先看看文档对于Scheduler的作用介绍
https://code4craft.gitbooks.io/webmagic-in-action/content/zh/posts/ch1-overview/architecture.html
之前我们也介绍过了，Scheduler主要负责爬虫的下一步爬取的规划，包括一些去重等功能。在主流程中也看到了Scheduler，现在来具体结合源码分析

源码

Scheduler是一个接口

public interface Scheduler {

/**
* add a url to fetch
*
* @param request
* @param task
*/
public void push(Request request, Task task);

/**
* get an url to crawl
*
* @param task the task of spider
* @return the url to crawl
*/
public Request poll(Task task);

}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
其主要的实现是DuplicateRemovedScheduler，使用模板模式定义了push的步骤。

public abstract class DuplicateRemovedScheduler implements Scheduler {

protected Logger logger = LoggerFactory.getLogger(getClass());

private DuplicateRemover duplicatedRemover = new HashSetDuplicateRemover();

public DuplicateRemover getDuplicateRemover() {
return duplicatedRemover;
}

public DuplicateRemovedScheduler setDuplicateRemover(DuplicateRemover duplicatedRemover) {
this.duplicatedRemover = duplicatedRemover;
return this;
}

@Override
public void push(Request request, Task task) {
logger.trace("get a candidate url {}", request.getUrl());
if (!duplicatedRemover.isDuplicate(request, task) || shouldReserved(request)) {
logger.debug("push to queue {}", request.getUrl());
pushWhenNoDuplicate(request, task);
}
}

protected boolean shouldReserved(Request request) {
return request.getExtra(Request.CYCLE_TRIED_TIMES) != null;
}

protected void pushWhenNoDuplicate(Request request, Task task) {

}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
我们来看看负责去重的接口DuplicateRemover，其实现类有HashSetDuplicateRemover使用HashSet来去重，RedisScheduler接触Redis来去重和BloomFilterDuplicateRemover使用BloomFilter去重。默认使用HashSetDuplicateRemover

public class HashSetDuplicateRemover implements DuplicateRemover {

private Set<String> urls = Sets.newSetFromMap(new ConcurrentHashMap<String, Boolean>());

@Override
public boolean isDuplicate(Request request, Task task) {
return !urls.add(getUrl(request));
}

protected String getUrl(Request request) {
return request.getUrl();
}

@Override
public void resetDuplicateCheck(Task task) {
urls.clear();
}

@Override
public int getTotalRequestsCount(Task task) {
return urls.size();
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
DuplicateRemovedScheduler抽象类有四个具体实现类QueueScheduler，PriorityScheduler，FileCacheQueueScheduler和RedisScheduler。默认使用QueueScheduler

@ThreadSafe
public class QueueScheduler extends DuplicateRemovedScheduler implements MonitorableScheduler {

private BlockingQueue<Request> queue = new LinkedBlockingQueue<Request>();

@Override
public void pushWhenNoDuplicate(Request request, Task task) {
queue.add(request);
}

@Override
public synchronized Request poll(Task task) {
return queue.poll();
}

@Override
public int getLeftRequestsCount(Task task) {
return queue.size();
}

@Override
public int getTotalRequestsCount(Task task) {
return getDuplicateRemover().getTotalRequestsCount(task);
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
其内部是使用了一个LinkedBlockingQueue这个无界队列来存储Request，我们应该看到了@ThreadSafe注解，那我抛一个问题吧。Scheduler是否存在线程同步问题呢，如果存在那是如何解决的呢？
再来看下一个

@ThreadSafe
public class PriorityScheduler extends DuplicateRemovedScheduler implements MonitorableScheduler {

public static final int INITIAL_CAPACITY = 5;

private BlockingQueue<Request> noPriorityQueue = new LinkedBlockingQueue<Request>();

private PriorityBlockingQueue<Request> priorityQueuePlus = new PriorityBlockingQueue<Request>(INITIAL_CAPACITY, new Comparator<Request>() {
@Override
public int compare(Request o1, Request o2) {
return -NumberUtils.compareLong(o1.getPriority(), o2.getPriority());
}
});

private PriorityBlockingQueue<Request> priorityQueueMinus = new PriorityBlockingQueue<Request>(INITIAL_CAPACITY, new Comparator<Request>() {
@Override
public int compare(Request o1, Request o2) {
return -NumberUtils.compareLong(o1.getPriority(), o2.getPriority());
}
});

@Override
public void pushWhenNoDuplicate(Request request, Task task) {
if (request.getPriority() == 0) {
noPriorityQueue.add(request);
} else if (request.getPriority() > 0) {
priorityQueuePlus.put(request);
} else {
priorityQueueMinus.put(request);
}
}

