/*

  * $Source$

  * $Revision$

  *

  * Copyright (C) 2000 William Chesters

  *

  * Part of Melati (http://melati.org), a framework for the rapid

  * development of clean, maintainable web applications.

  *

  * Melati is free software; Permission is granted to copy, distribute

  * and/or modify this software under the terms either:

  *

  * a) the GNU General Public License as published by the Free Software

  *    Foundation; either version 2 of the License, or (at your option)

  *    any later version,

  *

  *    or

  *

  * b) any version of the Melati Software License, as published

  *    at http://melati.org

  *

  * You should have received a copy of the GNU General Public License and

  * the Melati Software License along with this program;

  * if not, write to the Free Software Foundation, Inc.,

  * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA to obtain the

  * GNU General Public License and visit http://melati.org to obtain the

  * Melati Software License.

  *

  * Feel free to contact the Developers of Melati (http://melati.org),

  * if you would like to work out a different arrangement than the options

  * outlined here.  It is our intention to allow Melati to be used by as

  * wide an audience as possible.

  *

  * This program is distributed in the hope that it will be useful,

  * but WITHOUT ANY WARRANTY; without even the implied warranty of

  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

  * GNU General Public License for more details.

  *

  * Contact details for copyright holder:

  *

  *     William Chesters <williamc At paneris.org>

  *     http://paneris.org/~williamc

  *     Obrechtstraat 114, 2517VX Den Haag, The Netherlands

  */

 package org.melati;

 import java.io.IOException;

 import java.io.PrintWriter;

 import java.io.UnsupportedEncodingException;

 import java.lang.reflect.Constructor;

 import java.util.Vector;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 import javax.servlet.http.HttpSession;

 import org.melati.poem.Database;

 import org.melati.poem.Field;

 import org.melati.poem.NotInSessionPoemException;

 import org.melati.poem.Persistent;

 import org.melati.poem.PoemLocale;

 import org.melati.poem.PoemThread;

 import org.melati.poem.ReferencePoemType;

 import org.melati.poem.Table;

 import org.melati.poem.User;

 import org.melati.poem.util.StringUtils;

 import org.melati.servlet.Form;

 import org.melati.template.HTMLMarkupLanguage;

 import org.melati.template.MarkupLanguage;

 import org.melati.template.ServletTemplateContext;

 import org.melati.template.ServletTemplateEngine;

 import org.melati.template.TemplateContext;

 import org.melati.template.TemplateEngine;

 import org.melati.util.AcceptCharset;

 import org.melati.util.CharsetException;

 import org.melati.util.DatabaseInitException;

 import org.melati.util.HttpHeader;

 import org.melati.util.HttpUtil;

 import org.melati.util.MelatiBufferedWriter;

 import org.melati.util.MelatiBugMelatiException;

 import org.melati.util.MelatiIOException;

 import org.melati.util.MelatiSimpleWriter;

 import org.melati.util.MelatiStringWriter;

 import org.melati.util.MelatiWriter;

 import org.melati.util.UTF8URLEncoder;

 import org.melati.util.UnexpectedExceptionException;

 /**

  * This is the main entry point for using the Melati framework.

  * A Melati exists once per request, or command from an application.

  * <p>

  * It provides a central container for all the relevant objects that 

  * a Servlet or command line application needs to create textual 

  * output, optionally using a Template Engine or a Database.

  * <p>

  * You will need to create a MelatiConfig in order to construct a Melati.

  * <p>

  * If you are using servlets, you will want to construct a Melati with

  * a request and response object.  Otherwise, simply pass in a Writer.

  * <p>

  * If you are using a template engine outside of a servlets context you will 

  * still need the servlets jar in your classpath, annoyingly, as Velocity and 

  * WebMacro introspect all possible methods and throw a ClassNotFound exception 

  * if the servlets classes are not available.  

