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Google App Engine compliant variation of FreeMarker. 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.ext.jython;

import org.python.core.PySystemState;

import freemarker.template.utility.StringUtil;

/**
 * Holds the {@link JythonVersionAdapter} so that it's only initialized when this class is accessed.
 * This utilizes that the JVM is required to initialize the fields of a class not earlier than the class is
 * first accessed. Furthermore, it utilizes that the JVM guarantees that the objects created as part of the class
 * initialization will be visible with their after-initialization state for the threads that access it. 
 */
class JythonVersionAdapterHolder {
    
    final static JythonVersionAdapter INSTANCE;
    static {
        // Note: Only the textual version number is available in Jython 2.0. 
        int version;
        try {
            // Although PySystemState.version is present in all versions,
            // its type changes, so we must use reflection to get it.
            version = StringUtil.versionStringToInt(
                    PySystemState.class.getField("version").get(null).toString());
        } catch (Exception e) {
            throw new RuntimeException("Failed to get Jython version: " + e);
        }
        ClassLoader cl = JythonVersionAdapter.class.getClassLoader();
        try {
            if (version >= 2005000) {
                INSTANCE = (JythonVersionAdapter) cl.loadClass(
                        "freemarker.ext.jython._Jython25VersionAdapter")
                    .newInstance();
            } else if (version >= 2002000) {
                INSTANCE = (JythonVersionAdapter) cl.loadClass(
                        "freemarker.ext.jython._Jython22VersionAdapter")
                    .newInstance();
            } else {
                INSTANCE = (JythonVersionAdapter) cl.loadClass(
                        "freemarker.ext.jython._Jython20And21VersionAdapter")
                    .newInstance();
            }
        } catch (ClassNotFoundException | IllegalAccessException | InstantiationException e) {
            throw adapterCreationException(e);
        }
    }

    private static RuntimeException adapterCreationException(Exception e) {
        return new RuntimeException("Unexpected exception when creating JythonVersionAdapter", e);
    }

}