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Tridion Beanification - Expression Language Evaluator for JSP/JSTL Templates

By Unknown
The "beanification" I am presenting here -- which could in fact be called the "DGX's Java steroided big-brother" -- is the functionality that enables writing EL for the JSP/JSTL Tridion templates, part of the Java Mediator I am currently working on. I knew writing this piece would be fun, but I underestimated how much fun it would really be. I had a blast! :) <geek sneers here/>

EL simplifies writing rather complex expressions by providing the possibility of writing the expression just using the . and [ ] syntax. The dot (.) is used to specify 'Java Beans'-like properties on a given object, and the square brakets ([ ]) specify an indexed element on any property that returns a collection.

Example:
    ${Page.ComponentPresentations[1]}
looks up the Page object, then accesses the ComponentPresentations list property on it and finally returns the 2nd Component Presentation as denoted by [1].

The Tridion beanification, in fact an EL Evaluator, conforms closely to the Java Beans convention, with minor modifications to suit the JNI proxy methods from .NET into Java. If we wanted to evaluate ${base.propertyName}, default Java Bean standard would look up, using reflection, the methods getPropertyName() and isPropertyName() on the base object. Additionally, the Tridion EL evaluator will also lookup GetPropertyName(), IsPropertyName() and PropertyName() methods on the base object, that would be generated from .NET methods or properties.

Special handling is done for items in the Package and Engine -- expressions starting with Package or Engine will consider them as the base object in the EL.

Example:
    ${Engine.RenderMode} returns the Render Mode as string by invoking engine.getRenderMode()
    ${Package.Page.PublishLocationUrl} returns the URL where the page is published by invoking engine.GetObject(package.GetByName("Page")).GetPublishLocationUrl()

For all other expressions, the evaluator will try to first look it up in the Package and if found, try to retrieve its actual TOM.Java object (from the Engine), so expressions ${Engine.Page} and ${Page} are in fact the same, as both yield the same Page TOM.Java proxy object into .NET.

Some Tridion EL examples:
    ${Page.ContextRepository.Title}
    ${Page.Publication.Title}
    ${Page.Publication.Metadata.Link.Metadata.Type.Title} from current Page get Publication (ContextRepository), get Metadata Schema, field Link (Component Link) get actual linked Component, get Metadata field Type (a Category Keyword) and finally get the Keyword title;
    ${Page.Publication.Metadata.Link.Fields.Summary} linked Component, field Summary
    ${Page.Publication.Metadata.Embeddable[1].Number} Embeddable field, numeric field Number
    ${Page.Publication.Metadata.Embeddable[1].MMLink.Title} Embeddable Multimedia Link, get title
    ${Page.Publication.Metadata.Link.Title} Component Link title
I can even write loops now:
    <c:forEach var="cp" items="${Page.ComponentPresentationsList}">
        Component: ${cp.Component.Id} '${cp.Component.Title}'
        Component Template: ${cp.ComponentTemplate.Id} '${cp.ComponentTemplate.Title}'
    </c:forEach>
Notice the usage of ComponentPresentationsList instead of ComponentPresentations -- this is because the List version returns a java.util.List wrapper, so it can be used in a for each iteration.

Implementation Details

I implemented the whole logic in an javax.el.ELResolver class, which gets wired up in the Page Context's getELContext method.

