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Google App Engine compliant variation of FreeMarker. FreeMarker is a "template engine"; a generic tool to generate text output based on templates.

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package freemarker.ext.jython;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.List;
import java.util.Map;

import org.python.core.Py;
import org.python.core.PyDictionary;
import org.python.core.PyFloat;
import org.python.core.PyInteger;
import org.python.core.PyJavaInstance;
import org.python.core.PyLong;
import org.python.core.PyNone;
import org.python.core.PyObject;
import org.python.core.PySequence;
import org.python.core.PyStringMap;

import freemarker.ext.beans.BeansWrapper;
import freemarker.ext.beans.DateModel;
import freemarker.ext.util.ModelCache;
import freemarker.template.TemplateModel;

class JythonModelCache extends ModelCache
{
    private final JythonWrapper wrapper;
    
    JythonModelCache(JythonWrapper wrapper) {
        this.wrapper = wrapper;
    }

    protected boolean isCacheable(Object object) {
        return true;
    }
    
    protected TemplateModel create(Object obj) {
        boolean asHash = false;
        boolean asSequence = false;
        if(obj instanceof PyJavaInstance) {
            Object jobj = ((PyJavaInstance)obj).__tojava__(java.lang.Object.class);
            // FreeMarker-aware, Jython-wrapped Java objects are left intact 
            if(jobj instanceof TemplateModel) {
                return (TemplateModel)jobj; 
            }
            if(jobj instanceof Map) {
                asHash = true;
            }
            if (jobj instanceof Date) {
                return new DateModel((Date) jobj, BeansWrapper.getDefaultInstance());
            }
            else if(jobj instanceof Collection) {
                asSequence = true;
                // FIXME: This is an ugly hack, but AFAIK, there's no better
                // solution if we want to have Sets and other non-List
                // collections managed by this layer, as Jython quite clearly
                // doesn't support sets.  
                if(!(jobj instanceof List)) {
                    obj = new ArrayList((Collection)jobj); 
                }
            }
        }
        
        // If it's not a PyObject, first make a PyObject out of it.
        if(!(obj instanceof PyObject)) {
            obj = Py.java2py(obj);
        }
        if(asHash || obj instanceof PyDictionary || obj instanceof PyStringMap) {
            return JythonHashModel.FACTORY.create(obj, wrapper);
        }
        if(asSequence || obj instanceof PySequence) {
            return JythonSequenceModel.FACTORY.create(obj, wrapper);
        }
        if(obj instanceof PyInteger || obj instanceof PyLong || obj instanceof PyFloat) {
            return JythonNumberModel.FACTORY.create(obj, wrapper);
        }
        if(obj instanceof PyNone) {
            return null;
        }
        return JythonModel.FACTORY.create(obj, wrapper);
    }
}