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From Christopher Oliver <res1c...@verizon.net>
Subject Re: Groovy support in Cocoon
Date Mon, 05 Apr 2004 16:47:56 GMT
Stephan Michels wrote:

>Am Mo, den 05.04.2004 schrieb Christopher Oliver um 4:42:
>  
>
>>The current implementation needs some work to qualify as "generalized 
>>Java continuations". It would be nice to make it work more like Scheme 
>>continuations:
>>
>>1) When you access the "current" continuation, it captures the call 
>>stack up to but not including the current method (and makes a (shallow) 
>>copy of it). The current method continues as normal. 
>>    
>>
>
>What to you mean with "continues as normal". The current impl. stops the
>execution of the method with null as return value, if the cuntinuation
>is suspended. Then the continuation captures the object of the method
>call and the current frame.
>  
>
OK, IIUC there are three modes of execution of the instrumented code:

1) isCapturing
During this mode the JVM call stack is unwound and its state captured in 
the continuation call stack.
2) isRestoring
During this mode a saved call stack in a continuation is recreated on 
the JVM stack.
3) !isCapturing && !isRestoring
This is the "normal" execution of the JVM  - no modifications are made 
to the JVM call stack or to the continuation call stack.

This is achieved by inserting code before each invoke instruction to 
copy local variables from the continuation stack to the JVM stack and to 
reset the program counter to the value saved in the continuation stack 
if "isRestoring" is true, and by inserting code after each invoke 
instruction to copy the values of local variables from the JVM call 
stack as well as the program counter into the continuation call stack if 
"isCapturing" is true.

To make continuations more "Scheme-like" it must be possible to 
construct a continuation without modifying the JVM call stack (thus the 
current method invocation can "continue as normal").

To make this possible the byte-code transformer could work like this:

1) Insert code before an invoke instruction to push local vars and the 
pc onto the continuation stack if (!isRestoring && !isCapturing)
2) Insert code after an invoke instruction to pop those values from the 
continuation stack if (!isRestoring && !isCapturing)
3) If (isRestoring == true) the pre-invoke code would work as it does now.
4) If (isCapturing == true) the post-invoke code would simply cause the 
current method invocation to return to its caller without modifying the 
continuation stack - since it is already set up from (1). When the JVM 
stack is completely unwound another continuation will take the place of 
this one and that new continuation would be in "isRestoring" mode until 
it is recreated on the JVM stack at which point it would enter 
(!isRestoring && !isCapturing) mode.

The user interface to this might look something like this:

public class Continuation {
        // Empty continuation for use as an escape procedure
        public static final Continuation SUICIDE;

        // Invoke a captured continuation - it will replace the current 
continuation. I.e. the current JVM call stack will be unwound, the call 
stack saved in this object will be recreated on the JVM call stack, and 
the method invocation in which this object was created will return with 
"returnValue".
        public void invoke(Object returnValue);
        // Get the current continuation -
        public static Continuation getCurrentContinuation();
        // Entry point to the instrumented code (precondition: cannot be 
called recursively and method.getDeclaringClass() must be instrumented)
        public static Object invoke(Object obj, Method method, Object[] 
args);
}


Below is some pseudo-code that shows how this would work with Cocoon:

public class JavaInterpreter {
        // ...
        public void callFunction(String function, List params, 
Redirector redirector)
            throws Exception {
            if (!initialized)
                initialize();

            Method method = (Method) methods.get(function);
            Request request = ContextHelper.getRequest(this.avalonContext);
            Session session = request.getSession(true);
            HashMap userScopes = (HashMap) 
session.getAttribute(USER_GLOBAL_SCOPE);
            if (userScopes == null)
               userScopes = new HashMap();

            Object flow = (Object) 
userScopes.get(method.getDeclaringClass());

            if (flow == null) {
                if (getLogger().isDebugEnabled())
                   getLogger().debug("create new instance of 
\""+method.getDeclaringClass()+"\"");

            flow =  method.getDeclaringClass().newInstance();
            userScopes.put(method.getDeclaringClass(), flow);
            int size = params == null ? 0 : params.size();
            Object[] args = new Object[size];
            for (int i = 0; i < size; i++) {
                    Interpreter.Argument arg = 
(Interpreter.Argument)params.get(i);
                    args[i] = arg.value;
            }
            Continuation.invoke(flow, method, args);
        }

        public void handleContinuation(String id, List params, 
Redirector redirector)
            throws Exception {
            if (!initialized)
                initialize();

            WebContinuation wk = continuationsMgr.lookupWebContinuation(id);
            Continuation k = (Continuation) wk.getContinuation();
             k.invoke(wk);
         }

}

public class Cocoon {
     // ...
     public WebContinuation sendPageAndWait(String uri, Object biz, long 
ttl) {
              Continuation kont = Continuation.getCurrentContinuation();
              WebContinuation wk = 
continuationsMgr.createWebContinuation(kont,
                                           parentWebContinuation,
                                           ttl,
                                           null);
               sendPage(uri, biz);
               Continuation.SUICIDE.invoke(null);
               return wk; // never reached
     }

      public void sendPage(String uri, Object biz) {
           // ...
      }
}


Chris

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