How to create a Search Crawler in Java

Search Crawler in Java

With Search Crawler shown in this tip, you can enter search criteria and then search the Web in real time, URL by URL, looking for matches to the criteria.

The content of the SearchCrawler.java file is written below:
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import java.util.*;
import java.util.regex.*;
import javax.swing.*;
import javax.swing.table.*;

// The Search Web Crawler
public class SearchCrawler extends JFrame {

// Max URLs drop down values.
private static final String[] MAX_URLS =
{"50", "100", "500", "1000"};

// Cache of robot disallow lists.
private HashMap disallowListCache = new HashMap();

// Search GUI controls.
private JTextField startTextField;
private JComboBox maxComboBox;
private JCheckBox limitCheckBox;
private JTextField logTextField;
private JTextField searchTextField;
private JCheckBox caseCheckBox;
private JButton searchButton;

// Search stats GUI controls.
private JLabel crawlingLabel2;
private JLabel crawledLabel2;
private JLabel toCrawlLabel2;
private JProgressBar progressBar;
private JLabel matchesLabel2;

// Table listing search matches.
private JTable table;

// Flag for whether or not crawling is underway.
private boolean crawling;

// Matches log file print writer.
private PrintWriter logFileWriter;

// Constructor for Search Web Crawler.
public SearchCrawler() {
// Set application title.
setTitle("Search Crawler");

// Set window size.
setSize(600, 600);

// Handle window closing events.
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
actionExit();
}
});

// Set up file menu.
JMenuBar menuBar = new JMenuBar();
JMenu fileMenu = new JMenu("File");
fileMenu.setMnemonic(KeyEvent.VK_F);
JMenuItem fileExitMenuItem = new JMenuItem("Exit",
KeyEvent.VK_X);
fileExitMenuItem.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionExit();
}
});
fileMenu.add(fileExitMenuItem);
menuBar.add(fileMenu);
setJMenuBar(menuBar);

// Set up search panel.
JPanel searchPanel = new JPanel();
GridBagConstraints constraints;
GridBagLayout layout = new GridBagLayout();
searchPanel.setLayout(layout);

JLabel startLabel = new JLabel("Start URL:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(startLabel, constraints);
searchPanel.add(startLabel);

startTextField = new JTextField();
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(startTextField, constraints);
searchPanel.add(startTextField);

JLabel maxLabel = new JLabel("Max URLs to Crawl:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(maxLabel, constraints);
searchPanel.add(maxLabel);

maxComboBox = new JComboBox(MAX_URLS);
maxComboBox.setEditable(true);
constraints = new GridBagConstraints();
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(maxComboBox, constraints);
searchPanel.add(maxComboBox);

limitCheckBox =
new JCheckBox("Limit crawling to Start URL site");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.WEST;
constraints.insets = new Insets(0, 10, 0, 0);
layout.setConstraints(limitCheckBox, constraints);
searchPanel.add(limitCheckBox);

JLabel blankLabel = new JLabel();
constraints = new GridBagConstraints();
constraints.gridwidth = GridBagConstraints.REMAINDER;
layout.setConstraints(blankLabel, constraints);
searchPanel.add(blankLabel);

JLabel logLabel = new JLabel("Matches Log File:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(logLabel, constraints);
searchPanel.add(logLabel);

String file =
System.getProperty("user.dir") +
System.getProperty("file.separator") +
"crawler.log";
logTextField = new JTextField(file);
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(logTextField, constraints);
searchPanel.add(logTextField);

JLabel searchLabel = new JLabel("Search String:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(searchLabel, constraints);
searchPanel.add(searchLabel);

searchTextField = new JTextField();
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.insets = new Insets(5, 5, 0, 0);
constraints.gridwidth= 2;
constraints.weightx = 1.0d;
layout.setConstraints(searchTextField, constraints);
searchPanel.add(searchTextField);

caseCheckBox = new JCheckBox("Case Sensitive");
constraints = new GridBagConstraints();
constraints.insets = new Insets(5, 5, 0, 5);
constraints.gridwidth = GridBagConstraints.REMAINDER;
layout.setConstraints(caseCheckBox, constraints);
searchPanel.add(caseCheckBox);

searchButton = new JButton("Search");
searchButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionSearch();
}
});
constraints = new GridBagConstraints();
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 5, 5);
layout.setConstraints(searchButton, constraints);
searchPanel.add(searchButton);

JSeparator separator = new JSeparator();
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 5, 5);
layout.setConstraints(separator, constraints);
searchPanel.add(separator);

JLabel crawlingLabel1 = new JLabel("Crawling:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(crawlingLabel1, constraints);
searchPanel.add(crawlingLabel1);

crawlingLabel2 = new JLabel();
crawlingLabel2.setFont(
crawlingLabel2.getFont().deriveFont(Font.PLAIN));
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(crawlingLabel2, constraints);
searchPanel.add(crawlingLabel2);

