visual basic 6.0 barcode generator 6: Crawling the Web with Java in Java

Make PDF-417 2d barcode in Java 6: Crawling the Web with Java

6: Crawling the Web with Java
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remove any leading or trailing space characters. Similar to comments, extraneous space characters will trip up comparisons, so they are removed. Finally, the disallow path is added to the list of disallow paths. After the disallow path list has been created, it is added to the disallow list cache, as shown here:
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// Add new disallow list to cache. disallowListCache.put(host, disallowList); } catch (Exception e) { /* Assume robot is allowed since an exception is thrown if the robot file doesn't exist. */ return true; } }
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The disallow path is added to the disallow list cache so that subsequent requests for the list can be quickly retrieved from cache instead of having to be downloaded again. If an error occurs while opening the input stream to the robot file URL or while reading the contents of the file, an exception will be thrown. Since an exception will be thrown if the robots.txt file does not exist, we ll assume robots are allowed if an exception is thrown. Normally, the error checking in this scenario should be more robust; however, for simplicity and brevity s sake, we ll make the blanket decision that robots are allowed. Next, the following code iterates over the disallow list to see if the urlToCheck is allowed or not:
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/* Loop through disallow list to see if the crawling is allowed for the given URL. */ String file = urlToCheck.getFile(); for (int i = 0; i < disallowList.size(); i++) { String disallow = (String) disallowList.get(i); if (file.startsWith(disallow)) { return false; } } return true;
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Each iteration of the for loop checks to see if the file portion of the urlToCheck is found in the disallow list. If the urlToCheck s file does in fact match one of the statements in the disallow list, then false is returned, indicating that crawlers are not allowed to crawl the given URL. However, if the list is iterated over and no match is made, true is returned, indicating that crawling is allowed.
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The downloadPage( ) Method
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The downloadPage( ) method, shown here, simply does as its name implies: it downloads the Web page at the given URL and returns the contents of the page as a large string:
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// Download page at given URL. private String downloadPage(URL pageUrl) { try { // Open connection to URL for reading. BufferedReader reader = new BufferedReader(new InputStreamReader( pageUrl.openStream())); // Read page into buffer. String line; StringBuffer pageBuffer = new StringBuffer(); while ((line = reader.readLine()) != null) { pageBuffer.append(line); } return pageBuffer.toString(); } catch (Exception e) { } return null; }
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Downloading Web pages from the Internet in Java is quite simple, as evidenced by this method. First, a BufferedReader object is created for reading the contents of the page at the given URL. The BufferedReader s constructor is passed an instance of InputStreamReader, whose constructor is passed the InputStream object returned from calling pageUrl.openStream( ). Next, a while loop is used to read the contents of the page, line by line, until the reader.readLine( ) method returns null, signaling that all lines have been read. Each line that is read with the while loop is added to the pageBuffer StringBuffer instance. After the page has been downloaded, its contents are returned as a String by calling pageBuffer.toString( ). If an error occurs when opening the input stream to the page URL or while reading the contents of the Web page, an exception will be thrown. This exception will be caught by the empty catch block. The catch block has purposefully been left blank so that execution will continue to the remaining return null line. A return value of null from this method indicates to callers that an error occurred.
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