Which Java libraries are most commonly used for web scraping?

In Java, web scraping is often performed using libraries that can handle HTTP requests, parse HTML, and extract data from web pages. Here are some of the most commonly used Java libraries for web scraping:

• Jsoup: Jsoup is a Java library for working with real-world HTML. It provides a very convenient API for extracting and manipulating data, using the best of DOM, CSS, and jQuery-like methods.

   Document doc = Jsoup.connect("http://example.com/").get();
   String title = doc.title();
   Elements links = doc.select("a[href]");
   for (Element link : links) {
       System.out.println("\nlink : " + link.attr("href"));
       System.out.println("text : " + link.text());
   }

• HtmlUnit: HtmlUnit is a "GUI-less browser for Java programs." It models HTML documents and provides an API that allows you to invoke pages, fill out forms, click links, etc., just like you do in your "normal" browser.

   WebClient webClient = new WebClient();
   HtmlPage page = webClient.getPage("http://example.com/");
   String pageAsXml = page.asXml();
   String pageAsText = page.asText();
   webClient.close();

• Selenium WebDriver: Although Selenium is primarily used for automating web applications for testing purposes, it can also be used for web scraping. It's particularly useful when you need to interact with web pages that heavily rely on JavaScript.

   WebDriver driver = new ChromeDriver();
   driver.get("http://example.com/");
   WebElement element = driver.findElement(By.name("q"));
   element.sendKeys("Cheese!");
   element.submit();
   driver.quit();

Note that using Selenium requires a browser driver (e.g., ChromeDriver for Google Chrome, GeckoDriver for Firefox) to be installed and configured.

• Apache HttpClient: HttpClient is a library for client-side HTTP communication. It's not specifically designed for web scraping, but you can use it to send HTTP requests, handle cookies, and then parse the responses with a different library like Jsoup.

   CloseableHttpClient httpclient = HttpClients.createDefault();
   HttpGet httpGet = new HttpGet("http://example.com/");
   CloseableHttpResponse response = httpclient.execute(httpGet);

   try {
       HttpEntity entity = response.getEntity();
       if (entity != null) {
           InputStream inputStream = entity.getContent();
           // Process the input stream.
       }
   } finally {
       response.close();
   }

• Jaunt: Jaunt is a Java library for web scraping and JSON querying. It provides a headless browser that allows you to navigate and search through web pages.

   UserAgent userAgent = new UserAgent();
   userAgent.visit("http://example.com/");
   Element div = userAgent.doc.findFirst("<div>");
   System.out.println(div.innerText());

When choosing a library for web scraping in Java, consider the complexity of the web pages you want to scrape, whether you need to execute JavaScript, and how much control you need over the HTTP requests. Libraries like Jsoup are great for simple HTML parsing, while tools like Selenium WebDriver or HtmlUnit are better suited for dealing with complex pages that rely on JavaScript.