Java实现多线程断点下载功能可以用于大文件的下载，可以提高下载速度，增加用户体验。以下是实现的完整攻略：
1. 分析
- 当前文件大小：文件已下载的长度
- 总文件大小：文件在服务器上的长度
- 当前已下载部分的起点和终点
- 每条线程要下载的文件块大小
2. 算法流程
- 获得URL连接对象，获取文件大小
- 计算出每条线程要下载的大小
- 检查下载目录是否存在，若不存在则创建
- 创建多条线程，每条线程下载一段文件块
- 等待所有线程下载完成后进行合并
3. 示例说明
示例一：多线程下载一个.txt文件
假设需要下载一个大小为500MB的txt文件，以下为示例代码：
public class MultiThreadDownloadDemo {
private static final int THREAD_COUNT = 4; //线程数
private static final String URL_PATH = "http://example.com/file.txt"; //文件下载路径
private static final String FILE_PATH = "D:/downloads/file.txt"; //文件保存路径
private static final int BUFFER_SIZE = 1024*1024; //缓冲区大小
private static final long BLOCK_SIZE = 1024*1024*50; //每条线程下载文件块大小
public static void main(String[] args) {
try {
URL url = new URL(URL_PATH);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
long totalSize = conn.getContentLength();
conn.disconnect();
File parent = new File(FILE_PATH).getParentFile();
if (!parent.exists()) {
parent.mkdirs();
}
RandomAccessFile raf = new RandomAccessFile(FILE_PATH, "rw");
raf.setLength(totalSize);
long blockSize = (totalSize % THREAD_COUNT == 0) ? (totalSize / THREAD_COUNT) :
((totalSize / THREAD_COUNT) + 1);
List<DownloadThread> threads = new ArrayList<>();
for (int i = 0; i < THREAD_COUNT; i++) {
long start = i * blockSize;
long end = (i+1) * blockSize - 1;
if (end >= totalSize) {
end = totalSize - 1;
}
DownloadThread thread = new DownloadThread(i+1, URL_PATH, FILE_PATH,
start, end, BLOCK_SIZE, BUFFER_SIZE);
threads.add(thread);
thread.start();
}
for (DownloadThread thread : threads) {
thread.join();
}
raf.close();
System.out.println("下载完成");
} catch (Exception e) {
e.printStackTrace();
}
}
public static class DownloadThread extends Thread {
private int index; //线程编号
private String urlPath; //文件下载路径
private String filePath; //保存路径
private long start; //开始位置
private long end; //结束位置
private long blockSize; //每条线程下载文件块大小
private int bufferSize; //缓冲区大小
public DownloadThread(int index, String urlPath, String filePath,
long start, long end, long blockSize, int bufferSize) {
this.index = index;
this.urlPath = urlPath;
this.filePath = filePath;
this.start = start;
this.end = end;
this.blockSize = blockSize;
this.bufferSize = bufferSize;
}
@Override
public void run() {
try {
URL url = new URL(urlPath);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setRequestProperty("Connection", "Keep-Alive");
conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
InputStream is = conn.getInputStream();
RandomAccessFile raf = new RandomAccessFile(filePath, "rw");
raf.seek(start);
byte[] buffer = new byte[bufferSize];
int len;
while ((len = is.read(buffer)) != -1) {
raf.write(buffer, 0, len);
synchronized (DownloadThread.class) {
start += len;
if (start > end) {
System.out.println("线程" + index + "下载完成");
return;
}
}
}
raf.close();
is.close();
conn.disconnect();
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
示例二：多线程下载图片文件并显示
假设需要下载一个图片文件并在Swing程序中展示图片，以下为示例代码：
public class MultiThreadDownloadAndDisplayDemo extends JFrame {
private static final int THREAD_COUNT = 4; //线程数
private static final String URL_PATH = "http://example.com/image.jpg"; //文件下载路径
private static final String FILE_PATH = "D:/downloads/image.jpg"; //文件保存路径
private static final int BUFFER_SIZE = 1024*1024; //缓冲区大小
private static final long BLOCK_SIZE = 1024*1024*50; //每条线程下载文件块大小
private static final int WIDTH = 800; //窗口宽度
private static final int HEIGHT = 600; //窗口高度
public MultiThreadDownloadAndDisplayDemo() {
setLayout(new BorderLayout());
setSize(WIDTH, HEIGHT);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
ImageIcon icon = downloadImage();
JLabel label = new JLabel(icon);
add(label);
setLocationRelativeTo(null);
setVisible(true);
}
private ImageIcon downloadImage() {
ImageIcon icon = null;
try {
URL url = new URL(URL_PATH);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
long totalSize = conn.getContentLength();
conn.disconnect();
File parent = new File(FILE_PATH).getParentFile();
if (!parent.exists()) {
parent.mkdirs();
}
RandomAccessFile raf = new RandomAccessFile(FILE_PATH, "rw");
raf.setLength(totalSize);
long blockSize = (totalSize % THREAD_COUNT == 0) ? (totalSize / THREAD_COUNT) :
((totalSize / THREAD_COUNT) + 1);
List<DownloadThread> threads = new ArrayList<>();
for (int i = 0; i < THREAD_COUNT; i++) {
long start = i * blockSize;
long end = (i+1) * blockSize - 1;
if (end >= totalSize) {
end = totalSize - 1;
}
DownloadThread thread = new DownloadThread(i+1, URL_PATH, FILE_PATH,
start, end, BLOCK_SIZE, BUFFER_SIZE);
threads.add(thread);
thread.start();
}
for (DownloadThread thread : threads) {
thread.join();
}
raf.close();
InputStream is = new FileInputStream(FILE_PATH);
byte[] bytes = new byte[is.available()];
is.read(bytes);
is.close();
icon = new ImageIcon(bytes);
} catch (Exception e) {
e.printStackTrace();
}
return icon;
}
public static class DownloadThread extends Thread {
private int index; //线程编号
private String urlPath; //文件下载路径
private String filePath; //保存路径
private long start; //开始位置
private long end; //结束位置
private long blockSize; //每条线程下载文件块大小
private int bufferSize; //缓冲区大小
public DownloadThread(int index, String urlPath, String filePath,
long start, long end, long blockSize, int bufferSize) {
this.index = index;
this.urlPath = urlPath;
this.filePath = filePath;
this.start = start;
this.end = end;
this.blockSize = blockSize;
this.bufferSize = bufferSize;
}
@Override
public void run() {
try {
URL url = new URL(urlPath);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setRequestProperty("Connection", "Keep-Alive");
conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
InputStream is = conn.getInputStream();
RandomAccessFile raf = new RandomAccessFile(filePath, "rw");
raf.seek(start);
byte[] buffer = new byte[bufferSize];
int len;
while ((len = is.read(buffer)) != -1) {
raf.write(buffer, 0, len);
synchronized (DownloadThread.class) {
start += len;
if (start > end) {
System.out.println("线程" + index + "下载完成");
return;
}
}
}
raf.close();
is.close();
conn.disconnect();
} catch (Exception e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
new MultiThreadDownloadAndDisplayDemo();
}
}
以上是多线程断点下载的完整攻略，其中示例一演示了在控制台中下载文件的过程，示例二演示了在Swing程序中下载并展示图片的过程。
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