  * <p>

  * Melati is typically used with Servlets, POEM (Persistent Object Engine for

  * Melati) and a Template Engine

  *

  * @see org.melati.MelatiConfig

  * @see org.melati.servlet.ConfigServlet

  * @see org.melati.servlet.PoemServlet

  * @see org.melati.servlet.TemplateServlet

  */

 public class Melati {

   /** UTF-8. */

   public static final String DEFAULT_ENCODING = "UTF-8";

   private MelatiConfig config;

   private PoemContext poemContext;

   private HttpServletRequest request;

   private HttpServletResponse response;

   private Database database = null;

   private Table<?> table = null;

   private Persistent object = null;

   private MarkupLanguage markupLanguage = null;

   private String[] arguments;

   // the template engine that is in use (if any)

   private TemplateEngine templateEngine;

   // the object that is used by the template engine to expand the template

   // against

   private TemplateContext templateContext;

   // are we manually flushing the output

   private boolean flushing = false;

   // are we buffering the output

   private boolean buffered= true;

   // the output writer

   private MelatiWriter writer;

   private String encoding;

   /**

    * Construct a Melati for use with Servlets.

    *

    * @param config - the MelatiConfig

    * @param request - the Servlet Request

    * @param response - the Servlet Response

    */

   public Melati(MelatiConfig config,

                 HttpServletRequest request,

                 HttpServletResponse response) {

     this.request = request;

     this.response = response;

     this.config = config;

   }

   /** Convenience constructor. */ 

   public Melati(MelatiWriter writer) {

     this.config = new MelatiConfig();

     this.writer = writer;

   }

   /**

    * Construct a Melati for use in 'stand alone' mode.

    * NB: you will not have access to servlet related stuff (eg sessions)

    *

    * @param config - the MelatiConfig

    * @param writer - the Writer that all output is written to

    */

   public Melati(MelatiConfig config, MelatiWriter writer) {

     this.config = config;

     this.writer = writer;

   }

   /**

    * Get the servlet request object.

    *

    * @return the Servlet Request

    */

   public HttpServletRequest getRequest() {

     return request;

   }

   /**

    * It is sometimes convenient to reconstruct the request object and

    * reset it, for example when returning from a log-in page.

    *

    * @see org.melati.login.HttpSessionAccessHandler

    * @param request - new request object

    */

   public void setRequest(HttpServletRequest request) {

     this.request = request;

   }

   /**

    * Used to set response mock in tests.

    * @see org.melati.login.HttpSessionAccessHandler

    * @param response - mock response object

    */

   public void setResponse(HttpServletResponse response) {

     this.response = response;

   }

   /**

    * Get the servlet response object.

    *

    * @return - the Servlet Response

    */

   public HttpServletResponse getResponse() {

     return response;

   }

   /**

    * Set the {@link PoemContext} for this request.  If the Context has a

    * LogicalDatabase set, this will be used to establish a connection

    * to the database.

    *

    * @param context - a PoemContext

    * @throws DatabaseInitException - if the database fails to initialise for

    *                                 some reason

    * @see org.melati.LogicalDatabase

    * @see org.melati.servlet.PoemServlet

    */

   public void setPoemContext(PoemContext context)

       throws DatabaseInitException {

     this.poemContext = context;

     if (poemContext.getLogicalDatabase() != null)

       database = LogicalDatabase.getDatabase(poemContext.getLogicalDatabase());

   }

   /**

    * Load a POEM Table and POEM Object for use in this request.  This is useful

    * as often Servlet requests are relevant for a single Table and/or Object.

    *

    * The Table name and Object id are set from the PoemContext.

    *

    * @see org.melati.admin.Admin

    * @see org.melati.servlet.PoemServlet

    */

   public void loadTableAndObject() {

     if (database != null)

       if (poemContext.getTable() != null ) {

         table = database.getTable(poemContext.getTable());

         if (poemContext.getTroid() != null)

           object = table.getObject(poemContext.getTroid().intValue());

         else

           object = null;

       }

   }

   /**

    * Get the PoemContext for this Request.

    *

    * @return - the PoemContext for this Request

    */

   public PoemContext getPoemContext() {

     return poemContext;

   }

   /**

    * Get the POEM Database for this Request.

    *

    * @return - the POEM Database for this Request

    * @see #setPoemContext

    */

   public Database getDatabase() {

     return database;

   }

   /**

    * @return the name of the Database 

    */

   public String getDatabaseName() { 

     return getPoemContext().getLogicalDatabase();  

   }

   /**

    * Return the names of other databases known at the moment. 

