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<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V2.0//EN" "http://forrest.apache.org/dtd/document-v20.dtd">
<document>
  <header>
    <title>Apache™ FOP: Servlets</title>
    <subtitle>How to use Apache™ FOP in a Servlet</subtitle>
    <version>$Revision$</version>
  </header>
  <body>
    <section id="overview">
      <title>Overview</title>
      <p>
        This page discusses topic all around using Apache™ FOP in a servlet environment.
      </p>
    </section>
    <section id="example-servlets">
      <title>Example Servlets in the FOP distribution</title>
      <p>
        In the directory {fop-dir}/src/java/org/apache/fop/servlet, you'll find a working example
        of a FOP-enabled servlet.
      </p>
      <p>
        The servlet is automatically built when you build Apache FOP using the supplied Ant script. After building 
        the servlet, drop fop.war into the webapps directory of Apache Tomcat (or any other web container). Then, you can use 
        URLs like the following to generate PDF files:
      </p>
      <ul>
        <li>http://localhost:8080/fop/fop?fo=/home/path/to/fofile.fo</li>
        <li>http://localhost:8080/fop/fop?xml=/home/path/to/xmlfile.xml&amp;xsl=/home/path/to/xslfile.xsl</li>
      </ul>
      <p/>
      <p>The source code for the servlet can be found under {fop-dir}/src/java/org/apache/fop/servlet/FopServlet.java.</p>
      <note>
        This example servlet should not be used on a public web server connected to the Internet as it does not contain
        any measures to prevent Denial-of-Service-Attacks. It is provided as an example and as a starting point for
        your own servlet.
      </note>
    </section>
    <section id="servlet">
      <title>Create your own Servlet</title>
      <note>
        This section assumes you are familiar with <a href="embedding.html">embedding FOP</a>.
      </note>
      <section id="minimal-servlet">
        <title>A minimal Servlet</title>
        <p>
          Here is a minimal code snippet to demonstrate the basics:
        </p>
        <source>private FopFactory fopFactory = FopFactory.newInstance();
private TransformerFactory tFactory = TransformerFactory.newInstance();
        
public void doGet(HttpServletRequest request,
                   HttpServletResponse response) throws ServletException {
    try {
        response.setContentType("application/pdf");
        Fop fop = fopFactory.newFop(MimeConstants.MIME_PDF, response.getOutputStream());
        Transformer transformer = tFactory.newTransformer();
        Source src = new StreamSource("foo.fo");
        Result res = new SAXResult(fop.getDefaultHandler());
        transformer.transform(src, res);
    } catch (Exception ex) {
        throw new ServletException(ex);
    }
}</source>
        <note>
          There are numerous problems with the code snippet above.
          Its purpose is only to demonstrate the basic concepts.
          See below for details.
        </note>
      </section>
      <section id="xslt">
        <title>Adding XSL tranformation (XSLT)</title>
        <p>
          A common requirement is to transform an XML source to
          XSL-FO using an XSL transformation. It is recommended to use
          JAXP for this task. The following snippet shows the basic
          code:
        </p>
        <source>private FopFactory fopFactory = FopFactory.newInstance();
private TransformerFactory tFactory = TransformerFactory.newInstance();

public void init() throws ServletException {
    //Optionally customize the FopFactory and TransformerFactory here
}

[..]

    //Setup a buffer to obtain the content length
    ByteArrayOutputStream out = new ByteArrayOutputStream();

    //Setup FOP
    Fop fop = fopFactory.newFop(MimeConstants.MIME_PDF, out);

    //Setup Transformer
    Source xsltSrc = new StreamSource(new File("foo-xml2fo.xsl"));
    Transformer transformer = tFactory.newTransformer(xsltSrc);

    //Make sure the XSL transformation's result is piped through to FOP
    Result res = new SAXResult(fop.getDefaultHandler());

    //Setup input
    Source src = new StreamSource(new File("foo.xml"));

