Request.java

/*
 * Copyright 2008 Google Inc.
 * 
 * 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 com.google.gwt.http.client;

import com.google.gwt.core.client.impl.Impl;
import com.google.gwt.core.shared.GWT;
import com.google.gwt.xhr.client.XMLHttpRequest;

/**
 * An HTTP request that is waiting for a response. Requests can be queried for
 * their pending status or they can be canceled.
 *
 * <h3>Required Module</h3> Modules that use this class should inherit
 * <code>com.google.gwt.http.HTTP</code>.
 *
 * {@gwt.include
 * com/google/gwt/examples/http/InheritsExample.gwt.xml}
 *
 */
public class Request {

    /**
     * Native implementation associated with {@link Request}. User classes should not use this class
     * directly.
     */
    static class RequestImpl {

        /**
         * Creates a {@link Response} instance for the given JavaScript XmlHttpRequest object.
         *
         * @param xmlHttpRequest xmlHttpRequest object for which we need a response
         * @return a {@link Response} object instance
         */
        Response createResponse(final XMLHttpRequest xmlHttpRequest) {
            return new ResponseImpl(xmlHttpRequest);
        }
    }

    /**
     * Special {@link RequestImpl} for IE6-9 to work around some IE specialities.
     */
    static class RequestImplIE6To9 extends RequestImpl {

        @Override
        Response createResponse(XMLHttpRequest xmlHttpRequest) {
            return new ResponseImpl(xmlHttpRequest) {

                @Override
                public int getStatusCode() {
                  /*
                   * http://code.google.com/p/google-web-toolkit/issues/detail?id=5031
                   *
                   * The XMLHTTPRequest object in IE will return a status code of 1223 and drop some
                   * response headers if the server returns a HTTP/204.
                   *
                   * This issue is fixed in IE10.
                   */
                    int statusCode = super.getStatusCode();
                    return (statusCode == 1223) ? SC_NO_CONTENT : statusCode;
                }
            };
        }
    }

    /*
     * Although Request is a client-side class, it's a transitive dependency of
     * some GWT servlet code.  Because GWT.create() isn't safe to call on the
     * server, we use the "Initialization On Demand Holder" idiom to lazily
     * initialize the RequestImpl.
     */
    private static class ImplHolder {
        private static final RequestImpl impl = GWT.create(RequestImpl.class);

        public static RequestImpl get() {
            return impl;
        }
    }

    /**
     * Creates a {@link Response} instance for the given JavaScript XmlHttpRequest
     * object.
     *
     * @param xmlHttpRequest xmlHttpRequest object for which we need a response
     * @return a {@link Response} object instance
     */
    private static Response createResponse(final XMLHttpRequest xmlHttpRequest) {
        return ImplHolder.get().createResponse(xmlHttpRequest);
    }

    private static native int createTimeout(Request request, RequestCallback callback, int timeoutMillis) /*-{
        return @com.google.gwt.core.client.impl.Impl::setTimeout(Lcom/google/gwt/core/client/JavaScriptObject;I)(
            $entry(function() {
                request.@com.google.gwt.http.client.Request::fireOnTimeout(Lcom/google/gwt/http/client/RequestCallback;)(callback);
            }),
            timeoutMillis);
    }-*/;

    /**
     * The number of milliseconds to wait for this HTTP request to complete.
     */
    private final int timeoutMillis;

    /**
     * ID of the timer used to force HTTPRequest timeouts. Only meaningful if
     * timeoutMillis > 0.
     */
    private final int timerId;

    /**
     * JavaScript XmlHttpRequest object that this Java class wraps. This field is
     * not final because we transfer ownership of it to the HTTPResponse object
     * and set this field to null.
     */
    private XMLHttpRequest xmlHttpRequest;

    /**
     * Only used for building a
     * {@link com.google.gwt.user.client.rpc.impl.FailedRequest}.
     */
    protected Request() {
        timeoutMillis = 0;
        xmlHttpRequest = null;
        timerId = 0;
    }

    /**
     * Constructs an instance of the Request object.
     *
     * @param xmlHttpRequest JavaScript XmlHttpRequest object instance
     * @param timeoutMillis number of milliseconds to wait for a response
     * @param callback callback interface to use for notification
     *
     * @throws IllegalArgumentException if timeoutMillis &lt; 0
     * @throws NullPointerException if xmlHttpRequest, or callback are null
     */
    public Request(XMLHttpRequest xmlHttpRequest, int timeoutMillis,
            final RequestCallback callback) {
        if (xmlHttpRequest == null) {
            throw new NullPointerException();
        }

        if (callback == null) {
            throw new NullPointerException();
        }

        if (timeoutMillis < 0) {
            throw new IllegalArgumentException();
        }

        this.timeoutMillis = timeoutMillis;

        this.xmlHttpRequest = xmlHttpRequest;

        if (timeoutMillis > 0) {
            // create and schedule a cancel command
            timerId = createTimeout(this, callback, timeoutMillis);
        } else {
            // no Timer required
            timerId = 0;
        }
    }

