Java Modules and Multi-Release Jar

Alan Bateman Alan.Bateman at
Thu Oct 4 16:52:56 UTC 2018

On 04/10/2018 08:04, Luke Hutchison wrote:
> :
> I see in JEP 238 that multi-release jars "will contain" the attribute 
> "Multi-Release: true". Should this "will" be interpreted as "MUST"? 
> i.e. if a manifest file is missing this attribute, but the jar 
> contains some version layers in "META-INF/versions", will the JRE 
> (possibly in some future version) ignore the version layers, or will 
> it detect the version layers and use them anyway?
> (I'm trying to match the semantics and the intent, including any 
> possible future behavior if multi-release jars become the norm, and 
> I'm wondering if I should ignore everything in "META-INF/versions" if 
> the Multi-Release attribute is missing.)
If the JAR file does not contain "Multi-Release" attribute in its main 
manifest then it is not a multi-release JAR. The wording in the JarFile 
API spec might be a bit clearer.


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