Frontier是Heritrix最核心的组成部分之一,也是最复杂的组成部分.它主要功能是为处理链接的线程提供URL,并负责链接处理完成后的一些后续调度操作.并且为了提高效率,它在内部使用了Berkeley DB.本节将对它的内部机理进行详细解剖.
在Heritrix的官方文档上有一个Frontier的例子,虽然很简单,但是它却解释Frontier实现的基本原理.在这里就不讨论,有兴趣的读者可以参考相应文档.但是不得不提它的三个核心方法:
(1)next(int timeout):为处理线程提供一个链接.Heritrix的所有处理线程(ToeThread)都是通过调用该方法获取链接的.
(2)schedule(CandidateURI caURI):调度待处理的链接.
(3)finished(CrawlURI cURI):完成一个已处理的链接.
整体结构如下:
BdbMultipleWorkQueues:
它是对Berkeley DB的简单封装.在内部有一个Berkeley Database,存放所有待处理的链接.
<!---->
package org.archive.crawler.frontier;public class BdbMultipleWorkQueues {
//存放所有待处理的URL的数据库 private Database pendingUrisDB = null; //由key获取一个链接 public CrawlURI get(DatabaseEntry headKey)
throws DatabaseException { DatabaseEntry result = new DatabaseEntry(); // From Linda Lee of sleepycat: // "You want to check the status returned from Cursor.getSearchKeyRange // to make sure that you have OperationStatus.SUCCESS. In that case, // you have found a valid data record, and result.getData() // (called by internally by the binding code, in this case) will be // non-null. The other possible status return is // OperationStatus.NOTFOUND, in which case no data record matched // the criteria. " //由key获取相应的链接 OperationStatus status = getNextNearestItem(headKey, result); CrawlURI retVal = null; if (status != OperationStatus.SUCCESS) { LOGGER.severe("See '1219854 NPE je-2.0 " + "entryToObject
'. OperationStatus " + " was not SUCCESS: " + status + ", headKey " + BdbWorkQueue.getPrefixClassKey(headKey.getData())); return null;
} try { retVal = (CrawlURI)crawlUriBinding.entryToObject(result); } catch (RuntimeExceptionWrapper rw) { LOGGER.log( Level.SEVERE, "expected object missing in queue " + BdbWorkQueue.getPrefixClassKey(headKey.getData()), rw); return null; } retVal.setHolderKey(headKey); return retVal;//返回链接 } //从等处理列表获取一个链接 protected OperationStatus getNextNearestItem(DatabaseEntry headKey, DatabaseEntry result) throws DatabaseException { Cursor cursor = null; OperationStatus status; try { //打开游标 cursor = this.pendingUrisDB.openCursor(null, null); // get cap; headKey at this point should always point to // a queue-beginning cap entry (zero-length value) status = cursor.getSearchKey(headKey, result, null); if(status!=OperationStatus.SUCCESS || result.getData().length > 0) { // cap missing throw new DatabaseException("bdb queue cap missing"); } // get next item (real first item of queue) status = cursor.getNext(headKey,result,null); } finally { if(cursor!=null) { cursor.close(); } } return status; } /** * Put the given CrawlURI in at the appropriate place. * 添加URL到数据库 * @param curi * @throws DatabaseException */ public void put(CrawlURI curi, boolean overwriteIfPresent) throws DatabaseException { DatabaseEntry insertKey = (DatabaseEntry)curi.getHolderKey(); if (insertKey == null) { insertKey = calculateInsertKey(curi); curi.setHolderKey(insertKey); } DatabaseEntry value = new DatabaseEntry(); crawlUriBinding.objectToEntry(curi, value); // Output tally on avg. size if level is FINE or greater. if (LOGGER.isLoggable(Level.FINE)) { tallyAverageEntrySize(curi, value); } OperationStatus status; if(overwriteIfPresent) { //添加 status = pendingUrisDB.put(null, insertKey, value); } else { status = pendingUrisDB.putNoOverwrite(null, insertKey, value); } if(status!=OperationStatus.SUCCESS) { LOGGER.severe("failed; "+status+ " "+curi); } } 
}
BdbWorkQueue:
代表一个链接队列,该队列中所有的链接都具有相同的键值.它实际上是通过调用BdbMultipleWorkQueues的get方法从等处理链接数据库中取得一个链接的.
<!---->package org.archive.crawler.frontier;
public class BdbWorkQueue extends WorkQueue
implements Comparable, Serializabl
{
//获取一个URL
protected CrawlURI peekItem(final WorkQueueFrontier frontier)
throws IOException {
/**
* 关键:从BdbFrontier中返回pendingUris
*/
final BdbMultipleWorkQueues queues = ((BdbFrontier) frontier)
.getWorkQueues();
DatabaseEntry key = new DatabaseEntry(origin);
CrawlURI curi = null;
int tries = 1;
while(true) {
try {
//获取链接
curi = queues.get(key);
} catch (DatabaseException e) {
LOGGER.log(Level.SEVERE,"peekItem failure; retrying",e);
}
return curi;
}
}
WorkQueueFrontier:
实现了最核心的三个方法.
