Lesson 25: Downloading From the Internet

When in the course of human events a developer needs to write a Java program to download the Declaration of Independence to a file and then display that file in a web browser, all he or she needs to code is something like this:

public class PageDownload implements Runnable {
    private static final String DECLARATION_URL = "https://www.archives.gov/founding-docs/declaration-transcript";

    public static void main(String[] args) throws Exception {
            // Create a File to receive the downloaded web page.
            File htmlDir = new File("html");
            htmlDir.mkdirs();
            File htmlFile = File.createTempFile("temp_", ".html", htmlDir);
            htmlFile.deleteOnExit();
            BufferedWriter wtr = new BufferedWriter(new FileWriter(htmlFile));

            // URL, URLConnection, and the latter's getInputStream are the basis
            // for reading a file on the Internet. In fact, URL has an openInputStream()
            // method which accomplishes the same thing.
            URL url = new URL(address);
            URLConnection conn = url.openConnection();
            InputStream in = conn.getInputStream();
            BufferedReader rdr = new BufferedReader(new InputStreamReader(in));

            // We're going to copy the file from the Internet to a
            // temporary file on disk, and then display it in our
            // web browser.
            StringBuilder html = new StringBuilder();
            char[] buffer = new char[2048];
            int length = 0;
            while ((length = rdr.read(buffer)) >= 0) {
                html.append(buffer, 0, length);
                wtr.write(buffer, 0, length);
            }
            System.out.println(html.length() + " bytes were read from " + address + ".");

            rdr.close();
            wtr.flush();
            wtr.close();

            // If a web browser is available, call the browse() method
            // of Desktop to display the HTML file we've written.
            if (Desktop.isDesktopSupported() && Desktop.getDesktop().isSupported(Desktop.Action.BROWSE)) {
                Desktop.getDesktop().browse(htmlFile.toURI());
            } else {
                System.out.println("Browsing is not supported on your system.");
            }

        } catch (Throwable e) {
            this.exception = e;
        }

Wasn’t that easy? Now try creating a project in your workspace that contains the code above and try running it. Good luck!

The Desktop class provides access to some application features on your system. You might also like to know more about URL and URLConnection.

What You Need to Know

We’ll be using this code in Lesson 26: Threads