    //Start the transformation and rendering process
    transformer.transform(src, res);

    //Prepare response
    response.setContentType("application/pdf");
    response.setContentLength(out.size());
    
    //Send content to Browser
    response.getOutputStream().write(out.toByteArray());
    response.getOutputStream().flush();</source>
        <note>
          Buffering the generated PDF in a ByteArrayOutputStream is done to avoid potential 
          problems with the Acrobat Reader Plug-in in Microsoft Internet Explorer.
        </note>
        <p>
          The <code>Source</code> instance used above is simply an
          example.  If you have to read the XML from a string, supply
          a <code>new StreamSource(new
          StringReader(xmlstring))</code>. Constructing and reparsing
          an XML string is generally less desirable than using a
          SAXSource if you generate your XML.  You can alternatively
          supply a DOMSource as well.  You may also use dynamically
          generated XSL if you like.
        </p>
        <p>
          Because you have an explicit <code>Transformer</code> object, you can also use it to 
          explicitely set parameters for the transformation run.
        </p>
      </section>
      <section id="cfg">
        <title>Custom configuration</title>
        <p>
          You can easily set up your own FOUserAgent as demonstrated on the <a href="embedding.html">Embedding page</a>.
        </p>
      </section>
      <section id="performance">
        <title>Improving performance</title>
        <p>
          There are several options to consider:
        </p>
        <ul>
          <li>
            Instead of java.io.ByteArrayOutputStream consider using the ByteArrayOutputStream
            implementation from the <a href="ext:commons-io">Jakarta Commons IO project</a> which allocates less memory.
            The full class name is: <code>org.apache.commons.io.output.ByteArrayOutputStream</code>
          </li>
          <li>
            In certain cases it can help to write the generated PDF to a temporary file so
            you can quickly reuse the file. This is especially useful, if Internet Explorer
            calls the servlet multiple times with the same request or if you often generate
            equal PDFs.
          </li>
        </ul>
        <p>
          Of course, the 
          <a href="embedding.html#performance">performance hints from the Embedding page</a>
          apply here, too.
        </p>
      </section>
      <section id="uriresolver">
        <title>Accessing resources in your web application</title>
        <p>
          Often, you will want to use resources (stylesheets, images etc.) which are bundled with
          your web application. FOP provides a URIResolver implementation that lets you access
          files via the Servlet's ServletContext. The class is called 
          <code>org.apache.fop.servlet.ServletContextURIResolver</code>.
        </p>
        <p>
          Here's how to set it up in your servlet. Instantiate a new instance in the servlet's 
          init() method:
        </p>
        <source><![CDATA[
    /** URIResolver for use by this servlet */
    protected URIResolver uriResolver; 

    public void init() throws ServletException {
        this.uriResolver = new ServletContextURIResolver(getServletContext());
        [..]
    }]]></source>
        <p>
          The ServletContextURIResolver reacts on URIs beginning with "servlet-context:". If you
          want to access an image in a subdirectory of your web application, you could, for
          example, use: "servlet-context:/images/myimage.png". Don't forget the leading slash
          after the colon!
        </p>
        <p>
          Further down, you can use the URIResolver for various things:
        </p>
        <ul>
          <li>
            With the Transformer (JAXP/XSLT) so things like document() functions can resolver 
            "servlet-context:" URIs.
          </li>
          <li>
            With the FopFactory so every resource FOP loads can be loaded using a "servlet-context:"
            URI.
          </li>
          <li>
            You can the ServletContextURIResolver yourself in your servlet code to access
            stylesheets or XML files bundled with your web application.
          </li>
        </ul>
        <p>
          Here are some example snippets:
        </p>
        <source><![CDATA[
//Setting up the JAXP TransformerFactory
this.transFactory = TransformerFactory.newInstance();
this.transFactory.setURIResolver(this.uriResolver);

[..]

//Setting up the FOP factory
this.fopFactory = FopFactory.newInstance();
this.fopFactory.setURIResolver(this.uriResolver);

[..]