    /**
     * Cancels a pending request. If the request has already been canceled or if
     * it has timed out no action is taken.
     */
    public void cancel() {
    /*
     * There is a strange race condition that occurs on Mozilla when you cancel
     * a request while the response is coming in. It appears that in some cases
     * the onreadystatechange handler is still called after the handler function
     * has been deleted and during the call to XmlHttpRequest.abort(). So we
     * null the xmlHttpRequest here and that will prevent the
     * fireOnResponseReceived method from calling the callback function.
     * 
     * Setting the onreadystatechange handler to null gives us the correct
     * behavior in Mozilla but crashes IE. That is why we have chosen to fixed
     * this in Java by nulling out our reference to the XmlHttpRequest object.
     */
        if (xmlHttpRequest != null) {
            XMLHttpRequest xmlHttp = xmlHttpRequest;
            xmlHttpRequest = null;

            xmlHttp.clearOnReadyStateChange();
            xmlHttp.abort();

            cancelTimer();
        }
    }

    /**
     * Returns true if this request is waiting for a response.
     *
     * @return true if this request is waiting for a response
     */
    public boolean isPending() {
        if (xmlHttpRequest == null) {
            return false;
        }

        int readyState = xmlHttpRequest.getReadyState();

    /*
     * Because we are doing asynchronous requests it is possible that we can
     * call XmlHttpRequest.send and still have the XmlHttpRequest.getReadyState
     * method return the state as XmlHttpRequest.OPEN. That is why we include
     * open although it is nottechnically true since open implies that the
     * request has not been sent.
     */
        switch (readyState) {
            case XMLHttpRequest.OPENED:
            case XMLHttpRequest.HEADERS_RECEIVED:
            case XMLHttpRequest.LOADING:
                return true;
        }

        return false;
    }

    /*
     * Method called when the JavaScript XmlHttpRequest object's readyState
     * reaches 4 (LOADED).
     */
    public void fireOnResponseReceived(RequestCallback callback) {
        if (xmlHttpRequest == null) {
            // the request has timed out at this point
            return;
        }

        cancelTimer();

    /*
     * We cannot use cancel here because it would clear the contents of the
     * JavaScript XmlHttpRequest object so we manually null out our reference to
     * the JavaScriptObject
     */
        final XMLHttpRequest xhr = xmlHttpRequest;
        xmlHttpRequest = null;

        String errorMsg = getBrowserSpecificFailure(xhr);
        if (errorMsg != null) {
            Throwable exception = new RuntimeException(errorMsg);
            callback.onError(this, exception);
        } else {
            Response response = createResponse(xhr);
            callback.onResponseReceived(this, response);
        }
    }

    /*
     * Stops the current HTTPRequest timer if there is one.
     */
    private void cancelTimer() {
        if (timeoutMillis > 0) {
            Impl.clearTimeout(timerId);
        }
    }

    /*
     * Method called when this request times out.
     *
     * NOTE: this method is called from JSNI
     */
    private void fireOnTimeout(RequestCallback callback) {
        if (xmlHttpRequest == null) {
            // the request has been received at this point
            return;
        }

        cancel();

        callback.onError(this, new RequestTimeoutException(this, timeoutMillis));
    }

    /**
     * Tests if the JavaScript <code>XmlHttpRequest.status</code> property is
     * readable. This can return failure in two different known scenarios:
     *
     * <ol>
     * <li>On Mozilla, after a network error, attempting to read the status code
     * results in an exception being thrown. See <a
     * href="https://bugzilla.mozilla.org/show_bug.cgi?id=238559"
     * >https://bugzilla.mozilla.org/show_bug.cgi?id=238559</a>.</li>
     * <li>On Safari, if the HTTP response does not include any response text. See
     * <a
     * href="http://bugs.webkit.org/show_bug.cgi?id=3810">http://bugs.webkit.org
     * /show_bug.cgi?id=3810</a>.</li>
     * </ol>
     *
     * @param xhr the JavaScript <code>XmlHttpRequest</code> object to test
     * @return a String message containing an error message if the
     *         <code>XmlHttpRequest.status</code> code is unreadable or null if
     *         the status code could be successfully read.
     */
    private native String getBrowserSpecificFailure(XMLHttpRequest xhr) /*-{
        try {
            if (xhr.status === undefined) {
                return "XmlHttpRequest.status == undefined, please see Safari bug " +
                    "http://bugs.webkit.org/show_bug.cgi?id=3810 for more details";
            }
            return null;
        } catch (e) {
            return "Unable to read XmlHttpRequest.status; likely causes are a " +
                "networking error or bad cross-domain request. Please see " +
                "https://bugzilla.mozilla.org/show_bug.cgi?id=238559 for more " +
                "details";
        }
    }-*/;
}