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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boiler plate code between the web browser and your Java code.
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/*
 * Copyright 2005 Joe Walker
 *
 * 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 org.directwebremoting.servlet;

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.extend.Handler;
import org.directwebremoting.util.CopyUtils;

/**
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public abstract class CachingFileHandler implements Handler
{
    /* (non-Javadoc)
     * @see org.directwebremoting.Handler#handle(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
     */
    public void handle(HttpServletRequest request, HttpServletResponse response) throws IOException
    {
        if (isUpToDate(request))
        {
            response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
            return;
        }

        String output;

        synchronized (scriptCache)
        {
            String url = request.getPathInfo();
            output = scriptCache.get(url);
            if (output == null)
            {
                output = generateCachableContent(request, response);
            }
            scriptCache.put(url, output);
        }

        response.setContentType(mimeType);
        response.setDateHeader(HttpConstants.HEADER_LAST_MODIFIED, CONTAINER_START_TIME);
        response.setHeader(HttpConstants.HEADER_ETAG, ETAG);

        PrintWriter out = response.getWriter();
        out.println(output);
    }

    /**
     * Create a String which can be cached and sent as a 302
     * @param request The HTTP request data
     * @param response Where we write the HTTP response data
     * @return The string to output for this resource
     * @throws IOException
     */
    protected abstract String generateCachableContent(HttpServletRequest request, HttpServletResponse response) throws IOException;

    /**
     * An easy way to implement {@link #generateCachableContent(HttpServletRequest, HttpServletResponse)}
     * is to simply return {@link #readResource(String)}; using the
     * path to some resource provided in dwr.jar.
     * @param resource The fully qualified path (i.e. includes package) to a resource in dwr.jar
     * @return The contents of the resource as a string
     * @throws IOException If the resource can not be found or read
     */
    protected String readResource(String resource) throws IOException
    {
        InputStream raw = getClass().getResourceAsStream(resource);
        if (raw == null)
        {
            throw new IOException("Failed to find resource: " + resource);
        }

        StringWriter sw = new StringWriter();
        CopyUtils.copy(raw, sw);

        return sw.toString();
    }

    /**
     * Do we need to send the content for this file
     * @param req The HTTP request
     * @return true iff the ETags and If-Modified-Since headers say we have not changed
     */
    protected boolean isUpToDate(HttpServletRequest req)
    {
        if (ignoreLastModified)
        {
            return false;
        }

        long modifiedSince = -1;
        try
        {
            // HACK: Websphere appears to get confused sometimes
            modifiedSince = req.getDateHeader(HttpConstants.HEADER_IF_MODIFIED);
        }
        catch (RuntimeException ex)
        {
            // TODO: Check for "length" and re-parse
            // Normally clients send If-Modified-Since in rfc-complaint form
            // ("If-Modified-Since: Tue, 13 Mar 2007 13:11:09 GMT") some proxies
            // or browsers add length to this header so it comes like
            // ("If-Modified-Since: Tue, 13 Mar 2007 13:11:09 GMT; length=35946")
            // Servlet spec says container can throw IllegalArgumentException
            // if header value can not be parsed as http-date.
            // We might want to check for "; length=" and then do our own parsing
            // See: http://getahead.org/bugs/browse/DWR-20
            // And: http://www-1.ibm.com/support/docview.wss?uid=swg1PK20062
        }

        if (modifiedSince != -1)
        {
            // Browsers are only accurate to the second
            modifiedSince -= modifiedSince % 1000;
        }
        String givenEtag = req.getHeader(HttpConstants.HEADER_IF_NONE);
        String pathInfo = req.getPathInfo();

        // Deal with missing etags
        if (givenEtag == null)
        {
            // There is no ETag, just go with If-Modified-Since
            if (modifiedSince > CONTAINER_START_TIME)
            {
                if (log.isDebugEnabled())
                {
                    log.debug("Sending 304 for " + pathInfo + " If-Modified-Since=" + modifiedSince + ", Last-Modified=" + CONTAINER_START_TIME);
                }
                return true;
            }

            // There are no modified settings, carry on
            return false;
        }

        // Deal with missing If-Modified-Since
        if (modifiedSince == -1)
        {
            if (!ETAG.equals(givenEtag))
            {
                // There is an ETag, but no If-Modified-Since
                if (log.isDebugEnabled())
                {
                    log.debug("Sending 304 for " + pathInfo + ", If-Modified-Since=-1, Old ETag=" + givenEtag + ", New ETag=" + ETAG);
                }
                return true;
            }

            // There are no modified settings, carry on
            return false;
        }

        // Do both values indicate that we are in-date?
        if (ETAG.equals(givenEtag) && modifiedSince < CONTAINER_START_TIME)
        {
            if (log.isDebugEnabled())
            {
                log.debug("Sending 304 for " + pathInfo + ", If-Modified-Since=" + modifiedSince + ", Container Start=" + CONTAINER_START_TIME + ", Old ETag=" + givenEtag + ", New ETag=" + ETAG);
            }
            return true;
        }

        log.debug("Sending content for " + pathInfo + ", If-Modified-Since=" + modifiedSince + ", Container Start=" + CONTAINER_START_TIME + ", Old ETag=" + givenEtag + ", New ETag=" + ETAG);
        return false;
    }

    /**
     * @param ignoreLastModified The ignoreLastModified to set.
     */
    public void setIgnoreLastModified(boolean ignoreLastModified)
    {
        this.ignoreLastModified = ignoreLastModified;
    }

    /**
     * The mime type to send the output under
     * @param mimeType the mimeType to set
     */
    public void setMimeType(String mimeType)
    {
        this.mimeType = mimeType;
    }

    /**
     * @return the current mime type
     */
    public String getMimeType()
    {
        return mimeType;
    }

    /**
     * The time on the script files
     */
    private static final long CONTAINER_START_TIME;

    /**
     * The ETAG (=time for us) on the script files
     */
    private static final String ETAG;

    /**
     * Initialize the container start time
     */
    static
    {
        // Browsers are only accurate to the second
        long now = System.currentTimeMillis();
        CONTAINER_START_TIME = now - (now % 1000);

        ETAG = "\"" + CONTAINER_START_TIME + '\"';
    }

    /**
     * The mime type to send the output under
     */
    private String mimeType;

    /**
     * We cache the script output for speed
     */
    private final Map scriptCache = new HashMap();

    /**
     * Do we ignore all the Last-Modified/ETags blathering?
     */
    private boolean ignoreLastModified = false;

    /**
     * The log stream
     */
    private static final Log log = LogFactory.getLog(CachingFileHandler.class);
}