JLabel crawledLabel1 = new JLabel("Crawled URLs:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(crawledLabel1, constraints);
searchPanel.add(crawledLabel1);

crawledLabel2 = new JLabel();
crawledLabel2.setFont(
crawledLabel2.getFont().deriveFont(Font.PLAIN));
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(crawledLabel2, constraints);
searchPanel.add(crawledLabel2);

JLabel toCrawlLabel1 = new JLabel("URLs to Crawl:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(toCrawlLabel1, constraints);
searchPanel.add(toCrawlLabel1);

toCrawlLabel2 = new JLabel();
toCrawlLabel2.setFont(
toCrawlLabel2.getFont().deriveFont(Font.PLAIN));
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(toCrawlLabel2, constraints);
searchPanel.add(toCrawlLabel2);

JLabel progressLabel = new JLabel("Crawling Progress:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 0, 0);
layout.setConstraints(progressLabel, constraints);
searchPanel.add(progressLabel);

progressBar = new JProgressBar();
progressBar.setMinimum(0);
progressBar.setStringPainted(true);
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 0, 5);
layout.setConstraints(progressBar, constraints);
searchPanel.add(progressBar);

JLabel matchesLabel1 = new JLabel("Search Matches:");
constraints = new GridBagConstraints();
constraints.anchor = GridBagConstraints.EAST;
constraints.insets = new Insets(5, 5, 10, 0);
layout.setConstraints(matchesLabel1, constraints);
searchPanel.add(matchesLabel1);

matchesLabel2 = new JLabel();
matchesLabel2.setFont(
matchesLabel2.getFont().deriveFont(Font.PLAIN));
constraints = new GridBagConstraints();
constraints.fill = GridBagConstraints.HORIZONTAL;
constraints.gridwidth = GridBagConstraints.REMAINDER;
constraints.insets = new Insets(5, 5, 10, 5);
layout.setConstraints(matchesLabel2, constraints);
searchPanel.add(matchesLabel2);

// Set up matches table.
table =
new JTable(new DefaultTableModel(new Object[][]{},
new String[]{"URL"}) {
public boolean isCellEditable(int row, int column) {
return false;
}
});

// Set up matches panel.
JPanel matchesPanel = new JPanel();
matchesPanel.setBorder(
BorderFactory.createTitledBorder("Matches"));
matchesPanel.setLayout(new BorderLayout());
matchesPanel.add(new JScrollPane(table),
BorderLayout.CENTER);

// Add panels to display.
getContentPane().setLayout(new BorderLayout());
getContentPane().add(searchPanel, BorderLayout.NORTH);
getContentPane().add(matchesPanel, BorderLayout.CENTER);
}

// Exit this program.
private void actionExit() {
System.exit(0);
}

// Handle search/stop button being clicked.
private void actionSearch() {
// If stop button clicked, turn crawling flag off.
if (crawling) {
crawling = false;
return;
}

ArrayList errorList = new ArrayList();

// Validate that start URL has been entered.
String startUrl = startTextField.getText().trim();
if (startUrl.length() < 1) {
errorList.add("Missing Start URL.");
}
// Verify start URL.
else if (verifyUrl(startUrl) == null) {
errorList.add("Invalid Start URL.");
}

// Validate that max URLs is either empty or is a number.
int maxUrls = 0;
String max = ((String) maxComboBox.getSelectedItem()).trim();
if (max.length() > 0) {
try {
maxUrls = Integer.parseInt(max);
} catch (NumberFormatException e) {
}
if (maxUrls < 1) {
errorList.add("Invalid Max URLs value.");
}
}

// Validate that matches log file has been entered.
String logFile = logTextField.getText().trim();
if (logFile.length() < 1) {
errorList.add("Missing Matches Log File.");
}

// Validate that search string has been entered.
String searchString = searchTextField.getText().trim();
if (searchString.length() < 1) {
errorList.add("Missing Search String.");
}

// Show errors, if any, and return.
if (errorList.size() > 0) {
StringBuffer message = new StringBuffer();

// Concatenate errors into single message.
for (int i = 0; i < errorList.size(); i++) {
message.append(errorList.get(i));
if (i + 1 < errorList.size()) {
message.append("\n");
}
}

showError(message.toString());
return;
}

// Remove "www" from start URL if present.
startUrl = removeWwwFromUrl(startUrl);

// Start the search crawler.
search(logFile, startUrl, maxUrls, searchString);
}

private void search(final String logFile, final String startUrl,
final int maxUrls, final String searchString) {
// Start the search in a new thread.
Thread thread = new Thread(new Runnable() {
public void run() {
// Show hour glass cursor while crawling is under way.
setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));

