Http下载文件的Java代码

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 codes.   public static final int DOWNLOADING = 0;   public static final int PAUSED = 1;   public static final int COMPLETE = 2;   public static final int CANCELLED = 3;   public static final int ERROR = 4;     private URL url; // download URL   private int size; // size of download in bytes   private int downloaded; // number of bytes downloaded   private int status; // current status of download     // Constructor for Download.   public Download(URL url) {    this.url = url;    size = -1;    downloaded = 0;    status = DOWNLOADING;      // Begin the download.    download();   }     // Get this download's URL.   public String getUrl() {    return url.toString();   }     // Get this download's size.   public int getSize() {    return size;   }     // Get this download's progress.   public float getProgress() {    return ((float) downloaded / size) * 100;   }     // Get this download's status.   public int getStatus() {    return status;   }     // Pause this download.   public void pause() {    status = PAUSED;    stateChanged();   }     // Resume this download.   public void resume() {    status = DOWNLOADING;    stateChanged();    download();   }     // Cancel this download.   public void cancel() {    status = CANCELLED;    stateChanged();   }     // Mark this download as having an error.   private void error() {    status = ERROR;    stateChanged();   }     // Start or resume downloading.   private void download() {    Thread thread = new Thread(this);    thread.start();   }     // Get file name portion of URL.   private String getFileName(URL url) {    String fileName = url.getFile();    return fileName.substring(fileName.lastIndexOf('/') + 1);   }     // Download file.   public void run() {    RandomAccessFile file = null;    InputStream stream = null;    try {     // Open connection to URL.     HttpURLConnection connection = (HttpURLConnection) url       .openConnection();     // Specify what portion of file to download.     connection.setRequestProperty("Range", "bytes=" + downloaded + "-");     // Connect to server.     connection.connect();     // Make sure response code is in the 200 range.     if (connection.getResponseCode() / 100 != 2) {      error();     }     // Check for valid content length.     int contentLength = connection.getContentLength();     if (contentLength < 1) {      error();     }     // Set the size for this download if it hasn't been already set.     if (size == -1) {      size = contentLength;      stateChanged();     }     // Open file and seek to the end of it.     file = new RandomAccessFile(getFileName(url), "rw");     file.seek(downloaded);       stream = connection.getInputStream();     while (status == DOWNLOADING) {      // Size buffer according to how much of the file is left to      // download.      byte buffer[];      if (size - downloaded > MAX_BUFFER_SIZE) {       buffer = new byte[MAX_BUFFER_SIZE];      } else {       buffer = new byte[size - downloaded];      }        // Read from server into buffer.      int read = stream.read(buffer);      if (read == -1)       break;        // Write buffer to file.      file.write(buffer, 0, read);      downloaded += read;      stateChanged();     }       // Change status to complete if this point was reached because     // downloading has finished.     if (status == DOWNLOADING) {      status = COMPLETE;      stateChanged();     }    } catch (Exception e) {     error();    } finally {     // Close file.     if (file != null) {      try {       file.close();      } catch (Exception e) {      }     }       // Close connection to server.     if (stream != null) {      try {       stream.close();      } catch (Exception e) {      }     }    }   }     // Notify observers that this download's status has changed.   private void stateChanged() {    setChanged();    notifyObservers();   }     public static void main(String args[]) {    try {     Download d = new Download(new URL(       "http://csdnimg.cn/www/images/csdnindex_logo.gif"));     d.run();    } catch (MalformedURLException e) {     e.printStackTrace();    }   }  }