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/**
 * Copyright (C) 2010 Mycila ([email protected])
 *
 * 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.inject.servlet;

import com.google.common.base.Preconditions;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Sets;
import com.google.inject.Binding;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.Singleton;
import com.google.inject.TypeLiteral;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;
import java.io.IOException;
import java.util.List;
import java.util.Set;

/**
 * A wrapping dispatcher for servlets, in much the same way as {@link ManagedFilterPipeline} is for
 * filters.
 *
 * @author [email protected] (Dhanji R. Prasanna)
 */
@Singleton
class ManagedServletPipeline {
    private final ServletDefinition[] servletDefinitions;
    private static final TypeLiteral SERVLET_DEFS =
        TypeLiteral.get(ServletDefinition.class);

    @Inject
    public ManagedServletPipeline(Injector injector) {
        this.servletDefinitions = collectServletDefinitions(injector);
    }

    boolean hasServletsMapped() {
        return servletDefinitions.length > 0;
    }

    /**
     * Introspects the injector and collects all instances of bound {@code List}
     * into a master list.
     * 
     * We have a guarantee that {@link com.google.inject.Injector#getBindings()} returns a map
     * that preserves insertion order in entry-set iterators.
     */
    private ServletDefinition[] collectServletDefinitions(Injector injector) {
        List servletDefinitions = Lists.newArrayList();
        for (Binding entry : injector.findBindingsByType(SERVLET_DEFS)) {
            servletDefinitions.add(entry.getProvider().get());
        }

        // Copy to a fixed size array for speed.
        return servletDefinitions.toArray(new ServletDefinition[servletDefinitions.size()]);
    }

    public void init(ServletContext servletContext, Injector injector) throws ServletException {
        Set initializedSoFar
            = Sets.newSetFromMap(Maps.newIdentityHashMap());

        for (ServletDefinition servletDefinition : servletDefinitions) {
            servletDefinition.init(servletContext, injector, initializedSoFar);
        }
    }

    public boolean service(ServletRequest request, ServletResponse response)
        throws IOException, ServletException {

        //stop at the first matching servlet and service
        for (ServletDefinition servletDefinition : servletDefinitions) {
            if (servletDefinition.service(request, response)) {
                return true;
            }
        }

        //there was no match...
        return false;
    }

    public void destroy() {
        Set destroyedSoFar
            = Sets.newSetFromMap(Maps.newIdentityHashMap());
        for (ServletDefinition servletDefinition : servletDefinitions) {
            servletDefinition.destroy(destroyedSoFar);
        }
    }

    /**
     * @return Returns a request dispatcher wrapped with a servlet mapped to
     *         the given path or null if no mapping was found.
     */
    RequestDispatcher getRequestDispatcher(String path) {
        final String newRequestUri = path;

        // TODO(dhanji): check servlet spec to see if the following is legal or not.
        // Need to strip query string if requested...

        for (final ServletDefinition servletDefinition : servletDefinitions) {
            if (servletDefinition.shouldServe(path)) {
                return new RequestDispatcher() {
                    public void forward(ServletRequest servletRequest, ServletResponse servletResponse)
                        throws ServletException, IOException {
                        Preconditions.checkState(!servletResponse.isCommitted(),
                            "Response has been committed--you can only call forward before"
                                + " committing the response (hint: don't flush buffers)");

                        // clear buffer before forwarding
                        servletResponse.resetBuffer();

                        ServletRequest requestToProcess;
                        if (servletRequest instanceof HttpServletRequest) {
                            requestToProcess = new RequestDispatcherRequestWrapper(servletRequest, newRequestUri);
                        } else {
                            // This should never happen, but instead of throwing an exception
                            // we will allow a happy case pass thru for maximum tolerance to
                            // legacy (and internal) code.
                            requestToProcess = servletRequest;
                        }

                        servletRequest.setAttribute(REQUEST_DISPATCHER_REQUEST, Boolean.TRUE);

                        // now dispatch to the servlet
                        try {
                            servletDefinition.doService(requestToProcess, servletResponse);
                        } finally {
                            servletRequest.removeAttribute(REQUEST_DISPATCHER_REQUEST);
                        }
                    }

                    public void include(ServletRequest servletRequest, ServletResponse servletResponse)
                        throws ServletException, IOException {
                        servletRequest.setAttribute(REQUEST_DISPATCHER_REQUEST, Boolean.TRUE);

                        // route to the target servlet
                        try {
                            servletDefinition.doService(servletRequest, servletResponse);
                        } finally {
                            servletRequest.removeAttribute(REQUEST_DISPATCHER_REQUEST);
                        }
                    }
                };
            }
        }

        //otherwise, can't process
        return null;
    }

    /**
     * A Marker constant attribute that when present in the request indicates to Guice servlet that
     * this request has been generated by a request dispatcher rather than the servlet pipeline.
     * In accordance with section 8.4.2 of the Servlet 2.4 specification.
     */
    public static final String REQUEST_DISPATCHER_REQUEST = "javax.servlet.forward.servlet_path";

    private static class RequestDispatcherRequestWrapper extends HttpServletRequestWrapper {
        private final String newRequestUri;

        public RequestDispatcherRequestWrapper(ServletRequest servletRequest, String newRequestUri) {
            super((HttpServletRequest) servletRequest);
            this.newRequestUri = newRequestUri;
        }

        @Override
        public String getRequestURI() {
            return newRequestUri;
        }
    }
}