// Disable search controls.
startTextField.setEnabled(false);
maxComboBox.setEnabled(false);
limitCheckBox.setEnabled(false);
logTextField.setEnabled(false);
searchTextField.setEnabled(false);
caseCheckBox.setEnabled(false);

// Switch search button to "Stop."
searchButton.setText("Stop");

// Reset stats.
table.setModel(new DefaultTableModel(new Object[][]{},
new String[]{"URL"}) {
public boolean isCellEditable(int row, int column) {
return false;
}
});
updateStats(startUrl, 0, 0, maxUrls);

// Open matches log file.
try {
logFileWriter = new PrintWriter(new FileWriter(logFile));
} catch (Exception e) {
showError("Unable to open matches log file.");
return;
}

// Turn crawling flag on.
crawling = true;

// Perform the actual crawling.
crawl(startUrl, maxUrls, limitCheckBox.isSelected(),
searchString, caseCheckBox.isSelected());

// Turn crawling flag off.
crawling = false;

// Close matches log file.
try {
logFileWriter.close();
} catch (Exception e) {
showError("Unable to close matches log file.");
}

// Mark search as done.
crawlingLabel2.setText("Done");

// Enable search controls.
startTextField.setEnabled(true);
maxComboBox.setEnabled(true);
limitCheckBox.setEnabled(true);
logTextField.setEnabled(true);
searchTextField.setEnabled(true);
caseCheckBox.setEnabled(true);

// Switch search button back to "Search."
searchButton.setText("Search");

// Return to default cursor.
setCursor(Cursor.getDefaultCursor());

// Show message if search string not found.
if (table.getRowCount() == 0) {
JOptionPane.showMessageDialog(SearchCrawler.this,
"Your Search String was not found. Please try another.",
"Search String Not Found",
JOptionPane.WARNING_MESSAGE);
}
}
});
thread.start();
}

// Show dialog box with error message.
private void showError(String message) {
JOptionPane.showMessageDialog(this, message, "Error",
JOptionPane.ERROR_MESSAGE);
}

// Update crawling stats.
private void updateStats(
String crawling, int crawled, int toCrawl, int maxUrls) {
crawlingLabel2.setText(crawling);
crawledLabel2.setText("" + crawled);
toCrawlLabel2.setText("" + toCrawl);

// Update progress bar.
if (maxUrls == -1) {
progressBar.setMaximum(crawled + toCrawl);
} else {
progressBar.setMaximum(maxUrls);
}
progressBar.setValue(crawled);

matchesLabel2.setText("" + table.getRowCount());
}

// Add match to matches table and log file.
private void addMatch(String url) {
// Add URL to matches table.
DefaultTableModel model =
(DefaultTableModel) table.getModel();
model.addRow(new Object[]{url});

// Add URL to matches log file.
try {
logFileWriter.println(url);
} catch (Exception e) {
showError("Unable to log match.");
}
}

// Verify URL format.
private URL verifyUrl(String url) {
// Only allow HTTP URLs.
if (!url.toLowerCase().startsWith("http://"))
return null;

// Verify format of URL.
URL verifiedUrl = null;
try {
verifiedUrl = new URL(url);
} catch (Exception e) {
return null;
}

return verifiedUrl;
}

// Check if robot is allowed to access the given URL.
private boolean isRobotAllowed(URL urlToCheck) {
String host = urlToCheck.getHost().toLowerCase();

// Retrieve host's disallow list from cache.
ArrayList disallowList =
(ArrayList) disallowListCache.get(host);

// If list is not in the cache, download and cache it.
if (disallowList == null) {
disallowList = new ArrayList();

try {
URL robotsFileUrl =
new URL("http://" + host + "/robots.txt");

// Open connection to robot file URL for reading.
BufferedReader reader =
new BufferedReader(new InputStreamReader(
robotsFileUrl.openStream()));

// Read robot file, creating list of disallowed paths.
String line;
while ((line = reader.readLine()) != null) {
if (line.indexOf("Disallow:") == 0) {
String disallowPath =
line.substring("Disallow:".length());

// Check disallow path for comments and
// remove if present.
int commentIndex = disallowPath.indexOf("#");
if (commentIndex != - 1) {
disallowPath =
disallowPath.substring(0, commentIndex);
}

// Remove leading or trailing spaces from
// disallow path.
disallowPath = disallowPath.trim();

// Add disallow path to list.
disallowList.add(disallowPath);
}
}

// 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;
}
}

/* 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;
}

// 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;
}

// Remove leading "www" from a URL's host if present.
private String removeWwwFromUrl(String url) {
int index = url.indexOf("://www.");
if (index != -1) {
return url.substring(0, index + 3) +
url.substring(index + 7);
}

return (url);
}

// Parse through page contents and retrieve links.
private ArrayList retrieveLinks(
URL pageUrl, String pageContents, HashSet crawledList,
boolean limitHost) {
// Compile link matching pattern.
Pattern p =
Pattern.compile("]",
Pattern.CASE_INSENSITIVE);
Matcher m = p.matcher(pageContents);

