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Turbine is a servlet based framework that allows experienced Java developers to quickly build secure web applications. Parts of Turbine can also be used independently of the web portion of Turbine as well. In other words, we strive to make portions of Turbine easily available for use in other applications.
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package org.apache.turbine.modules.screens;

/*
 * 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.
 */

import org.apache.commons.lang3.StringUtils;
import org.apache.commons.lang3.exception.ExceptionUtils;
import org.apache.turbine.TurbineConstants;
import org.apache.turbine.annotation.TurbineConfiguration;
import org.apache.turbine.annotation.TurbineService;
import org.apache.turbine.pipeline.PipelineData;
import org.apache.turbine.services.template.TemplateService;
import org.apache.turbine.services.velocity.VelocityService;
import org.apache.turbine.util.RunData;
import org.apache.velocity.context.Context;

/**
 * Base Velocity Screen.  The buildTemplate() assumes the template
 * parameter has been set in the PipelineData object.  This provides the
 * ability to execute several templates from one Screen.
 *
 * 
 *
 * If you need more specific behavior in your application, extend this
 * class and override the doBuildTemplate() method.
 *
 * @author Dave Bryson
 * @author Henning P. Schmiedehausen
 * @author Peter Courcoux
 * @version $Id: VelocityScreen.java 1854797 2019-03-04 20:41:39Z tv $
 */
public class VelocityScreen
    extends TemplateScreen
{
    /** The prefix for lookup up screen pages */
    protected static final String prefix = PREFIX + "/";

    /** Injected service instance */
    @TurbineService
    protected VelocityService velocity;

    /** Injected service instance */
    @TurbineService
    protected TemplateService templateService;

    @TurbineConfiguration( TurbineConstants.TEMPLATE_ERROR_KEY )
    protected String templateError = TurbineConstants.TEMPLATE_ERROR_VM;

    /**
     * Velocity Screens extending this class should override this
     * method to perform any particular business logic and add
     * information to the context.
     *
     * @param pipelineData Turbine information.
     * @param context Context for web pages.
     * @throws Exception a generic exception.
     */
    protected void doBuildTemplate(PipelineData pipelineData,
                                   Context context)
            throws Exception
    {
        // empty
    }

    /**
     * Needs to be implemented to make TemplateScreen like us.  The
     * actual method that you should override is the one with the
     * context in the parameter list.
     *
     * @param pipelineData Turbine information.
     * @throws Exception a generic exception.
     */
    @Override
    protected void doBuildTemplate(PipelineData pipelineData)
            throws Exception
    {
        doBuildTemplate(pipelineData, velocity.getContext(pipelineData));
    }

    /**
     * This builds the Velocity template.
     *
     * @param pipelineData Turbine information.
     * @return the content of the screen
     * @throws Exception a generic exception.
     */
    @Override
    public String buildTemplate(PipelineData pipelineData)
        throws Exception
    {
        RunData data = pipelineData.getRunData();
        String screenData = null;

        Context context = velocity.getContext(pipelineData);

        String screenTemplate = data.getTemplateInfo().getScreenTemplate();
        String templateName
            = templateService.getScreenTemplateName(screenTemplate);

        // The Template Service could not find the Screen
        if (StringUtils.isEmpty(templateName))
        {
            log.error("Screen " + screenTemplate + " not found!");
            throw new Exception("Could not find screen for " + screenTemplate);
        }

        try
        {
            // if a layout has been defined return the results, otherwise
            // send the results directly to the output stream.
            if (getLayout(pipelineData) == null)
            {
                velocity.handleRequest(context,
                        prefix + templateName,
                        data.getResponse().getOutputStream());
            }
            else
            {
                screenData =
                    velocity.handleRequest(context, prefix + templateName);
            }
        }
        catch (Exception e)
        {
            // If there is an error, build a $processingException and
            // attempt to call the error.vm template in the screens
            // directory.
            context.put (TurbineConstants.PROCESSING_EXCEPTION_PLACEHOLDER, e.toString());
            context.put (TurbineConstants.STACK_TRACE_PLACEHOLDER, ExceptionUtils.getStackTrace(e));

            screenData = velocity.handleRequest(context, prefix + templateError);
        }

        return screenData;
    }
}