    *  

    * @return a Vector of database names

    */

   public Vector<String> getKnownDatabaseNames() {

     return LogicalDatabase.

                getInitialisedDatabaseNames();

   }

   /**

    * Get the POEM Table (if any) in use for this Request.

    *

    * @return the POEM Table for this Request

    * @see #loadTableAndObject

    */

   public Table<?> getTable() {

     return table;

   }

   /**

    * Get the POEM Object (if any) in use for this Request.

    *

    * @return the POEM Object for this Request

    * @see #loadTableAndObject

    */

   public Persistent getObject() {

     return object;

   }

   /**

    * Get the Method (if any) that has been set for this Request.

    *

    * @return the Method for this Request

    * @see org.melati.PoemContext

    * @see org.melati.servlet.ConfigServlet#poemContext

    * @see org.melati.servlet.PoemServlet#poemContext

    */

   public String getMethod() {

     return poemContext.getMethod();

   }

   /**

    * Set the template engine to be used for this Request.

    *

    * @param te - the template engine to be used

    * @see org.melati.servlet.TemplateServlet

    */

   public void setTemplateEngine(TemplateEngine te) {

     templateEngine = te;

   }

   /**

    * Get the template engine in use for this Request.

    *

    * @return - the template engine to be used

    */

   public TemplateEngine getTemplateEngine() {

     return templateEngine;

   }

   /**

    * Set the TemplateContext to be used for this Request.

    *

    * @param tc - the template context to be used

    * @see org.melati.servlet.TemplateServlet

    */

   public void setTemplateContext(TemplateContext tc) {

     templateContext = tc;

   }

   /**

    * Get the TemplateContext used for this Request.

    *

    * @return - the template context being used

    */

   public TemplateContext getTemplateContext() {

     return templateContext;

   }

   /**

    * Get the TemplateContext used for this Request.

    *

    * @return - the template context being used

    */

   public ServletTemplateContext getServletTemplateContext() {

     return (ServletTemplateContext)templateContext;

   }

   /**

    * Get the MelatiConfig associated with this Request.

    *

    * @return - the configuration being used

    */

   public MelatiConfig getConfig() {

     return config;

   }

   /**

    * Get the PathInfo for this Request split into Parts by '/'.

    *

    * @return - an array of the parts found on the PathInfo

    */

   public String[] getPathInfoParts() {

     String pathInfo = request.getPathInfo();

     if (pathInfo == null || pathInfo.length() < 1) return new String[0];

     pathInfo = pathInfo.substring(1);

     return StringUtils.split(pathInfo, '/');

   }

   /**

    * Set the aruments array from the commandline.

    *

    * @param args the arguments to set

    */

   public void setArguments(String[] args) {

     arguments = args;

   }

   /**

    * Get the Arguments array.

    *

    * @return the arguments array

    */

   public String[] getArguments() {

     return arguments;

   }

   /**

    * Get the Session for this Request.

    *

    * @return - the Session for this Request

    */

   public HttpSession getSession() {

     return getRequest().getSession(true);

   }

   /**

    * Get a named context utility eg org.melati.admin.AdminUtils.

    *  

    * @param className Name of a class with a single argument Melati constructor 

    * @return the instantiated class

    */

   public Object getContextUtil(String className) {

     Constructor<?> c;

     try {

         c  = Class.forName(className).getConstructor(new Class[] {this.getClass()});

     } catch (NoSuchMethodException e) {

       try { 

         c  = Class.forName(className).getConstructor(new Class[] {});

         try {

           return c.newInstance(new Object[] {});

         } catch (Exception e2) {

             throw new MelatiBugMelatiException("Class " + className + 

                     " cannot be instantiated ", e2);

         }

       } catch (Exception e2) {

           throw new MelatiBugMelatiException("Class " + className + 

                   " cannot be instantiated ", e2);

       }

     } catch (Exception e) {

           throw new MelatiBugMelatiException("Class " + className + 

                 " cannot be instantiated ", e);

     }  

     try {

       return c.newInstance(new Object[] {this});

     } catch (Exception e) {

         throw new MelatiBugMelatiException("Class " + className + 

                 " cannot be instantiated ", e);

     }

   }

   /**

    * Get a named context utility eg org.melati.admin.AdminUtils.

