I'm attemptin开发者_如何学编程g to make my first program in Java. The goal is to write a program that browses to a website and downloads a file for me. However, I don't know how to use Java to interact with the internet. Can anyone tell me what topics to look up/read about or recommend some good resources?

The simplest solution (without depending on any third-party library or platform) is to create a URL instance pointing to the web page / link you want to download, and read the content using streams.

For example:

    import java.io.BufferedReader;
    import java.io.IOException;
    import java.io.InputStream;
    import java.io.InputStreamReader;
    import java.net.URL;
    import java.net.URLConnection;
    
    
    public class DownloadPage {
    
        public static void main(String[] args) throws IOException {
            
            // Make a URL to the web page
            URL url = new URL("http://stackoverflow.com/questions/6159118/using-java-to-pull-data-from-a-webpage");
            
            // Get the input stream through URL Connection
            URLConnection con = url.openConnection();
            InputStream is = con.getInputStream();
            
            // Once you have the Input Stream, it's just plain old Java IO stuff.
            
            // For this case, since you are interested in getting plain-text web page
            // I'll use a reader and output the text content to System.out.
            
            // For binary content, it's better to directly read the bytes from stream and write
            // to the target file.          
            
            try(BufferedReader br = new BufferedReader(new InputStreamReader(is))) {
                String line = null;
            
                // read each line and write to System.out
                while ((line = br.readLine()) != null) {
                    System.out.println(line);
                }
            }
        }
    }

Hope this helps.

The Basics

Look at these to build a solution more or less from scratch:

• Start from the basics: The Java Tutorial's chapter on Networking, including Working With URLs
• Make things easier for yourself: Apache HttpComponents (including HttpClient)

The Easily Glued-Up and Stitched-Up Stuff

You always have the option of calling external tools from Java using the exec() and similar methods. For instance, you could use wget, or cURL.

The Hardcore Stuff

Then if you want to go into more fully-fledged stuff, thankfully the need for automated web-testing as given us very practical tools for this. Look at:

• HtmlUnit (powerful and simple)
• Selenium, Selenium-RC
• WebDriver/Selenium2 (still in the works)
• JBehave with JBehave Web

Some other libs are purposefully written with web-scraping in mind:

• JSoup
• Jaunt

Some Workarounds

Java is a language, but also a platform, with many other languages running on it. Some of which integrate great syntactic sugar or libraries to easily build scrapers.

Check out:

• Groovy (and its XmlSlurper)
• or Scala (with great XML support as presented here and here)

If you know of a great library for Ruby (JRuby, with an article on scraping with JRuby and HtmlUnit) or Python (Jython) or you prefer these languages, then give their JVM ports a chance.

Some Supplements

Some other similar questions:

• Scrape data from HTML using Java
• Options for HTML Scraping

Here's my solution using URL and try with resources phrase to catch the exceptions.

/**
 * Created by mona on 5/27/16.
 */
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
public class ReadFromWeb {
    public static void readFromWeb(String webURL) throws IOException {
        URL url = new URL(webURL);
        InputStream is =  url.openStream();
        try( BufferedReader br = new BufferedReader(new InputStreamReader(is))) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        }
        catch (MalformedURLException e) {
            e.printStackTrace();
            throw new MalformedURLException("URL is malformed!!");
        }
        catch (IOException e) {
            e.printStackTrace();
            throw new IOException();
        }

    }
    public static void main(String[] args) throws IOException {
        String url = "https://madison.craigslist.org/search/sub";
        readFromWeb(url);
    }

}

You could additionally save it to file based on your needs or parse it using XML or HTML libraries.

Since Java 11 the most convenient way it to use java.net.http.HttpClient from the standard library.

Example:

HttpClient client = HttpClient.newBuilder()
     .version(Version.HTTP_1_1)
     .followRedirects(Redirect.NORMAL)
     .connectTimeout(Duration.ofSeconds(20))
     .proxy(ProxySelector.of(new InetSocketAddress("proxy.example.com", 80)))
     .authenticator(Authenticator.getDefault())
     .build();

HttpRequest request = HttpRequest.newBuilder()
     .uri(URI.create("httpss://foo.com/"))
     .timeout(Duration.ofMinutes(2))
     .GET()
     .build();

HttpResponse<String> response = client.send(request, BodyHandlers.ofString());

System.out.println(response.statusCode());

System.out.println(response.body());

I use the following code for my API:

try {
        URL url = new URL("https://stackoverflow.com/questions/6159118/using-java-to-pull-data-from-a-webpage");
        InputStream content = url.openStream();
        int c;
        while ((c = content.read())!=-1) System.out.print((char) c);
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException ie) {
        ie.printStackTrace();
    }

You can catch the characters and convert them to string.