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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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/*
 * Copyright 2014 Attila Szegedi, Daniel Dekany, Jonathan Revusky
 * 
 * Licensed 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.cache;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.Reader;
import java.net.MalformedURLException;
import java.net.URL;

import javax.servlet.ServletContext;

import freemarker.log.Logger;
import freemarker.template.utility.StringUtil;

/**
 * A {@link TemplateLoader} that uses streams reachable through 
 * {@link ServletContext#getResource(String)} as its source of templates.
 */
public class WebappTemplateLoader implements TemplateLoader
{
    private static final Logger logger = Logger.getLogger("freemarker.cache");
    
    private final ServletContext servletContext;
    private final String subdirPath;

    private Boolean urlConnectionUsesCaches;
    
    /**
     * Creates a resource template cache that will use the specified servlet
     * context to load the resources. It will use the base path of 
     * "/" meaning templates will be resolved relative to the 
     * servlet context root location.
     * @param servletContext the servlet context whose
     * {@link ServletContext#getResource(String)} will be used to load the
     * templates.
     */
    public WebappTemplateLoader(ServletContext servletContext) {
        this(servletContext, "/");
    }

    /**
     * Creates a template loader that will use the specified servlet
     * context to load the resources. It will use the specified base path,
     * which is interpreted relatively to the context root (does not mater
     * if you start it with "/" or not). Path components
     * should be separated by forward slashes independently of the separator 
     * character used by the underlying operating system.
     * @param servletContext the servlet context whose
     * {@link ServletContext#getResource(String)} will be used to load the
     * templates.
     * @param subdirPath the base path to template resources.
     */
    public WebappTemplateLoader(ServletContext servletContext, String subdirPath) {
        if(servletContext == null) {
            throw new IllegalArgumentException("servletContext == null");
        }
        if(subdirPath == null) {
            throw new IllegalArgumentException("path == null");
        }
        
        subdirPath = subdirPath.replace('\\', '/');
        if(!subdirPath.endsWith("/")) {
            subdirPath += "/";
        }
        if (!subdirPath.startsWith("/")) {
            subdirPath = "/" + subdirPath;
        }
        this.subdirPath = subdirPath;
        this.servletContext = servletContext;
    }

    public Object findTemplateSource(String name) throws IOException {
        String fullPath = subdirPath + name;
        // First try to open as plain file (to bypass servlet container resource caches).
        try {
            String realPath = servletContext.getRealPath(fullPath);
            if (realPath != null) {
                File file = new File(realPath);
                if(!file.isFile()) {
                    return null;
                }
                if(file.canRead()) {                    
                    return file;
                }
            }
        } catch (SecurityException e) {
            ;// ignore
        }
            
        // If it fails, try to open it with servletContext.getResource.
        URL url = null;
        try {
            url = servletContext.getResource(fullPath);
        } catch(MalformedURLException e) {
            logger.warn("Could not retrieve resource " + StringUtil.jQuoteNoXSS(fullPath),
                    e);
            return null;
        }
        return url == null ? null : new URLTemplateSource(url, getURLConnectionUsesCaches());
    }
    
    public long getLastModified(Object templateSource) {
        if (templateSource instanceof File) {
            return ((File) templateSource).lastModified();
        } else {
            return ((URLTemplateSource) templateSource).lastModified();
        }
    }
    
    public Reader getReader(Object templateSource, String encoding)
    throws IOException {
        if (templateSource instanceof File) {
            return new InputStreamReader(
                    new FileInputStream((File) templateSource),
                    encoding);
        } else {
            return new InputStreamReader(
                    ((URLTemplateSource) templateSource).getInputStream(),
                    encoding);
        }
    }
    
    public void closeTemplateSource(Object templateSource) throws IOException {
        if (templateSource instanceof File) {
            // Do nothing.
        } else {
            ((URLTemplateSource) templateSource).close();
        }
    }
    
    /**
     * Getter pair of {@link #setURLConnectionUsesCaches(Boolean)}.
     * 
     * @since 2.3.21
     */
    public Boolean getURLConnectionUsesCaches() {
        return urlConnectionUsesCaches;
    }

    /**
     * It does the same as {@link URLTemplateLoader#setURLConnectionUsesCaches(Boolean)}; see there.
     * 
     * @since 2.3.21
     */
    public void setURLConnectionUsesCaches(Boolean urlConnectionUsesCaches) {
        this.urlConnectionUsesCaches = urlConnectionUsesCaches;
    }

    /**
     * Show class name and some details that are useful in template-not-found errors.
     * 
     * @since 2.3.21
     */
    public String toString() {
        return "WebappTemplateLoader(servletContext.name="
                + StringUtil.jQuote(servletContext.getServletContextName()) + ", subdirPath=" + StringUtil.jQuote(subdirPath)
                + ")";
    }

    
}