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//  Copyright (c) 1995-2014 Mort Bay Consulting Pty. Ltd.
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//  and Apache License v2.0 which accompanies this distribution.
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//      The Eclipse Public License is available at
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//      The Apache License v2.0 is available at
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//  You may elect to redistribute this code under either of these licenses.
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package org.eclipse.jetty.continuation;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.Servlet;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.ServletResponseWrapper;

/* ------------------------------------------------------------ */
/**
 * Continuation.
 * 
 * A continuation is a mechanism by which a HTTP Request can be suspended and
 * restarted after a timeout or an asynchronous event has occurred.
 * <p>
 * The continuation mechanism is a portable mechanism that will work 
 * asynchronously without additional configuration of all jetty-7, 
 * jetty-8 and Servlet 3.0 containers.   With the addition of 
 * the {@link ContinuationFilter}, the mechanism will also work
 * asynchronously on jetty-6 and non-asynchronously on any 
 * servlet 2.5 container.
 * <p>
 * The Continuation API is a simplification of the richer async API
 * provided by the servlet-3.0 and an enhancement of the continuation
 * API that was introduced with jetty-6. 
 * </p>
 * <h1>Continuation Usage</h1>
 * <p>
 * A continuation object is obtained for a request by calling the 
 * factory method {@link ContinuationSupport#getContinuation(ServletRequest)}.
 * The continuation type returned will depend on the servlet container
 * being used.
 * </p> 
 * <p>
 * There are two distinct style of operation of the continuation API.
 * </p>
 * <h3>Suspend/Resume Usage</h3> 
 * <p>The suspend/resume style is used when a servlet and/or
 * filter is used to generate the response after a asynchronous wait that is
 * terminated by an asynchronous handler.
 * </p>
 * <pre>
 * <b>Filter/Servlet:</b>
 *   // if we need to get asynchronous results
 *   Object results = request.getAttribute("results);
 *   if (results==null)
 *   {
 *     Continuation continuation = ContinuationSupport.getContinuation(request);
 *     continuation.suspend();
 *     myAsyncHandler.register(continuation);
 *     return; // or continuation.undispatch();
 *   }
 * 
 * async wait ...
 * 
 * <b>Async Handler:</b>
 *   // when the waited for event happens
 *   continuation.setAttribute("results",event);
 *   continuation.resume();
 *   
 * <b>Filter/Servlet:</b>
 *   // when the request is redispatched 
 *   if (results==null)
 *   {
 *     ... // see above
 *   }
 *   else
 *   {
 *     response.getOutputStream().write(process(results));
 *   }
 * </pre> 
 * <h3>Suspend/Complete Usage</h3> 
 * <p>
 * The suspend/complete style is used when an asynchronous handler is used to 
 * generate the response:
 * </p>
 * <pre>
 * <b>Filter/Servlet:</b>
 *   // when we want to enter asynchronous mode
 *   Continuation continuation = ContinuationSupport.getContinuation(request);
 *   continuation.suspend(response); // response may be wrapped
 *   myAsyncHandler.register(continuation);
 *   return; // or continuation.undispatch();
 *
 * <b>Wrapping Filter:</b>
 *   // any filter that had wrapped the response should be implemented like:
 *   try
 *   {
 *     chain.doFilter(request,wrappedResponse);
 *   }
 *   finally
 *   {
 *     if (!continuation.isResponseWrapped())
 *       wrappedResponse.finish()
 *     else
 *       continuation.addContinuationListener(myCompleteListener)
 *   }
 *
 * async wait ...
 *
 * <b>Async Handler:</b>
 *   // when the async event happens
 *   continuation.getServletResponse().getOutputStream().write(process(event));
 *   continuation.complete()
 * </pre>
 * 
 * <h1>Continuation Timeout</h1>
 * <p>
 * If a continuation is suspended, but neither {@link #complete()} or {@link #resume()} is
 * called during the period set by {@link #setTimeout(long)}, then the continuation will
 * expire and {@link #isExpired()} will return true. 
 * </p>
 * <p>
 * When a continuation expires, the {@link ContinuationListener#onTimeout(Continuation)}
 * method is called on any {@link ContinuationListener} that has been registered via the
 * {@link #addContinuationListener(ContinuationListener)} method. The onTimeout handlers 
 * may write a response and call {@link #complete()}. If {@link #complete()} is not called, 
 * then the container will redispatch the request as if {@link #resume()} had been called,
 * except that {@link #isExpired()} will be true and {@link #isResumed()} will be false.
 * </p>
 * 
 * @see ContinuationSupport
 * @see ContinuationListener
 * 
 */
public interface Continuation
{
	public final static String ATTRIBUTE = "org.eclipse.jetty.continuation";

	/* ------------------------------------------------------------ */
	/**
	 * Set the continuation timeout.
	 * 
	 * @param timeoutMs
	 *            The time in milliseconds to wait before expiring this
	 *            continuation after a call to {@link #suspend()} or {@link #suspend(ServletResponse)}.
	 *            A timeout of <=0 means the continuation will never expire.
	 */
	void setTimeout(long timeoutMs);

