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/*
* Copyright 2014 Attila Szegedi, Daniel Dekany, Jonathan Revusky
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package freemarker.template;
import freemarker.template.utility.ClassUtil;
/**
* The common super-interface of the interfaces that stand for the FreeMarker Template Language data types.
* The template language only deals with {@link TemplateModel}-s, not with plain objects. This is why the data-model
* (aka. the "template context" in other languages) is (automatically) mapped to a tree of {@link TemplateModel}-s.
*
* Mapping the plain Java objects to {@link TemplateModel}-s (or the other way around sometimes) is the
* responsibility of the {@link ObjectWrapper} (can be set via {@link Configuration#setObjectWrapper(ObjectWrapper)}).
* But not all {@link TemplateModel}-s are for wrapping a plain object. For example, a value created within a template
* is not made to wrap an earlier existing object; it's a value that has always existed in the template language's
* domain. Users can also write {@link TemplateModel} implementations and put them directly into the data-model for
* full control over how that object is seen from the template. Certain {@link TemplateModel} interfaces may doesn't
* even have equivalent in Java. For example the directive type ({@link TemplateDirectiveModel}) is like that.
*
*
Because {@link TemplateModel} "subclasses" are all interfaces, a value in the template language can have multiple
* types. However, to prevent ambiguous situations, it's not recommended to make values that implement more than one of
* these types: string, number, boolean, date. The intended applications are like string+hash, string+method,
* hash+sequence, etc.
*
* @see ClassUtil#getFTLTypeDescription(TemplateModel)
*/
public interface TemplateModel {
/**
* A general-purpose object to represent nothing. It acts as
* an empty string, false, empty sequence, empty hash, and
* null-returning method model.
*/
TemplateModel NOTHING = GeneralPurposeNothing.getInstance();
}