@Override
public synchronized Request poll(Task task) {
Request poll = priorityQueuePlus.poll();
if (poll != null) {
return poll;
}
poll = noPriorityQueue.poll();
if (poll != null) {
return poll;
}
return priorityQueueMinus.poll();
}

@Override
public int getLeftRequestsCount(Task task) {
return noPriorityQueue.size();
}

@Override
public int getTotalRequestsCount(Task task) {
return getDuplicateRemover().getTotalRequestsCount(task);
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
我们看到了两个PriorityBlockingQueue和一个LinkedBlockingQueue。在poll的时候存在一个顺序。
继续

public class FileCacheQueueScheduler extends DuplicateRemovedScheduler implements MonitorableScheduler {

private String filePath = System.getProperty("java.io.tmpdir");

private String fileUrlAllName = ".urls.txt";

private Task task;

private String fileCursor = ".cursor.txt";

private PrintWriter fileUrlWriter;

private PrintWriter fileCursorWriter;

private AtomicInteger cursor = new AtomicInteger();

private AtomicBoolean inited = new AtomicBoolean(false);

private BlockingQueue<Request> queue;

private Set<String> urls;

public FileCacheQueueScheduler(String filePath) {
if (!filePath.endsWith("/") && !filePath.endsWith("\\")) {
filePath += "/";
}
this.filePath = filePath;
}

private void flush() {
fileUrlWriter.flush();
fileCursorWriter.flush();
}

private void init(Task task) {
this.task = task;
File file = new File(filePath);
if (!file.exists()) {
file.mkdirs();
}
readFile();
initWriter();
initFlushThread();
inited.set(true);
logger.info("init cache scheduler success");
}

private void initFlushThread() {
Executors.newScheduledThreadPool(1).scheduleAtFixedRate(new Runnable() {
@Override
public void run() {
flush();
}
}, 10, 10, TimeUnit.SECONDS);
}

private void initWriter() {
try {
fileUrlWriter = new PrintWriter(new FileWriter(getFileName(fileUrlAllName), true));
fileCursorWriter = new PrintWriter(new FileWriter(getFileName(fileCursor), false));
} catch (IOException e) {
throw new RuntimeException("init cache scheduler error", e);
}
}

private void readFile() {
try {
queue = new LinkedBlockingQueue<Request>();
urls = new LinkedHashSet<String>();
readCursorFile();
readUrlFile();
} catch (FileNotFoundException e) {
//init
logger.info("init cache file " + getFileName(fileUrlAllName));
} catch (IOException e) {
logger.error("init file error", e);
}
}

private void readUrlFile() throws IOException {
String line;
BufferedReader fileUrlReader = null;
try {
fileUrlReader = new BufferedReader(new FileReader(getFileName(fileUrlAllName)));
int lineReaded = 0;
while ((line = fileUrlReader.readLine()) != null) {
urls.add(line.trim());
lineReaded++;
if (lineReaded > cursor.get()) {
queue.add(new Request(line));
}
}
} finally {
if (fileUrlReader != null) {
IOUtils.closeQuietly(fileUrlReader);
}
}
}

private void readCursorFile() throws IOException {
BufferedReader fileCursorReader = null;
try {
fileCursorReader = new BufferedReader(new FileReader(getFileName(fileCursor)));
String line;
//read the last number
while ((line = fileCursorReader.readLine()) != null) {
cursor = new AtomicInteger(NumberUtils.toInt(line));
}
} finally {
if (fileCursorReader != null) {
IOUtils.closeQuietly(fileCursorReader);
}
}
}

private String getFileName(String filename) {
return filePath + task.getUUID() + filename;
}

@Override
protected void pushWhenNoDuplicate(Request request, Task task) {
if (!inited.get()) {
init(task);
}
queue.add(request);
fileUrlWriter.println(request.getUrl());
}

@Override
public synchronized Request poll(Task task) {
if (!inited.get()) {
init(task);
}
fileCursorWriter.println(cursor.incrementAndGet());
return queue.poll();
}

@Override
public int getLeftRequestsCount(Task task) {
return queue.size();
}

@Override
public int getTotalRequestsCount(Task task) {
return getDuplicateRemover().getTotalRequestsCount(task);
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
会将url和已经执行的url指针存在两个文件中，创建了scheduleExecutor定期的flush，所有内存中的url还是存在BlockingQueue中。
RedisScheduler不是很懂。。目前还没有接触过：）

使用

具体使用过程还是需要自己根据自己的爬虫特点然后选择特定的Scheduler及DuplicateRemover，只有懂得其原理才能选择最合适的组件。
WebMagic组件都可以自行设置这点真的太棒了~

Scheduler