What is a deployment overlay and how do I use it?

Issue

• Need to change any file of a deployment at runtime so that the application need not be edited.
• Patch deployment in JBOSS - How to change *.class file of a deployment at runtime so that the application need not be edited.
• How to do deployment overlays in JBoss EAP 6.1
• Is it possible to overlay a deployment descriptor of a sub-deployment inside an ear?
• How to change the content of web.xml file at runtime in JBoss EAP 6.1 ?
• Is it possible to define separate context-roots for multiple applications when jboss-web.xml are not defined for the applications?
• Tried to use the overlay functionality over a directory instead over a single file. The CLI command is as follows:

  deployment-overlay add --name=jspOverlay --content=/test_jsp/=/a/testing_jsp/ --deployments=*.war --redeploy-affected

Here /a/testing_jsp/ and /test_jsp/ are directories not a file hence getting the following ERROR stacktrace:

java.lang.RuntimeException: java.io.FileNotFoundException: /a/testing_jsp/ (It's a directory)
        at org.jboss.as.controller.client.OperationBuilder.addFileAsAttachment(OperationBuilder.java:81)
        at org.jboss.as.cli.handlers.DeploymentOverlayHandler.add(DeploymentOverlayHandler.java:708)
        at org.jboss.as.cli.handlers.DeploymentOverlayHandler.doHandle(DeploymentOverlayHandler.java:381)
        at org.jboss.as.cli.handlers.CommandHandlerWithHelp.handle(CommandHandlerWithHelp.java:86)
        at org.jboss.as.cli.impl.CommandContextImpl.handle(CommandContextImpl.java:600)
        at org.jboss.as.cli.impl.CommandContextImpl.handleSafe(CommandContextImpl.java:616)
        at org.jboss.as.cli.impl.CommandContextImpl.interact(CommandContextImpl.java:1204)
        at org.jboss.as.cli.impl.CliLauncher.main(CliLauncher.java:263)
        at org.jboss.as.cli.CommandLineMain.main(CommandLineMain.java:34)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:601)
        at org.jboss.modules.Module.run(Module.java:292)
        at org.jboss.modules.Main.main(Main.java:455)
Caused by: java.io.FileNotFoundException: /a/testing_jsp/ (It's a directory)
        at java.io.FileInputStream.open(Native Method)
        at java.io.FileInputStream.<init>(FileInputStream.java:138)
        at org.jboss.as.controller.client.OperationBuilder$FileStreamEntry.<init>(OperationBuilder.java:158)
        at org.jboss.as.controller.client.OperationBuilder$FileStreamEntry.<init>(OperationBuilder.java:154)
        at org.jboss.as.controller.client.OperationBuilder.addFileAsAttachment(OperationBuilder.java:75)
        ... 14 more

• Is it possible to use this deployment overlays feature for overlaying directories as well?

• How can I creating custom class loader that loades classes from a jar in specific directory and later use this class in my application?
  Where can I find an example to this? The idea is to load jars at runtime in jboss.
  Is this functionality exist out of the box? for example, can I add new jars to modules at runtime or load them somehow other then creating my own custom
  classloader?