Issue: Unable to start XLR with custom plugins with Release 10.3.3

Issue

Release is run in a container environment with Release 10.3.3 hooked up to an external database, which is also running in a container (postgres:10.17-buster)

When we try to download the metrics plugin from https://github.com/xebialabs-community/xl-metrics-plugin/releases/download/v1.3.0/xl-metrics-plugin-1.3.0.jar and copy it to xl-release-server/plugins/__local__ and also when tried with xl-release-server/plugins/xlr-official, when we restart the container the below log output is observed:

2022-04-27 13:59:58.376 [main] {} ERROR o.s.boot.SpringApplication - Application run failed
org.springframework.context.ApplicationContextException: Unable to start web server; nested exception
    is org.springframework.boot.web.server.WebServerException: Unable to start embedded Jetty web
server
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(Servlet
   WebServerApplicationContext.java:163)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.
   java:577)
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWeb
   ServerApplicationContext.java:145)
.........
Caused by: org.springframework.boot.web.server.WebServerException: Unable to start embedded Jetty web
    server
at org.springframework.boot.web.embedded.jetty.JettyWebServer.initialize(JettyWebServer.java:129)
at org.springframework.boot.web.embedded.jetty.JettyWebServer.<init>(JettyWebServer.java:90)
at org.springframework.boot.web.embedded.jetty.JettyServletWebServerFactory.getJettyWebServer(Jetty
   ServletWebServerFactory.java:429)
at org.springframework.boot.web.embedded.jetty.JettyServletWebServerFactory.getWebServer(JettyServlet
   WebServerFactory.java:170)
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer
   (ServletWebServerApplicationContext.java:182)
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(Servlet
   WebServerApplicationContext.java:160)
... 7 common frames omitted
Caused by: java.lang.NoClassDefFoundError: scala/Serializable
at java.base/java.lang.ClassLoader.defineClass1(Native Method)
at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1017)
at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:878)
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.protected$defineClass(XlrPlugin
   ClassLoader.scala:80)
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.$anonfun$loadClassFromUrl$2(Xlr
   PluginClassLoader.scala:80)
at com.xebialabs.xlplatform.utils.ResourceManagement$.using(ResourceManagement.scala:54)
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.loadClassFromUrl(XlrPluginClass
   Loader.scala:74)
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.$anonfun$findClass$3(XlrPlugin
   ClassLoader.scala:61)
at scala.Option.map(Option.scala:242)
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.$anonfun$findClass$2(XlrPlugin
   ClassLoader.scala:61)
at com.xebialabs.xlplatform.utils.PerformanceLogging.logWithTime(PerformanceLogging.scala:11)
at com.xebialabs.xlplatform.utils.PerformanceLogging.logWithTime$(PerformanceLogging.scala:9)
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.logWithTime(XlrPluginClassLoader
   .scala:56)
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.findClass(XlrPluginClassLoader.
   scala:59)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:589)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
at com.xebialabs.xlplatform.metrics.MetricsProtocol.$init$(Metrics.scala:30)
at com.xebialabs.xlplatform.metrics.MetricsRoute.<init>(MetricsRoute.scala:29)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessor
   Impl.java:62)
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructor
   AccessorImpl.java:45)
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
at java.base/java.lang.Class.newInstance(Class.java:584)
at com.xebialabs.xlplatform.endpoints.routes.ScalaExtensionRoute.$anonfun$codedExtensionRoutes$2(Scala
   ExtensionRoute.scala:23)
at scala.collection.immutable.List.map(List.scala:246)
at scala.collection.immutable.List.map(List.scala:79)
at com.xebialabs.xlplatform.endpoints.routes.ScalaExtensionRoute.codedExtensionRoutes(ScalaExtension
   Route.scala:20)
at com.xebialabs.xlplatform.endpoints.routes.ScalaExtensionRoute.codedExtensionRoutes$(ScalaExtension
   Route.scala:16)
at com.xebialabs.xlplatform.endpoints.servlet.ServletBoot.codedExtensionRoutes(ServletBoot.scala:10)
at com.xebialabs.xlplatform.endpoints.routes.ScalaExtensionRoute.codedExtensionRoutes(ScalaExtension
   Route.scala:50)
at com.xebialabs.xlplatform.endpoints.routes.ScalaExtensionRoute.codedExtensionRoutes$(ScalaExtension
   Route.scala:49)
... 12 common frames omitted
Caused by: java.lang.ClassNotFoundException: A plugin could not be loaded due to a missing class
   (scala.Serializable).
   Please remove the offending plugin to successfully start the server.Classes related to JCR were
   removed from the server
   because of the migration from JCR to SQL.If the plugin depends on these classes and its functionality
   is required, please contact support to fix your configuration.scala.Serializable not found
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.$anonfun$findClass$4(XlrPlugin
   ClassLoader.scala:67)
at scala.Option.getOrElse(Option.scala:201)
at com.xebialabs.xlrelease.plugin.classloading.XlrPluginClassLoader.$anonfun$findClass$2(XlrPlugin
   ClassLoader.scala:63)
at com.xebialabs.xlplatform.utils.PerformanceLogging.logWithTime(PerformanceLogging.scala:11)
... 67 common frames omitted

Workaround/Resolution Action

If you see these errors during startup, it might be because the plugin needs an update and it has to be recompiled in JDK11 because the Java version of the latest Release application is JDK 11.