//The stylesheet for the JAXP Transfomer
Source xsltSrc = this.uriResolver.resolve(
    "servlet-context:/xslt/mystylesheet.xsl", null);
Transformer transformer = this.transFactory.newTransformer(xsltSrc);
transformer.setURIResolver(this.uriResolver);]]></source>
      </section>
    </section>
    <section id="ie">
      <title>Notes on Microsoft Internet Explorer</title>
      <p>
        Some versions of Internet Explorer will not automatically show the PDF or call the servlet multiple times.
        These are well-known limitations of Internet Explorer and are not a problem of the servlet.
        However, Internet Explorer can still be used to download the PDF so that it can be viewed later. 
        Here are some suggestions in this context:
      </p>
      <ul>
        <li>
          Use an URL ending in <code>.pdf</code>, like
          <code>http://myserver/servlet/stuff.pdf</code>. Yes, the servlet can
          be configured to handle this. If the URL has to contain parameters,
          try to have <strong>both</strong> the base URL as well as the last parameter end in
          <code>.pdf</code>, if necessary append a dummy parameter, like
          <code>http://myserver/servlet/stuff.pdf?par1=a&amp;par2=b&amp;d=.pdf</code>. The
          effect may depend on IEx version.
        </li>
        <li>
          Give IEx the opportunity to cache. In particular, ensure the
          server does not set any headers causing IEx not to cache the
          content. This may be a real problem if the document is sent
          over HTTPS, because most IEx installations will by default
          <em>not</em> cache any content retrieved over HTTPS.
          Setting the <code>Expires</code> header entry may help in
          this case:<br/> <code>response.setDateHeader("Expires",
          System.currentTimeMillis() + cacheExpiringDuration *
          1000);</code><br/> Consult your server manual and the
          relevant RFCs for further details on HTTP headers and
          caching.
        </li>
        <li>
          Cache in the server. It may help to include a parameter in
          the URL which has a timestamp as the value min order to
          decide whether a request is repeated. IEx is reported to
          retrieve a document up to three times, but never more often.
        </li>
      </ul>
    </section>
    <section id="servlet-engine">
      <title>Servlet Engines</title>
      <p>
        When using a servlet engine, there are potential CLASSPATH issues, and potential conflicts 
        with existing XML/XSLT libraries. Servlet containers also often use their own classloaders 
        for loading webapps, which can cause bugs and security problems.
      </p>
      <section id="tomcat">
        <title>Tomcat</title>
        <p>
          Check Tomcat's documentation for detailed instructions about installing FOP and Cocoon.
          There are known bugs that must be addressed, particularly for Tomcat 4.0.3.
        </p>
      </section>
      <section id="websphere">
        <title>WebSphere 3.5</title>
        <p>
          Put a copy of a working parser in some directory where WebSphere can access it.
          For example, if /usr/webapps/yourapp/servlets is the CLASSPATH for your servlets, 
          copy the Xerces jar into it (any other directory would also be fine).
          Do not add the jar to the servlet CLASSPATH, but add it to the CLASSPATH of the 
          application server which contains your web application.
          In the WebSphere administration console, click on the "environment" button in the 
          "general" tab. In the "variable name" box, enter "CLASSPATH".
          In the "value" box, enter the correct path to the parser jar file 
          (/usr/webapps/yourapp/servlets/Xerces.jar in our example here).
          Press "OK", then apply the change and restart the application server.
        </p>
      </section>
    </section>
    <section id="complex-usecases">
      <title>Handling complex use cases</title>
      <p>
        Sometimes the requirements for a servlet get quite sophisticated: SQL data sources, 
        multiple XSL transformations, merging of several datasources etc. In such a case 
        consider using <a class="fork" href="ext:cocoon">Apache Cocoon</a> instead 
        of a custom servlet to accomplish your goal.
      </p>
    </section>
  </body>
</document>