Why does Scalatra give me a FlashMap casting error sometimes?

Question

When running Scalatra in code reload mode, if I load the page before the scalate engine has reinitialized I get a 500 error.

If I watch the terminal until it looks like the engine has fully reloaded, it works fine, but I think this happens when I load the page between a) successful compile, and b) the container restart.

This won't correct itself until I change something else and force a new compile and code reload.

I can't seem to find why this is happening, does anyone have any ideas?

Here is how I'm doing code reloading:

./sbt "container:start" "~ ;copy-resources;aux-compile"

Here is the error I see:

HTTP ERROR 500

Problem accessing /. Reason:

    org.scalatra.FlashMap cannot be cast to org.scalatra.FlashMap
Caused by:

java.lang.ClassCastException: org.scalatra.FlashMap cannot be cast to org.scalatra.FlashMap
    at org.scalatra.FlashMapSupport$$anonfun$org$scalatra$FlashMapSupport$$getFlash$2$$anonfun$1.apply(flashMap.scala:182)
    at org.scalatra.FlashMapSupport$$anonfun$org$scalatra$FlashMapSupport$$getFlash$2$$anonfun$1.apply(flashMap.scala:182)
    at scala.Option.map(Option.scala:145)
    at org.scalatra.FlashMapSupport$$anonfun$org$scalatra$FlashMapSupport$$getFlash$2.apply(flashMap.scala:181)
    at org.scalatra.FlashMapSupport$$anonfun$org$scalatra$FlashMapSupport$$getFlash$2.apply(flashMap.scala:180)
    at scala.Option.getOrElse(Option.scala:120)
    at org.scalatra.FlashMapSupport$class.org$scalatra$FlashMapSupport$$getFlash(flashMap.scala:180)
    at org.scalatra.FlashMapSupport$class.flash(flashMap.scala:192)
    at beekeeper.controllers.HomeServlet.flash(HomeServlet.scala:13)
    at org.scalatra.FlashMapSupport$$anonfun$handle$1.apply$mcV$sp(flashMap.scala:137)
    at org.scalatra.FlashMapSupport$$anonfun$handle$1.apply(flashMap.scala:136)
    at org.scalatra.FlashMapSupport$$anonfun$handle$1.apply(flashMap.scala:136)
    at scala.util.DynamicVariable.withValue(DynamicVariable.scala:57)
    at org.scalatra.DynamicScope$class.withRequest(DynamicScope.scala:71)
    at org.scalatra.ScalatraServlet.withRequest(ScalatraServlet.scala:49)
    at org.scalatra.FlashMapSupport$class.handle(flashMap.scala:136)
    at beekeeper.controllers.HomeServlet.handle(HomeServlet.scala:13)
    at org.scalatra.ScalatraServlet.service(ScalatraServlet.scala:54)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
    at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:669)
    at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:455)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
    at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:560)
    at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
    at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1072)
    at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:382)
    at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
    at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1006)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
    at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:255)
    at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
    at org.eclipse.jetty.server.Server.handle(Server.java:365)
    at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:485)
    at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:926)
    at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:988)
    at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:635)
    at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
    at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
    at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:628)
    at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
    at java.lang.Thread.run(Thread.java:744)

Answer 1

This error says that there are 2 instances of the same class, loaded by different class loaders. This behavior rooted in java: classes from different class loaders are not the same. Basically equals method for classes is the function of a class name and a class loader. Classes will not be the same in this case even if they come from the same jar or folder.

When container reloads code it creates a new class loader, while your session still holds instance of flash map loaded using old class loader. This most likely the cause of the issue.

Answer 2

In Java every class is identified by its fully qualified class name and the class loader that has loaded it, this implies that:

ClassLoader1.my.package.MyClass != ClassLoader2.my.package.MyClass

This signifiy that it is not possible to cast an object of class MyClass loaded in ClassLoader1 to type MyClass loaded from ClassLoader2. You can find more details on

http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-5.html

On the other hand, it is not possible to load the same class twice in the same class loader, so you need a new class loader, there is no way around it. While I never wrote code that performs class reloading, one can suppose that class reloading means in practice:

• create a new class loader
• reload the class
• swap class loaders

Because of what described above, one needs to be careful when designing classes that are supposed to be reloaded: see http://tutorials.jenkov.com/java-reflection/dynamic-class-loading-reloading.html#designing