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RobotsTxtManager.java
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RobotsTxtManager.java
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/**
* Copyright 2015 Scott Mansfield
*
* 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.widowcrawler.core.siteattr;
import com.widowcrawler.terminator.*;
import com.widowcrawler.terminator.model.RobotsTxt;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.concurrent.*;
/**
* @author Scott Mansfield
*/
public class RobotsTxtManager {
private static final Logger logger = LoggerFactory.getLogger(RobotsTxtManager.class);
private static final ConcurrentMap<String, RobotsTxt> robotstxtByDomain;
private static final ConcurrentMap<String, Future<Boolean>> fetchesInProgress;
private static final Executor pool;
static {
robotstxtByDomain = new ConcurrentHashMap<>();
fetchesInProgress = new ConcurrentHashMap<>();
pool = Executors.newWorkStealingPool();
}
public static RobotsTxt getByDomain(String domain) {
try {
FutureTask<Boolean> fetchFuture;
// Double-check and fetch if needed
if (!fetchesInProgress.containsKey(domain)) {
synchronized (RobotsTxtManager.class) {
if (!fetchesInProgress.containsKey(domain)) {
fetchFuture = new FutureTask<>(() -> {
logger.info("Fetching and parsing robots.txt for domain " + domain);
RobotsTxt robotsTxt = Terminator.parse(makeRobotsTxtURL(domain));
robotstxtByDomain.put(domain, robotsTxt);
return Boolean.TRUE;
});
fetchesInProgress.put(domain, fetchFuture);
pool.execute(fetchFuture);
}
}
} else {
logger.info("Getting robots.txt for domain " + domain + " from cache");
}
// Past here, we should be guaranteed that the robotstxtByDomain map
// has at least something for the given domain, even if it's empty
fetchesInProgress.get(domain).get();
return robotstxtByDomain.get(domain);
} catch (InterruptedException ex) {
logger.warn("Interrupted while trying to get robots.txt for " + domain, ex);
Thread.currentThread().interrupt();
return null; // useless, but necessary to compile
} catch (ExecutionException ex) {
logger.error("ExecutionException while trying to get robots.txt fetch in progress", ex);
return null;
}
}
private static String makeRobotsTxtURL(String domain) {
return String.format("http://%s/robots.txt", domain);
}
}