    *  

    * @param className Name of a class with a single argument Melati constructor 

    * @return the instantiated class

    */

   public Object getInstance(String className) {

     Object util;

     try {

       Constructor<?> c  = Class.forName(className).getConstructor(new Class[] {this.getClass()});

       util = c.newInstance(new Object[] {this});

     } catch (Exception e) {

       throw new MelatiBugMelatiException("Class " + className + 

           " cannot be instantiated", e);

     }  

     return util;

   }

   /**

    * Get the URL for the Logout Page.

    *

    * @return - the URL for the Logout Page

    * @see org.melati.login.Logout

    */

   public String getLogoutURL() {

     StringBuffer url = new StringBuffer();

     HttpUtil.appendRelativeZoneURL(url, getRequest());

     url.append('/');

     url.append(MelatiConfig.getLogoutPageServletClassName());

     url.append('/');

     url.append(poemContext.getLogicalDatabase());

     return url.toString();

   }

   /**

    * Get the URL for the Login Page.

    *

    * @return - the URL for the Login Page

    * @see org.melati.login.Login

    */

   public String getLoginURL() {

     StringBuffer url = new StringBuffer();

     HttpUtil.appendRelativeZoneURL(url, getRequest());

     url.append('/');

     url.append(MelatiConfig.getLoginPageServletClassName());

     url.append('/');

     url.append(poemContext.getLogicalDatabase());

     return url.toString();

   }

   /**

    * Get the URL for this Servlet Zone.

    *

    * @return - the URL for this Servlet Zone

    * @see org.melati.util.HttpUtil#zoneURL

    */

   public String getZoneURL() {

     return HttpUtil.zoneURL(getRequest());

   }

   /**

    * @return the relative url for the Servlet Zone of the current request

    */

   public String getRelativeZoneURL() { 

     return HttpUtil.getRelativeRequestURL(getRequest());    

   }

   /**

    * Get the URL for this request.

    * Not used in Melati.

    *

    * @return - the URL for this request

    * @see org.melati.util.HttpUtil#servletURL

    */

   public String getServletURL() {

     return HttpUtil.servletURL(getRequest());

   }

   /**

    * Get the URL for the JavascriptLibrary.

    * Convenience method.

    * 

    * @return - the URL for the JavascriptLibrary

    * @see org.melati.MelatiConfig#getJavascriptLibraryURL

    */

   public String getJavascriptLibraryURL() {

     return config.getJavascriptLibraryURL();

   }

   /**

    * Returns a PoemLocale object based on the Accept-Language header

    * of this request.

    *

    * If no usable Accept-Language header is found or we are using 

    * Melati outside of a servlet context then the configured 

    * default locale is returned.

    *

    * @return a PoemLocale object

    */

   public PoemLocale getPoemLocale() {

     if (getRequest() == null)

        return MelatiConfig.getPoemLocale();

     else if(getRequest().getLocale() == null) {

       return MelatiConfig.getPoemLocale();

     } else 

       return PoemLocale.from(getRequest().getLocale());

   }

   /**

    * Suggest a response character encoding and if necessary choose a

    * request encoding.

    * <p>

    * If the request encoding is provided then we choose a response

    * encoding to meet our preferences on the assumption that the

    * client will also indicate next time what its request

    * encoding is.

    * The result can optionally be set in code or possibly in

    * templates using {@link #setResponseContentType(String)}.

    * <p>

    * Otherwise we tread carefully. We assume that the encoding is

    * the first supported encoding of the client's preferences for

    * responses, as indicated by Accept-Charsets, and avoid giving

    * it any reason to change.

    * <p>

    * Actually, the server preference is a bit dodgy for

    * the response because if it does persuade the client to

    * change encodings and future requests include query strings

    * that we are providing now then we may end up with the

    * query strings being automatically decoded using the wrong

    * encoding by request.getParameter(). But by the time we

    * end up with values in such parameters the client and

    * server will probably have settled on particular encodings.