// Create list of link matches.
ArrayList linkList = new ArrayList();
while (m.find()) {
String link = m.group(1).trim();

// Skip empty links.
if (link.length() < 1) {
continue;
}

// Skip links that are just page anchors.
if (link.charAt(0) == '#') {
continue;
}

// Skip mailto links.
if (link.indexOf("mailto:") != -1) {
continue;
}

// Skip JavaScript links.
if (link.toLowerCase().indexOf("javascript") != -1) {
continue;
}

// Prefix absolute and relative URLs if necessary.
if (link.indexOf("://") == -1) {
// Handle absolute URLs.
if (link.charAt(0) == '/') {
link = "http://" + pageUrl.getHost() + link;
// Handle relative URLs.
} else {
String file = pageUrl.getFile();
if (file.indexOf('/') == -1) {
link = "http://" + pageUrl.getHost() + "/" + link;
} else {
String path =
file.substring(0, file.lastIndexOf('/') + 1);
link = "http://" + pageUrl.getHost() + path + link;
}
}
}

// Remove anchors from link.
int index = link.indexOf('#');
if (index != -1) {
link = link.substring(0, index);
}

// Remove leading "www" from URL's host if present.
link = removeWwwFromUrl(link);

// Verify link and skip if invalid.
URL verifiedLink = verifyUrl(link);
if (verifiedLink == null) {
continue;
}

/* If specified, limit links to those
having the same host as the start URL. */
if (limitHost &&
!pageUrl.getHost().toLowerCase().equals(
verifiedLink.getHost().toLowerCase())) {
continue;
}

// Skip link if it has already been crawled.
if (crawledList.contains(link)) {
continue;
}

// Add link to list.
linkList.add(link);
}

return (linkList);
}

/* Determine whether or not search string is
matched in the given page contents. */
private boolean searchStringMatches(
String pageContents, String searchString,
boolean caseSensitive) {
String searchContents = pageContents;

/* If case sensitive search, lowercase
page contents for comparison. */
if (!caseSensitive) {
searchContents = pageContents.toLowerCase();
}

// Split search string into individual terms.
Pattern p = Pattern.compile("[\\s]+");
String[] terms = p.split(searchString);

// Check to see if each term matches.
for (int i = 0; i < terms.length; i++) {
if (caseSensitive) {
if (searchContents.indexOf(terms[i]) == -1) {
return false;
}
} else {
if (searchContents.indexOf(terms[i].toLowerCase()) == -1) {
return false;
}
}
}

return true;
}

// Perform the actual crawling, searching for the search string.
public void crawl(
String startUrl, int maxUrls, boolean limitHost,
String searchString, boolean caseSensitive) {
// Setup crawl lists.
HashSet crawledList = new HashSet();
LinkedHashSet toCrawlList = new LinkedHashSet();

// Add start URL to the to crawl list.
toCrawlList.add(startUrl);

/* Perform actual crawling by looping
through the to crawl list. */
while (crawling && toCrawlList.size() > 0) {
/* Check to see if the max URL count has
been reached, if it was specified.*/
if (maxUrls != -1) {
if (crawledList.size() == maxUrls) {
break;
}
}

// Get URL at bottom of the list.
String url = (String) toCrawlList.iterator().next();

// Remove URL from the to crawl list.
toCrawlList.remove(url);

// Convert string url to URL object.
URL verifiedUrl = verifyUrl(url);

// Skip URL if robots are not allowed to access it.
if (!isRobotAllowed(verifiedUrl)) {
continue;
}

// Update crawling stats.
updateStats(url, crawledList.size(), toCrawlList.size(),
maxUrls);

// Add page to the crawled list.
crawledList.add(url);

// Download the page at the given url.
String pageContents = downloadPage(verifiedUrl);

/* If the page was downloaded successfully, retrieve all of its
links and then see if it contains the search string. */
if (pageContents != null && pageContents.length() > 0) {
// Retrieve list of valid links from page.
ArrayList links =
retrieveLinks(verifiedUrl, pageContents, crawledList,
limitHost);

// Add links to the to crawl list.
toCrawlList.addAll(links);

/* Check if search string is present in
page and if so record a match. */
if (searchStringMatches(pageContents, searchString,
caseSensitive)) {
addMatch(url);
}
}

// Update crawling stats.
updateStats(url, crawledList.size(), toCrawlList.size(),
maxUrls);
}
}

// Run the Search Crawler.
public static void main(String[] args) {
SearchCrawler crawler = new SearchCrawler();
crawler.show();
}
}