为什么Scalatra有时会给我FlashMap投放错误？

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