public class TridionELResolver extends ELResolver {
    private static ELResolver instance;
    private static final String methodPrefixes[] = new String[] { "get", "Get", "is", "Is", "" };
    @Override
    public Object getValue(ELContext context, Object base, Object property) throws NullPointerException,
            PropertyNotFoundException, ELException {
        Object result = null;
        if (base == null) {
            result = getTopLevelProperty(context, property);
            if (result != null && !(result instanceof Item)) {
                context.setPropertyResolved(true);
            }
        } else {
            result = getBaseProperty(context, base, property);
            if (result == null) {
                String stringProperty = property.toString();
                String friendlyProperty = fixFriendlyProperty(base, stringProperty);
                if (!friendlyProperty.equals(stringProperty)) {
                    result = getBaseProperty(context, base, friendlyProperty);
                }
            }
            if (result != null) {
                context.setPropertyResolved(true);
            }
        }
        return result;
    }
    public static ELResolver getInstance() {
        if (instance == null) {
            instance = new TridionELResolver();
        }
        return instance;
    }
    private Object getTopLevelProperty(ELContext context, Object property) {
        FakePageContext pageContext = (FakePageContext) context.getContext(JspContext.class);
        Package _package = pageContext.getPackage();
        Engine engine = pageContext.getEngine();
        String propertyName = property.toString();
        Object result = null;
        if (propertyName.equals("Engine")) {
            result = engine;
        } else if (propertyName.equals("Package")) {
            result = _package;
        } else {
            result = getEngineOrPackageProperty(engine, _package, property);
        }
        return result;
    }
    private Object getEngineOrPackageProperty(ELContext context, Object property) {
        FakePageContext pageContext = (FakePageContext) context.getContext(JspContext.class);
        Package _package = pageContext.getPackage();
        Engine engine = pageContext.getEngine();
        return getEngineOrPackageProperty(engine, _package, property);
    }
    private Object getEngineOrPackageProperty(Engine engine, Package _package, Object property) {
        Object result = getPackageProperty(_package, property);
        if (result != null) {
            Object engineProperty = getEngineProperty(engine, result);
            if (engineProperty != null) {
                result = engineProperty;
            }
        }
        return result;
    }
    private Object getEngineProperty(Engine engine, Object property) {
        Object result = null;
        if (property instanceof Item) {
            Item item = (Item) property;
            String id = item.GetValue("ID");
            if (id == null) {
                return result;
            }
            result = engine.GetObject(item);
        } else {
            String stringProperty = property.toString();
            if (TcmUri.IsValid(stringProperty) || stringProperty.startsWith("/webdav")) {
                result = engine.GetObject(stringProperty);
            }
        }
        return result;
    }
    private Object getPackageProperty(Package _package, Object property) {
        Item item = _package.GetByName(property.toString());
        return item;
    }
    private Object getBaseProperty(ELContext context, Object base, Object property) {
        Object result = null;
        String propertyName = property.toString();
        if (base instanceof Component) {
            result = getComponentProperty(base, propertyName);
        } else if (base instanceof RepositoryLocalObject) {
            result = getRepositoryLocalObjectProperty(base, propertyName);
        } else if (base instanceof Repository) {
            result = getRepositoryProperty(base, propertyName);
        } else if (base instanceof Package) {
            result = getEngineOrPackageProperty(context, propertyName);
        } else if (base instanceof ItemFields) {
            Object[] bases = new Object[] { base };
            result = getItemFieldsProperty(bases, propertyName);
            base = bases[0];
        } else if (base instanceof List) {
            Object[] bases = new Object[] { base };
            result = getListProperty(bases, property);
            base = bases[0];
        } else if (base instanceof IList) {
            Object[] bases = new Object[] { base };
            result = getIListProperty(bases, property);
            base = bases[0];
        }
        if (result == null) {
            result = getBeanProperty(base, propertyName);
        }
        return result;
    }
    private Object getComponentProperty(Object base, String propertyName) {
        Object result = null;
        Component component = (Component) base;
        if (propertyName.equals("Fields")) {
            result = new ItemFields(component.getContent(), component.getSchema());
        } else if (propertyName.equals("Metadata")) {
            result = new ItemFields(component.getMetadata(), component.getMetadataSchema());
        }
        return result;
    }
    private Object getRepositoryLocalObjectProperty(Object base, String propertyName) {
        Object result = null;
        RepositoryLocalObject localObject = (RepositoryLocalObject) base;
        if (propertyName.equals("Metadata")) {
            result = new ItemFields(localObject.getMetadata(), localObject.getMetadataSchema());
        }
        return result;
    }
    private Object getRepositoryProperty(Object base, String propertyName) {
        Object result = null;
        Repository repository = (Repository) base;
        if (propertyName.equals("Metadata")) {
            result = new ItemFields(repository.getMetadata(), repository.getMetadataSchema());
        }
        return result;
    }
    private Object getItemFieldsProperty(Object[] bases, String propertyName) {
        Object result = null;
        Object base = bases[0];
        ItemFields itemFields = (ItemFields) base;
        base = bases[0] = itemFields.getItem(propertyName);
        ItemField itemField = (ItemField) base;
        if (itemField.getDefinition().getMaxOccurs() == 1) { // single-value
            result = getBeanProperty(base, "Value");
        } else { // multi-value
            result = getBeanProperty(base, "ValuesList");
            if (result == null) {
                result = getBeanProperty(base, "Values");
            }
        }
        if (itemField.getDefinition().getMinOccurs() == 0 && result == null) { // optional
            result = "";
        }
        return result;
    }
    private Object getListProperty(Object[] bases, Object property) {
        Object result = null;
        Object base = bases[0];
        List<?> list = (List<?>) base;
        int intProperty = 0;
        if (property instanceof Number) {
            Number numberProperty = (Number) property;
            intProperty = numberProperty.intValue();
            result = list.get(intProperty);
        } else {
            base = bases[0] = list.get(intProperty);
        }
        return result;
    }
    private Object getIListProperty(Object[] bases, Object property) {
        Object result = null;
        Object base = bases[0];
        IList list = (IList) base;
        int intProperty = 0;
        if (property instanceof Number) {
            Number numberProperty = (Number) property;
            intProperty = numberProperty.intValue();
            result = list.getItem(intProperty);
        } else {
            base = bases[0] = list.getItem(intProperty);
        }
        return result;
    }
    private Object getBeanProperty(Object base, String property, Object... methodArguments) {
        Object result = null;
        String propertyName = capitalize(property);
        for (String methodPrefix : methodPrefixes) {
            String methodName = methodPrefix + propertyName;
            Class<? extends Object> clazz = base.getClass();
            Method method = findMethod(clazz, methodName);
            if (method != null) {
                try {
                    result = method.invoke(base, methodArguments);
                    break;
                } catch (Exception e) {}
            }
        }
        return result;
    }
    private Method findMethod(Class<? extends Object> clazz, String methodName) {
        for (Method method : clazz.getMethods()) {
            if (method.getName().equals(methodName)) {
                return method;
            }
        }
        return null;
    }
    private String fixFriendlyProperty(Object base, String property) {
        if (base instanceof RepositoryLocalObject) {
            if (property.equals("Publication")) {
                property = "ContextRepository";
            }
        }
        return property;
    }
}
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