<!---->public CrawlURI next()
throws InterruptedException, EndedException {
while (true) { // loop left only by explicit return or exception
long now = System.currentTimeMillis();
// Do common checks for pause, terminate, bandwidth-hold
preNext(now);
synchronized(readyClassQueues) {
int activationsNeeded = targetSizeForReadyQueues() - readyClassQueues.size();
while(activationsNeeded > 0 && !inactiveQueues.isEmpty()) {
activateInactiveQueue();
activationsNeeded--;
}
}
WorkQueue readyQ = null;
Object key = readyClassQueues.poll(DEFAULT_WAIT,TimeUnit.MILLISECONDS);
if (key != null) {
readyQ = (WorkQueue)this.allQueues.get(key);
}
if (readyQ != null) {
while(true) { // loop left by explicit return or break on empty
CrawlURI curi = null;
synchronized(readyQ) {
/**取出一个URL,最终从子类BdbFrontier的
* pendingUris中取出一个链接
*/
curi = readyQ.peek(this);
if (curi != null) {
// check if curi belongs in different queue
String currentQueueKey = getClassKey(curi);
if (currentQueueKey.equals(curi.getClassKey())) {
// curi was in right queue, emit
noteAboutToEmit(curi, readyQ);
//加入正在处理队列中
inProcessQueues.add(readyQ);
return curi; //返回
}
// URI's assigned queue has changed since it
// was queued (eg because its IP has become
// known). Requeue to new queue.
curi.setClassKey(currentQueueKey);
readyQ.dequeue(this);//出队列
decrementQueuedCount(1);
curi.setHolderKey(null);
// curi will be requeued to true queue after lock
// on readyQ is released, to prevent deadlock
} else {
// readyQ is empty and ready: it's exhausted
// release held status, allowing any subsequent
// enqueues to again put queue in ready
readyQ.clearHeld();
break;
}
}
if(curi!=null) {
// complete the requeuing begun earlier
sendToQueue(curi);
}
}
} else {
// ReadyQ key wasn't in all queues: unexpected
if (key != null) {
logger.severe("Key "+ key +
" in readyClassQueues but not allQueues");
}
}
if(shouldTerminate) {
// skip subsequent steps if already on last legs
throw new EndedException("shouldTerminate is true");
}
if(inProcessQueues.size()==0) {
// Nothing was ready or in progress or imminent to wake; ensure
// any piled-up pending-scheduled URIs are considered
this.alreadyIncluded.requestFlush();
}
}
}
//将URL加入待处理队列
public void schedule(CandidateURI caUri) {
// Canonicalization may set forceFetch flag. See
// #canonicalization(CandidateURI) javadoc for circumstance.
String canon = canonicalize(caUri);
if (caUri.forceFetch()) {
alreadyIncluded.addForce(canon, caUri);
} else {
alreadyIncluded.add(canon, caUri);
}
}
BdbFrontier:
继承了WorkQueueFrontier,是Heritrix唯一个具有实际意义的链接工厂.
<!---->package org.archive.crawler.frontier;
public class BdbFrontier extends WorkQueueFrontier implements Serializable
{
/** 所有待抓取的链接*/
protected transient BdbMultipleWorkQueues pendingUris;
//初始化pendingUris,父类为抽象方法
protected void initQueue() throws IOException {
try {
this.pendingUris = createMultipleWorkQueues();
} catch(DatabaseException e) {
throw (IOException)new IOException(e.getMessage()).initCause(e);
}
}
private BdbMultipleWorkQueues createMultipleWorkQueues()
throws DatabaseException {
return new BdbMultipleWorkQueues(this.controller.getBdbEnvironment(),
this.controller.getBdbEnvironment().getClassCatalog(),
this.controller.isCheckpointRecover());
}
protected BdbMultipleWorkQueues getWorkQueues() {
return pendingUris;
}
}
BdbUriUniqFilter:
实际上是一个过滤器,它用来检查一个要进入等待队列的链接是否已经被抓取过.
<!---->//添加URL
protected boolean setAdd(CharSequence uri) {
DatabaseEntry key = new DatabaseEntry();
LongBinding.longToEntry(createKey(uri), key);
long started = 0;
OperationStatus status = null;
try {
if (logger.isLoggable(Level.INFO)) {
started = System.currentTimeMillis();
}
//添加到数据库
status = alreadySeen.putNoOverwrite(null, key, ZERO_LENGTH_ENTRY);
if (logger.isLoggable(Level.INFO)) {
aggregatedLookupTime +=
(System.currentTimeMillis() - started);
}
} catch (DatabaseException e) {
logger.severe(e.getMessage());
}
if (status == OperationStatus.SUCCESS) {
count++;
if (logger.isLoggable(Level.INFO)) {
final int logAt = 10000;
if (count > 0 && ((count % logAt) == 0)) {
logger.info("Average lookup " +
(aggregatedLookupTime / logAt) + "ms.");
aggregatedLookupTime = 0;
}
}
}
//如果存在,返回false
if(status == OperationStatus.KEYEXIST) {
return false; // not added
} else {
return true;
}
}