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architecturejakarta-eeclassloader

Java EE class loading standard

WebSphere comes with parent last and parent first. Is this Java EE compliant? Is this supported by all application servers that are Java EE 5 compliant?

Solution

  • I did my own research (going through the specs and few blogs) and below is what I've figured

    EAR

    The spec DOES NOT define or mandate how the class loaders should work within an EAR. It however defines that 

    1. there SHOULD be per thread context class loader for runtime loading of classes
    2. there MIGHT be a hierarchical class loading mechanism for resolving classes (app server vendors are free to implement whichever way they choose to)
    3. the top level class loader (WAR/EAR) MAY delegate to the low level class loaders (like Bootstrap, extension etc). This is in compliance with J2SE class loader delegation model (PARENT_FIRST in WAS)

    WAR

    Servlet specification defines and mandates the support of a PARENT_LAST (i.e. WAR/web-inf/classes and WAR/web-inf/lib take precedence over the libraries that come with the app server) class loading model. But this is just for WAR modules. The Servlet spec diverges from the standard J2SE delegation model of PARENT_FIRST in this case.

    Reference

    Spec: Servlet 2.3, Section: 9.7.2 Web Application Classloader

    Spec: Java EE 5, Section: EE.6.2.4.7 Context Class Loader

    App Server specifics

    Interestingly, though, it appears most major app servers support some mechanism of turning off delegation to isolate the application from the app server if necessary (because of conflicts or otherwise): WebSphere - "parent-last", GlassFish - <class-loader delegate="false">, JBoss - java2ParentDelegation=false, Geronimo - <java2-delgation-model>false</java2-delegation-model>