	/* ------------------------------------------------------------ */
	/**
	 * Suspend the processing of the request and associated
	 * {@link ServletResponse}.
	 * 
	 * <p>
	 * After this method has been called, the lifecycle of the request will be
	 * extended beyond the return to the container from the
	 * {@link Servlet#service(ServletRequest, ServletResponse)} method and
	 * {@link Filter#doFilter(ServletRequest, ServletResponse, FilterChain)}
	 * calls. When a suspended request is returned to the container after
	 * a dispatch, then the container will not commit the associated response
	 * (unless an exception other than {@link ContinuationThrowable} is thrown).
	 * </p>
	 * 
	 * <p>
	 * When the thread calling the filter chain and/or servlet has returned to
	 * the container with a suspended request, the thread is freed for other
	 * tasks and the request is held until either:
	 * <ul>
	 * <li>a call to {@link #resume()}.</li>
	 * <li>a call to {@link #complete()}.</li>
	 * <li>the timeout expires.</li>
	 * </ul>
	 * <p>
	 * Typically suspend with no arguments is uses when a call to {@link #resume()}
	 * is expected. If a call to {@link #complete()} is expected, then the 
	 * {@link #suspend(ServletResponse)} method should be used instead of this method.
	 * </p>
	 * 
	 * @exception IllegalStateException
	 *                If the request cannot be suspended
	 */
//	void suspend();
	
	
	/* ------------------------------------------------------------ */
	/**
	 * Suspend the processing of the request and associated
	 * {@link ServletResponse}.
	 * 
	 * <p>
	 * After this method has been called, the lifecycle of the request will be
	 * extended beyond the return to the container from the
	 * {@link Servlet#service(ServletRequest, ServletResponse)} method and
	 * {@link Filter#doFilter(ServletRequest, ServletResponse, FilterChain)}
	 * calls. When a suspended request is returned to the container after
	 * a dispatch, then the container will not commit the associated response
	 * (unless an exception other than {@link ContinuationThrowable} is thrown).
	 * </p>
	 * <p>
	 * When the thread calling the filter chain and/or servlet has returned to
	 * the container with a suspended request, the thread is freed for other
	 * tasks and the request is held until either:
	 * <ul>
	 * <li>a call to {@link #resume()}.</li>
	 * <li>a call to {@link #complete()}.</li>
	 * <li>the timeout expires.</li>
	 * </ul>
	 * <p>
	 * Typically suspend with a response argument is uses when a call to {@link #complete()}
	 * is expected. If a call to {@link #resume()} is expected, then the 
	 * {@link #suspend()} method should be used instead of this method.
	 * </p>
	 * <p>
	 * Filters that may wrap the response object should check {@link #isResponseWrapped()}
	 * to decide if they should destroy/finish the wrapper. If {@link #isResponseWrapped()}
	 * returns true, then the wrapped request has been passed to the asynchronous
	 * handler and the wrapper should not be destroyed/finished until after a call to 
	 * {@link #complete()} (potentially using a {@link ContinuationListener#onComplete(Continuation)}
	 * listener).
	 * 
	 * @param response The response to return via a call to {@link #getServletResponse()}
	 * @exception IllegalStateException
	 *                If the request cannot be suspended
	 */
//	void suspend(ServletResponse response);

	/* ------------------------------------------------------------ */
	/**
	 * Resume a suspended request.
	 * 
	 * <p>
	 * This method can be called by any thread that has been passed a reference
	 * to a continuation. When called the request is redispatched to the
	 * normal filter chain and servlet processing with {@link #isInitial()} false.
	 * </p>
	 * <p>
	 * If resume is called before a suspended request is returned to the
	 * container (ie the thread that called {@link #suspend()} is still
	 * within the filter chain and/or servlet service method), then the resume
	 * does not take effect until the call to the filter chain and/or servlet
	 * returns to the container. In this case both {@link #isSuspended()} and
	 * {@link #isResumed()} return true. Multiple calls to resume are ignored.
	 * </p>
	 * <p>
	 * Typically resume() is used after a call to {@link #suspend()} with
	 * no arguments. The dispatch after a resume call will use the original
	 * request and response objects, even if {@link #suspend(ServletResponse)} 
	 * had been passed a wrapped response.
	 * </p>
	 * 
	 * @see #suspend()
	 * @exception IllegalStateException if the request is not suspended.
	 * 
	 */
	void resume();

