freemarker-core/src/main/java/freemarker/template/TemplateTransformModel.java

/*
 * 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 java.io.FilterWriter;
import java.io.IOException;
import java.io.Writer;
import java.util.Map;

import freemarker.template.utility.DeepUnwrap;

/**
 * "transform" template language data type: user-defined directives 
 * (much like macros) specialized on filtering output; you should rather use the newer {@link TemplateDirectiveModel}
 * instead. This interface will certainly be deprecated in FreeMarker 2.4.
 */
public interface TemplateTransformModel extends TemplateModel {

    /**
     * Returns a writer that will be used by the engine to feed the transformation input to the transform. Each call to
     * this method must return a new instance of the writer so that the transformation is thread-safe.
     * <p>
     * This method should not throw {@link RuntimeException}, nor {@link IOException} that wasn't caused by writing to
     * the output. Such exceptions should be catched inside the method and wrapped inside a
     * {@link TemplateModelException}. (Note that setting {@link Configuration#setWrapUncheckedExceptions(boolean)} to
     * {@code true} can mitigate the negative effects of implementations that throw {@link RuntimeException}-s.)
     * 
     * @param out
     *            the character stream to which to write the transformed output
     * 
     * @param args
     *            the arguments (if any) passed to the transformation as a map of key/value pairs where the keys are
     *            strings and the arguments are {@link TemplateModel} instances. This is never {@code null}. (If you
     *            need to convert the template models to POJOs, you can use the utility methods in the
     *            {@link DeepUnwrap} class. Though it's recommended to work with {@link TemplateModel}-s directly.)
     * 
     * @return The {@link Writer} to which the engine will write the content to transform, or {@code null} if the
     *         transform does not support nested content (body). The returned {@link Writer} may implements the
     *         {@link TransformControl} interface if it needs advanced control over the evaluation of the nested
     *         content. FreeMarker will call {@link Writer#close()} after the transform end-tag. {@link Writer#close()}
     *         must not close the {@link Writer} received as the {@code out} parameter (so if you are using a
     *         {@link FilterWriter}, you must override {@link FilterWriter#close()}, as by default that closes the
     *         wrapped {@link Writer}). Since 2.3.27 its also allowed to return the {@code out} writer as is, in which
     *         case it won't be closed.
     * 
     * @throws TemplateModelException
     *             If any problem occurs that's not an {@link IOException} during writing the template output.
     * @throws IOException
     *             When writing to {@code out} (the parameter) fails. Other {@link IOException}-s should be catched in
     *             this method and wrapped into {@link TemplateModelException}.
     */
     Writer getWriter(Writer out, Map args) throws TemplateModelException, IOException;
}