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Support for Robots Exclusion Protocol, including parsing and matching against robot.txt directives.
There is a newer version: 1.2.1
package com.brandwatch.robots;

/*
 * #%L
 * Robots (core)
 * %%
 * Copyright (C) 2014 - 2015 Brandwatch
 * %%
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 * 
 * 1. Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer.
 * 
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 
 * 3. Neither the name of the Brandwatch nor the names of its contributors
 *    may be used to endorse or promote products derived from this software without
 *    specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
 * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 * #L%
 */

import com.brandwatch.robots.domain.Group;
import com.brandwatch.robots.domain.PathDirective;
import com.brandwatch.robots.domain.Robots;
import com.brandwatch.robots.matching.MatcherUtils;
import com.google.common.base.Optional;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.annotation.Nonnull;
import java.io.IOException;
import java.net.URI;

import static com.google.common.base.Preconditions.checkNotNull;

class RobotsServiceImpl implements RobotsService {

    private static final Logger log = LoggerFactory.getLogger(RobotsServiceImpl.class);

    private final RobotsLoader loader;
    private final RobotsUtilities utilities;
    private final MatcherUtils matcherUtils;

    public RobotsServiceImpl(@Nonnull RobotsLoader loader, @Nonnull RobotsUtilities utilities, MatcherUtils matcherUtils) {
        this.matcherUtils = matcherUtils;
        this.loader = checkNotNull(loader, "loader");
        this.utilities = checkNotNull(utilities, "utilities");
    }

    @Override
    public boolean isAllowed(@Nonnull String crawlerAgentString, @Nonnull URI resourceUri) {
        checkNotNull(crawlerAgentString, "crawlerAgentString is null");
        checkNotNull(resourceUri, "resourceUri is null");

        log.debug("Resolving robots URL for: {}", resourceUri);
        final URI robotsUri = utilities.getRobotsURIForResource(resourceUri);
        log.debug("Resolved robots URI to: {}", robotsUri);

        final Robots robots;
        try {
            robots = loader.load(robotsUri);
        } catch (Exception e) {
            log.debug("Download failure {}", e.getMessage());
            return allow(resourceUri);
        }

        if (robots.getGroups().isEmpty()) {
            log.debug("No agent groups found", resourceUri);
            return allow(resourceUri);
        }

        final Optional group = matcherUtils.getMostSpecificMatchingGroup(robots.getGroups(), crawlerAgentString);

        if (group.isPresent()) {
            if(log.isDebugEnabled()) {
                log.debug("Matched user-agent group: {}",
                        matcherUtils.getMostSpecificMatch(group.get(), crawlerAgentString).get().getValue());
            }
        } else {
            log.debug("No user-agent group matched");
            return allow(resourceUri);
        }

        final Optional bestMatch = matcherUtils.getMostSpecificMatch(
                group.get().getDirectives(PathDirective.class),
                utilities.getResourceLocalComponents(resourceUri));

        if (!bestMatch.isPresent()) {
            log.debug("No matching path directive");
            return allow(resourceUri);
        } else {
            final PathDirective directive = bestMatch.get();
            log.debug("Matched path directive {}:{}",  directive.getField(),  directive.getValue());
            return directive.isAllowed() ? allow(resourceUri) : disallow(resourceUri);
        }
    }

    private boolean allow(URI resourceUri) {
        log.debug("Allowing: {}", resourceUri);
        return true;
    }

    private boolean disallow(URI resourceUri) {
        log.debug("Disallowing: {}", resourceUri);
        return false;
    }

    @Override
    public void close() throws IOException {
        loader.close();
    }
}