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FreeMarker is a "template engine"; a generic tool to generate text output based on templates.

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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.utility;

import java.io.IOException;
import java.io.Writer;
import java.util.Map;

import org.python.core.PyObject;
import org.python.util.PythonInterpreter;

import freemarker.core.Environment;
import freemarker.template.TemplateTransformModel;

/**
 * A crude first pass at an embeddable Jython interpreter
 */

public class JythonRuntime extends PythonInterpreter
    implements TemplateTransformModel {
    public Writer getWriter(final Writer out,
                            final Map args) {
        final StringBuilder buf = new StringBuilder();
        final Environment env = Environment.getCurrentEnvironment();
        return new Writer() {
            @Override
            public void write(char cbuf[], int off, int len) {
                buf.append(cbuf, off, len);
            }

            @Override
            public void flush() throws IOException {
                interpretBuffer();
                out.flush();
            }

            @Override
            public void close() {
                interpretBuffer();
            }

            private void interpretBuffer() {
                synchronized (JythonRuntime.this) {
                    PyObject prevOut = systemState.stdout;
                    try {
                        setOut(out);
                        set("env", env);
                        exec(buf.toString());
                        buf.setLength(0);
                    } finally {
                        setOut(prevOut);
                    }
                }
            }
        };
    }
}