2009-11-08

Using Simple XML instead of JAXB on Google App Engine

In my previous post I said that I was going to try using a hack to get around the lack of support for JAXB in Google App Engine. Not only did this feel bad but also it didn't work for me, despite all the changes that had been made to circumvent the non-whitelisted classes. I still got various java.lang.NoClassDefFoundError exceptions.

So I decided to try Simple XML Serialization instead. This worked really well and caused no problems in the Local environment. However, when I deployed this to Google I was hit by the Sandbox limitations for Reflection. When Simple scans your classes to build its "schema" it calls setAccessible(true) on every method and constructor it finds all the way up the hierarchy to Object. This violates the sandbox restriction: "An application cannot reflect against any other classes not belonging to itself, and it can not use the setAccessible() method to circumvent these restrictions." App Engine throws a SecurityException when you try to call setAccessible(true) on one of its classes.

For my purposes, and probably the majority case, I do not need to serialize or deserialize to any non-public method of any superclasses other than my own. So given this I decided to absorb any SecurityExceptions thrown during the scanning process thus leaving those methods out of the "schema". Two minor changes are required to the source. The scan methods in org.simpleframework.xml.core.ClassScanner and org.simpleframework.xml.core.ConstructrorScanner both need a try..catch block added like so:
//ClassScanner
   private void scan(Class real, Class type) throws Exception {
      Method[] method = type.getDeclaredMethods();

      for(int i = 0; i < method.length; i++) {
         Method next = method[i];
         try { 
          if(!next.isAccessible()) {
             next.setAccessible(true);
          }
          scan(next);
        } catch (SecurityException e) {
   // Absorb this
        }

      }     
   }

//ConstructorScanner
   private void scan(Class type) throws Exception {
      Constructor[] array = type.getDeclaredConstructors();
      
      for(Constructor factory: array){
         ClassMap map = new ClassMap(type);
         
         try {
          if(!factory.isAccessible()) {
             factory.setAccessible(true);
          }
          scan(factory, map);
         } catch (SecurityException e) {
   // Absorb this
         }
          
      } 
   }
This works well now in the Local and Deployed environments. Perhaps Simple would benefit from a mode or an option on the @Root annotation to specify the depth of class scanning as an alternative to this work around. I will post this on the Simple mailing list and report back.

UPDATE:
The author of Simple has responded to my post on the mailing list saying that my fix above will be added to the next release.
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