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Understanding Internet Downloads
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To understand and appreciate the Download Manager, it s necessary to shed some light on how Internet downloads really work. Internet downloads in their simplest form are merely client/server transactions. The client, your browser, requests to download a file from a server on the Internet. The server then responds by sending the requested file to your browser. In order for clients to communicate with servers, they must have an established protocol for doing so. The most common protocols for downloading files are File Transfer Protocol (FTP) and Hypertext Transfer Protocol (HTTP). FTP is usually associated generically with exchanging files between computers, where as HTTP is usually associated specifically with transferring Web pages and their related files (i.e., graphics, sounds, and so on). Over time, as the World Wide Web has grown in popularity, HTTP has become the dominant protocol for downloading files from the Internet. FTP is definitely not extinct, though.
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4: Creating a Download Manager in Java
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For brevity s sake, the Download Manager developed in this chapter will only support HTTP downloads. Nonetheless, adding support for FTP would be an excellent exercise for extending the code. HTTP downloads come in two forms: resumable (HTTP 1.1) and nonresumable (HTTP 1.0). The difference between these two forms lies in the way files can be requested from servers. With the antiquated HTTP 1.0, a client can only request that a server send it a file, whereas with HTTP 1.1 a client can request that a server send it a file or only a specific portion of a file. This is the feature the Download Manager is built on.
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An Overview of the Download Manager
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The Download Manager uses a simple yet effective GUI interface built with Java s Swing libraries. The Download Manager window is shown in Figure 4-1. The use of Swing gives the interface a crisp, modern look and feel. The GUI maintains a list of downloads that are currently being managed. Each download in the list reports its URL, size of the file in bytes, progress as a percentage toward completion, and current status. The downloads can each be in one of the following different states: Downloading, Paused, Complete, Error, or Cancelled. The GUI also has controls for adding downloads to the list and for changing the state of each download in the list. When a download in the list is selected, depending on its current state, it can be paused, resumed, cancelled, or removed from the list altogether. The Download Manager is broken into a few classes for natural separation of functional components. These are the Download, DownloadsTableModel, ProgressRenderer, and DownloadManager classes, respectively. The DownloadManager class is responsible for the GUI interface and makes use of the DownloadsTableModel and ProgressRenderer classes for displaying the current list of downloads. The Download class represents a managed download and is responsible for performing the actual downloading of a file. In the following sections, we ll walk through each of these classes in detail, highlighting their inner workings and explaining how they relate to each other.
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Figure 4-1
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The Download Manager GUI Interface
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The Download Class
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The Download class is the workhorse of the Download Manager. Its primary purpose is to download a file and save that file s contents to disk. Each time a new download is added to the Download Manager, a new Download object is instantiated to handle the download. The Download Manager has the ability to download multiple files at once. To achieve this, it s necessary for each of the simultaneous downloads to run independently. It s also necessary for each individual download to manage its own state so that it can be reflected in the GUI. This is accomplished with the Download class. The entire code for Download is shown here. Notice that it extends Observable and implements Runnable. Each part is examined in detail in the sections that follow.
import java.io.*; import java.net.*; import java.util.*; // This class downloads a file from a URL. class Download extends Observable implements Runnable { // Max size of download buffer. private static final int MAX_BUFFER_SIZE = 1024; // These are the status names. public static final String STATUSES[] = {"Downloading", "Paused", "Complete", "Cancelled", "Error"}; // These are the status public static final int public static final int public static final int public static final int public static final int private private private private URL int int int codes. DOWNLOADING = 0; PAUSED = 1; COMPLETE = 2; CANCELLED = 3; ERROR = 4;
url; // download URL size; // size of download in bytes downloaded; // number of bytes downloaded status; // current status of download
// Constructor for Download. public Download(URL url) { this.url = url; size = -1; downloaded = 0; status = DOWNLOADING; // Begin the download.
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