    */

   public void establishCharsets() throws CharsetException {

     AcceptCharset ac;

     String acs = request.getHeader("Accept-Charset");

     //assert acs == null || acs.trim().length() > 0 :

     //  "Accept-Charset should not be empty but can be absent";

     // Having said that we don't want to split hairs once debugged

     if (acs != null && acs.trim().length() == 0) {

       acs = null;

     }

     try {

       ac = new AcceptCharset(acs, config.getPreferredCharsets());

     }

     catch (HttpHeader.HttpHeaderException e) {

       throw new CharsetException(

           "An error was detected in your HTTP request header, " +

           "response code: " +

           HttpServletResponse.SC_BAD_REQUEST +

           ": \"" + acs + '"', e);

     }

     if (request.getCharacterEncoding() == null) {

       responseCharset = ac.clientChoice();

       try {

         request.setCharacterEncoding(responseCharset);

       }

       catch (UnsupportedEncodingException e) {

         throw new MelatiBugMelatiException("This should already have been checked by AcceptCharset", e);

       }

     } else {

       responseCharset = ac.serverChoice();

     }

   }

   /**

    * Suggested character encoding for use in responses.

    */

   protected String responseCharset = null;

   /**

    * Sets the content type for use in the response.

    * <p>

    * Use of this method is optional and only makes sense in a 

    * Servlet context. If the response is null then this is a no-op.

    * <p>

    * If the type starts with "text/" and does not contain a semicolon

    * and a good response character set has been established based on

    * the request Accept-Charset header and server preferences, then this

    * and semicolon separator are automatically appended to the type.

    * <p>

    * Whether this function should be called at all may depend on

    * the application and templates.

    * <p>

    * It should be called before any calls to {@link #getEncoding()}

    * and before writing the response.

    *

    * @see #establishCharsets()

    */

   public void setResponseContentType(String type) {

     contentType = type;

     if (responseCharset != null) 

       if (type.startsWith("text/")) 

         if (type.indexOf(";") == -1)

           contentType += "; charset=" + responseCharset;

     if (response != null) {

       response.setContentType(contentType);

     }

   }

   protected String contentType = null;

   /**

    * @return the contentType

    */

   public String getContentType() {

     return contentType;

   }

   /**

    * Use this method if you wish to use a different 

    * MarkupLanguage, WMLMarkupLanguage for example. 

    * Cannot be set in MelatiConfig as MarkupLanguage 

    * does not have a no argument constructor.

    * @param ml The ml to set.

    */

   public void setMarkupLanguage(MarkupLanguage ml) {

     this.markupLanguage = ml;

   }

   /**

    * Get a {@link MarkupLanguage} for use generating output from templates.

    * Defaults to HTMLMarkupLanguage.

    *

    * @return - a MarkupLanguage, defaulting to HTMLMarkupLanguage

    * @see org.melati.template.TempletLoader

    * @see org.melati.poem.PoemLocale

    */

   public MarkupLanguage getMarkupLanguage() {

     if (markupLanguage == null) 

       markupLanguage = new HTMLMarkupLanguage(this,

                                   config.getTempletLoader(),

                                   getPoemLocale());

     return markupLanguage;

   }

   /**

    * Get a HTMLMarkupLanguage.

    * Retained for backward compatibility as there are a lot 

    * of uses in templates.

    *

    * @return - a HTMLMarkupLanguage

    */

   public HTMLMarkupLanguage getHTMLMarkupLanguage() {

     return (HTMLMarkupLanguage)getMarkupLanguage();

   }

   /**

    * The URL of the servlet request associated with this <TT>Melati</TT>, with

    * a modified or added form parameter setting (query string component).

    *

    * @param field   The name of the form parameter

    * @param value   The new value for the parameter (unencoded)

    * @return        The request URL with <TT>field=value</TT>.  If there is

    *                already a binding for <TT>field</TT> in the query string

    *                it is replaced, not duplicated.  If there is no query

    *                string, one is added.

    * @see org.melati.servlet.Form

    */

   public String sameURLWith(String field, String value) {

     return Form.sameURLWith(getRequest(), field, value);

   }

   /**

    * The URL of the servlet request associated with this <TT>Melati</TT>, with

    * a modified or added form flag setting (query string component).

    *

    * @param field   The name of the form parameter

    * @return        The request URL with <TT>field=1</TT>.  If there is

    *                already a binding for <TT>field</TT> in the query string

    *                it is replaced, not duplicated.  If there is no query

    *                string, one is added.

