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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.template.utility;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.util.List;

import freemarker.template.TemplateModelException;

/**
 * Gives FreeMarker the the ability to execute external commands. Will fork
 * a process, and inline anything that process sends to stdout in the template.
 * Based on a patch submitted by Peter Molettiere.
 *
 * BE CAREFUL! this tag, depending on use, may allow you
 * to set something up so that users of your web
 * application could run arbitrary code on your server.
 * This can only happen if you allow unchecked GET/POST
 * submissions to be used as the command string in the
 * exec tag.
 *
 * Usage:

 * From java:
 *  * SimpleHash root = new SimpleHash();
 *
 * root.put( "exec", new freemarker.template.utility.Execute() );
 *
 * ...
 * 
 *
 * From your FreeMarker template:
 *  *
 * The following is executed:
 * ${exec( "/usr/bin/ls" )}
 *
 * ...
 * 
 */
public class Execute implements freemarker.template.TemplateMethodModel {

    private final static int OUTPUT_BUFFER_SIZE = 1024;

    /**
     * Executes a method call.
     *
     * @param arguments a List of String objects containing the values
     * of the arguments passed to the method.
     * @return the TemplateModel produced by the method, or null.
     */
    public Object exec (List arguments) throws TemplateModelException {
        String aExecute;
        StringBuilder    aOutputBuffer = new StringBuilder();

        if ( arguments.size() < 1 ) {
            throw new TemplateModelException( "Need an argument to execute" );
        }

        aExecute = (String) (arguments.get(0));

        try {
            Process exec = Runtime.getRuntime().exec( aExecute );

            // stdout from the process comes in here
            InputStream execOut = exec.getInputStream();
            try {
                Reader execReader = new InputStreamReader( execOut );
    
                char[] buffer = new char[ OUTPUT_BUFFER_SIZE ];
                int bytes_read = execReader.read( buffer );
                while ( bytes_read > 0 ) {
                    aOutputBuffer.append( buffer, 0, bytes_read );
                    bytes_read = execReader.read( buffer );
                }
            } finally {
                execOut.close();
            }
        } catch ( IOException ioe ) {
            throw new TemplateModelException( ioe.getMessage() );
        }
        return aOutputBuffer.toString();
    }
}