Gülcü

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