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https://issues.apache.org/jira/browse/CONFIGURATION-831?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17941589#comment-17941589
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Gary D. Gregory commented on CONFIGURATION-831:
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We already support a syntax to provide a value when a key is absent, see
{{{}org.apache.commons.configuration2.interpol.TestConfigurationInterpolator.testInterpolationSingleVariableDefaultValue(){}}}:
{code:java}
assertEquals("42", interpolator.interpolate("${I_am_not_defined:-42}"));
{code}
> Unresolved (non-existend) environment variables should return null
> ------------------------------------------------------------------
>
> Key: CONFIGURATION-831
> URL: https://issues.apache.org/jira/browse/CONFIGURATION-831
> Project: Commons Configuration
> Issue Type: New Feature
> Components: Interpolation
> Affects Versions: 2.9.0
> Reporter: Wojtek
> Priority: Minor
>
> Currently Interpolation of environment variables
> (https://commons.apache.org/proper/commons-configuration/userguide/howto_basicfeatures.html#Variable_Interpolation)
> return un-interpolated string if the environment variable doesn't exists at
> all.
> It would be very convenient and (I feel) more consistent if, in that case,
> 'null' would be returned.
>
> Alternatively, option to specify default if the variable doesn't exist would
> also work.
> I.e. something along the lines:
> ```
> ${env:JAVA_HOME:<default value>}
> ```
>
> From my experiments, even if we try to register custom interpolator it's not
> possible to explicitly return `null` from it.
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