[logback-user] JaninoEventEvaluator and class loader

Glyn Normington gnormington at vmware.com
Thu Jan 5 13:09:49 CET 2012

I am attempting to get Janino 2.6.1 working with Logback 0.9.28 (or later, but that's the version we are using in Eclipse Virgo right now) - see [0] for background. Unfortunately Janino ends up using the thread context class loader as its "parent" class loader and fails with a runtime exception. I have discussed this ([1]) on the Janino mailing list and it seems that it is necessary to set the parent class loader in Janino. I am somewhat at the mercy of Logback here.

The surprising thing is that the code in Logback 0.9.24 looked pretty usable in this respect. JaninoEventEvaluator.start had the following sequence:

    ClassLoader cl = context.getClass().getClassLoader();
    ee = new ExpressionEvaluator(getDecoratedExpression(), EXPRESSION_TYPE,
        getParameterNames(), getParameterTypes(), THROWN_EXCEPTIONS, cl);

thus setting the parent class loader to a value which I could ensure would be capable of loading the necessary types.

In 0.9.28 this code has been replaced by:

    scriptEvaluator = new ScriptEvaluator(getDecoratedExpression(), EXPRESSION_TYPE,
    getParameterNames(), getParameterTypes(), THROWN_EXCEPTIONS);

which causes the current TCCL to be used as the parent class loader and ultimately results in Janino failing.

I can't control the TCCL that happens to be in use when start is called as that is driven out of a logging call which can come from an arbitrary thread.

I found the relevant commit ([2]), but I can't tell from that why this specific change was made.

If it is absolutely necessary to use ScriptEvaluator rather than ExpressionEvaluator, the following code sequence (based on a suggestion from Arno Unkrig) would reproduce the parent class loader behaviour of 0.9.24:

 ClassLoader cl = context.getClass().getClassLoader();
 scriptEvaluator = new ScriptEvaluator();

The alternative of using ExpressionEvaluator is much neater and seems to be close to the ScriptEvaluator variant since ExpressionEvaluator extends ScriptEvaluator.

Any suggestions gratefully received.

[0] https://bugs.eclipse.org/bugs/show_bug.cgi?id=333920
[1] mailing list archive link currently broken - if I can get a link to it, I'll post this later
[2] https://github.com/ceki/logback/commit/06a5b692f14560636bd92d7bd7cf1f85830f4e55#diff-4

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