    * @see org.melati.servlet.Form

    */

   public String sameURLWith(String field) {

     return sameURLWith(field, "1");

   }

   /**

    * The URL of the servlet request associated with this <TT>Melati</TT>.

    *

    * @return a string

    */

   public String getSameURL() {

     String qs = getRequest().getQueryString();

     return getRequest().getRequestURI() + (qs == null ? "" : '?' + qs);

   }

   /**

    * Turn off buffering of the output stream.

    *

    * By default, melati will buffer the output, which will not be written

    * to the output stream until you call melati.write();

    *

    * Buffering allows us to catch AccessPoemExceptions and redirect the user

    * to the login page.  This could not be done if any bytes had already  been written

    * to the client.

    *

    * @see org.melati.test.FlushingServletTest

    * @throws IOException if a writer has already been selected

    */

   public void setBufferingOff() throws IOException {

     if (writer != null)

       throw new IOException("You have already requested a Writer, " +

                             "and can't change it's properties now");

     buffered = false;

   }

   /**

    * Turn on flushing of the output stream.

    *

    * @throws IOException if there is a problem with the writer

    */

   public void setFlushingOn() throws IOException {

     if (writer != null)

       throw new IOException("You have already requested a Writer, " +

                             "and can't change it's properties now");

     flushing = true;

   }

   /**

    * Return the encoding that is used for URL encoded query

    * strings.

    * <p>

    * The requirement here is that parameters can be encoded in

    * query strings included in URLs in the body of responses.

    * User interaction may result in subsequent requests with such

    * a URL. The HTML spec. describes encoding of non-alphanumeric

    * ASCII using % and ASCII hex codes and, in the case of forms.

    * says the client may use the response encoding by default.

    * Sun's javadoc for <code>java.net.URLEncoder</code>

    * recommends UTF-8 but the default is the Java platform

    * encoding. Most significantly perhaps,

    * org.mortbay.http.HttpRequest uses the request encoding.

    * We should check that this is correct in the servlet specs.

    * <p>

    * So we assume that the servlet runner may dictate the

    * encoding that will work for multi-national characters in

    * field values encoded in URL's (but not necessarily forms).

    * <p>

    * If the request encoding is used then we have to try and

    * predict it. It will be the same for a session unless a client

    * has some reason to change it. E.g. if we respond to a request

    * in a different encoding and the client is influenced.

    * (See {@link #establishCharsets()}.

    * But that is only a problem if the first or second request

    * in a session includes field values encoded in the URL and

    * user options include manually entering the same in a form

    * or changing their browser configuration.

    * Or we can change the server configuration.

    * <p>

    * It would be better if we had control over what encoding

    * the servlet runner used to decode parameters.

    * Perhaps one day we will.

    * <p>

    * So this method implements the current policy and currently

    * returns the current request encoding.

    * It assumes {@link #establishCharsets()} has been called to

    * set the request encoding if necessary.

    *

    * @return the character encoding

    * @see #establishCharsets()

    * see also org.melati.admin.Admin#selection(ServletTemplateContext, Melati)

    */

   public String getURLQueryEncoding() {

     return request.getCharacterEncoding();

   }

   /**

    * Convenience method to URL encode a URL query string.

    *

    * See org.melati.admin.Admin#selection(ServletTemplateContext, Melati)

    */

   /**

    * @param string the String to encode

    * @return the encoded string

    */

   public String urlEncode(String string) {

     try {

       return UTF8URLEncoder.encode(string, getURLQueryEncoding());

     }

     catch (UnexpectedExceptionException e) {

       // Thrown if the encoding is not supported

       return string;

     }

   }

   /**

    * Return the encoding that is used for writing.

    * <p>

    * This should always return an encoding and it should be the same

    * for duration of use of an instance.

    *

    * @return Response encoding or a default in stand alone mode

    * @see #setResponseContentType(String)

    */

   public String getEncoding() {

     if (encoding == null)

       encoding = response == null ? DEFAULT_ENCODING :

                                     response.getCharacterEncoding();

     return encoding;

   }

   /**

    * Get a Writer for this request.

