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/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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 (FTL) data types.
* The template language only deals with {@link TemplateModel}-s, not directly with plain Java objects. (For example,
* it doesn't understand {@link java.lang.Number}, but {@link TemplateNumberModel}.) 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 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();
}