	/* ------------------------------------------------------------ */
	/**
	 * Complete a suspended request.
	 * 
	 * <p>
	 * This method can be called by any thread that has been passed a reference
	 * to a suspended request. When a request is completed, the associated
	 * response object committed and flushed. The request is not redispatched.
	 * </p>
	 * 
	 * <p>
	 * If complete is called before a suspended request is returned to the
	 * container (ie the thread that called {@link #suspend()} is still
	 * within the filter chain and/or servlet service method), then the complete
	 * does not take effect until the call to the filter chain and/or servlet
	 * returns to the container. In this case both {@link #isSuspended()} and
	 * {@link #isResumed()} return true.
	 * </p>
	 * 
	 * <p>
	 * Typically resume() is used after a call to {@link #suspend(ServletResponse)} with
	 * a possibly wrapped response. The async handler should use the response
	 * provided by {@link #getServletResponse()} to write the response before
	 * calling {@link #complete()}. If the request was suspended with a 
	 * call to {@link #suspend()} then no response object will be available via
	 * {@link #getServletResponse()}.
	 * </p>
	 * 
	 * <p>
	 * Once complete has been called and any thread calling the filter chain
	 * and/or servlet chain has returned to the container, the request lifecycle
	 * is complete. The container is able to recycle request objects, so it is
	 * not valid hold a request or continuation reference after the end of the 
	 * life cycle.
	 * 
	 * @see #suspend()
	 * @exception IllegalStateException
	 *                if the request is not suspended.
	 * 
	 */
	void complete();

	/* ------------------------------------------------------------ */
	/**
	 * @return true after {@link #suspend()} has been called and before the
	 *         request has been redispatched due to being resumed, completed or
	 *         timed out.
	 */
	boolean isSuspended();

	/* ------------------------------------------------------------ */
	/**
	 * @return true if the request has been redispatched by a call to
	 *         {@link #resume()}. Returns false after any subsequent call to
	 *         suspend
	 */
//	boolean isResumed();

	/* ------------------------------------------------------------ */
	/**
	 * @return true after a request has been redispatched as the result of a
	 *         timeout. Returns false after any subsequent call to suspend.
	 */
//	boolean isExpired();

	/* ------------------------------------------------------------ */
	/**
	 * @return true while the request is within the initial dispatch to the
	 *         filter chain and/or servlet. Will return false once the calling
	 *         thread has returned to the container after suspend has been
	 *         called and during any subsequent redispatch.
	 */
	boolean isInitial();

	/* ------------------------------------------------------------ */
	/**
	 * Is the suspended response wrapped.
	 * <p>
	 * Filters that wrap the response object should check this method to 
	 * determine if they should destroy/finish the wrapped response. If 
	 * the request was suspended with a call to {@link #suspend(ServletResponse)}
	 * that passed the wrapped response, then the filter should register
	 * a {@link ContinuationListener} to destroy/finish the wrapped response
	 * during a call to {@link ContinuationListener#onComplete(Continuation)}.
	 * @return True if {@link #suspend(ServletResponse)} has been passed a
	 * {@link ServletResponseWrapper} instance.
	 */
	boolean isResponseWrapped();


	/* ------------------------------------------------------------ */
	/**
	 * Get the suspended response.
	 * @return the {@link ServletResponse} passed to {@link #suspend(ServletResponse)}.
	 */
	ServletResponse getServletResponse();
	
	/* ------------------------------------------------------------ */
	/** 
	 * Add a ContinuationListener.
	 * 
	 * @param listener
	 */
	void addContinuationListener(ContinuationListener listener);
	
	/* ------------------------------------------------------------ */
	/** Set a request attribute.
	 * This method is a convenience method to call the {@link ServletRequest#setAttribute(String, Object)}
	 * method on the associated request object.
	 * This is a thread safe call and may be called by any thread.
	 * @param name the attribute name
	 * @param attribute the attribute value
	 */
	public void setAttribute(String name, Object attribute);
	
	/* ------------------------------------------------------------ */
	/** Get a request attribute.
	 * This method is a convenience method to call the {@link ServletRequest#getAttribute(String)}
	 * method on the associated request object.
	 * This is a thread safe call and may be called by any thread.
	 * @param name the attribute name
	 * @return the attribute value
	 */
	public Object getAttribute(String name);
	
	/* ------------------------------------------------------------ */
	/** Remove a request attribute.
	 * This method is a convenience method to call the {@link ServletRequest#removeAttribute(String)}
	 * method on the associated request object.
	 * This is a thread safe call and may be called by any thread.
	 * @param name the attribute name
	 */
	public void removeAttribute(String name);
	
	/* ------------------------------------------------------------ */
	/**
	 * Undispatch the request.
	 * <p>
	 * This method can be called on a suspended continuation in order
	 * to exit the dispatch to the filter/servlet by throwing a {@link ContinuationThrowable}
	 * which is caught either by the container or the {@link ContinuationFilter}.
	 * This is an alternative to simply returning from the dispatch in the case
	 * where filters in the filter chain may not be prepared to handle a suspended
	 * request.
	 * </p>
	 * This method should only be used as a last resort and a normal return is a prefereable
	 * solution if filters can be updated to handle that case.
	 * 
	 * @throws ContinuationThrowable thrown if the request is suspended. The instance of the 
	 * exception may be reused on subsequent calls, so the stack frame may not be accurate.
	 */
	public void undispatch() throws ContinuationThrowable;
}