    *

    * If you have not accessed the Writer, it is reasonable to assume that

    * nothing has been written to the output stream.

    *

    * @return - one of:

    *

    * - the Writer that was used to construct the Melati

    * - the Writer associated with the Servlet Response

    * - a buffered Writer

    * - a ThrowingPrintWriter

    */

   public MelatiWriter getWriter() {

     if (writer == null) writer = createWriter();

     return writer;

   }

   /**

    * @param writerP the MelatiWriter to set

    */

   public void setWriter(MelatiWriter writerP) {

     writer = writerP;

   }

   /**

    * Get a StringWriter.

    *

    * @return - one of:

    *

    * - a MelatiStringWriter from the template engine

    * - a new MelatiStringWriter if template engine not set

    *

    */

   public MelatiWriter getStringWriter() {

     if (templateEngine == null) {

       return new MelatiStringWriter();

     }

     return templateEngine.getStringWriter();

   }

   /**

    * Used in a servlet setting, where the class was not constructed with 

    * output set.

    * @return a response writer 

    */

   private MelatiWriter createWriter() {

     // first effort is to use the writer supplied by the template engine

     MelatiWriter writerL = null;

     if (response != null) {

       if (templateEngine != null &&

               templateEngine instanceof ServletTemplateEngine) {

         writerL = ((ServletTemplateEngine)templateEngine).getServletWriter(response, buffered);

       } else {

         PrintWriter printWriter = null;

         try { 

           printWriter = response.getWriter(); 

         } catch (IOException e) { 

           throw new MelatiIOException(e);

         }

         if (buffered) {

           writerL = new MelatiBufferedWriter(printWriter);

         } else {

           writerL = new MelatiSimpleWriter(printWriter);

         }

       }

       if (flushing) writerL.setFlushingOn();

     } else 

       throw new MelatiBugMelatiException("Method createWriter called when response was null.");

     return writerL;

   }

   /**

    * Write the buffered output to the Writer

    * we also need to stop the flusher if it has started.

    */

   public void write() {

     // only write stuff if we have previously got a writer

     if (writer != null)

       try {

         writer.close();

       } catch (IOException e) {

         System.err.println("Melati output already closed");

       }

   }

   /**

    * This allows an Exception to be handled inline during Template expansion

    * for example, if you would like to render AccessPoemExceptions to a

    * String to be displayed on the page that is returned to the client.

    * 

    * @see org.melati.template.MarkupLanguage#rendered(Object)

    * @see org.melati.poem.TailoredQuery

    */

   public void setPassbackExceptionHandling() { 

     templateContext.setPassbackExceptionHandling();

   }

   /**

    * The normal state of affairs: an exception is thrown and 

    * it is handled by the servlet.

    */

   public void setPropagateExceptionHandling() { 

     templateContext.setPropagateExceptionHandling();

   }

   /**

    * Get a User for this request (if they are logged in).

    * NOTE POEM studiously assumes there isn't necessarily a user, only

    * an AccessToken

    * @return - a User for this request

    */

   public User getUser() {

     try {

       return (User)PoemThread.accessToken();

     }

     catch (NotInSessionPoemException e) {

       return null;

     }

     catch (ClassCastException e) {

       // If the AccessToken is the RootAccessToken

       return null;

     }

   }

   /**

    * Establish if field is a ReferencePoemType field.

    * 

    * @param field

    *          the field to check

    * @return whether it is a reference poem type

    */

   public boolean isReferencePoemType(Field<?> field) {

     return field.getType() instanceof ReferencePoemType;

   }

   /**

    * Find a db specific template if it exists, otherwise a non-specific one, 

    * searching through all template paths.

    * 

    * @param key fileName of template, without extension

    * @return full resource name

    */

   public String templateName(String key) {

     String templateName = null;

     try {

       TemplateEngine te = getTemplateEngine(); 

       if (te == null)

         throw new MelatiBugMelatiException("Template engine null");

       Database db = getDatabase();

       templateName = te.getTemplateName(key, db == null ? null : db.getName());

     } catch (Exception e) {

       throw new MelatiBugMelatiException("Problem getting template named " + key  +

               " :" + e.toString(), e);

     }

     return templateName;

   }

 }