android实现多线程断点续传功能

Android实现多线程断点续传功能需要以下步骤：

1. 在AndroidManifest.xml中添加网络读写权限，以便应用程序能够进行网络请求.

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

1. 使用URLConnection建立网络连接，并设置连接属性。其中，设置请求的Range头部，以实现断点下载的功能

URL url = new URL(downloadUrl);
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setConnectTimeout(5000);
connection.setRequestMethod("GET");
//设置请求range头部，可以实现断点下载
connection.setRequestProperty("Range", "bytes=" + start + "-" + end);

1. 判断文件是否存在，如果存在，则获取文件大小，计算出已经下载的文件的大小，然后设置请求头的Range。

File file = new File(filePath);
if (file.exists()) {
    downloadedSize = file.length();
    connection.setRequestProperty("Range", "bytes=" + downloadedSize + "-");
}

1. 接收HttpURLConnection返回的Header，进行解析，获取响应头的文件长度，计算出文件的总长度用于进度显示。

long contentLength = connection.getContentLength();
if (contentLength == -1) {
    // 获取失败，退出下载
    return;
} else {
    totalSize = contentLength + downloadedSize;
}

1. 初始化随机访问文件流（RandomAccessFile），如果是断点下载，指定从文件剩余部分续传；如果是新下载，初始化文件流。

RandomAccessFile raf = new RandomAccessFile(filePath, "rw");
if (downloadedSize > 0) {
     raf.seek(downloadedSize);
} else {
     // 如果是新下载，清空文件
     raf.setLength(0);
}

1. 使用线程池进行多线程下载。根据文件的大小和线程数，进行分块下载。

ExecutorService threadPool = Executors.newCachedThreadPool();
List<Future<Long>> futures = new ArrayList<>();
for (int i = 0; i < threadCount; i++) {
    BlockDownloader downloader = new BlockDownloader(connection, filePath, threadCount, i, raf);
    Future<Long> future = threadPool.submit(downloader);
    futures.add(future);
}

1. 在下载过程中，更新进度显示和速度显示，并将下载结果返回给主线程。每一个单独的线程都会返回其下载的字节数，主线程根据所有线程返回的结果计算出整个文件的下载速度，并且更新界面显示。

for (Future<Long> future : futures) {
    downloadedSize += future.get();
}
threadPool.shutdown();
raf.close();

示例1：单线程下载示例

try {
    URL url = new URL(downloadUrl);
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    connection.setConnectTimeout(5000);
    connection.setRequestMethod("GET");
    InputStream is = connection.getInputStream();
    File file = new File(filePath);
    FileOutputStream fos = new FileOutputStream(file);
    byte[] buffer = new byte[1024 * 1024];
    int length;
    while ((length = is.read(buffer)) != -1) {
        fos.write(buffer, 0, length);
    }
    fos.close();
    is.close();
} catch (Exception e) {
    e.printStackTrace();
}

示例2：多线程下载示例

public class BlockDownloader implements Callable<Long> {
    private HttpURLConnection connection;
    private String filePath;
    private int threadCount;
    private int blockIndex;
    private RandomAccessFile raf;

    public BlockDownloader(HttpURLConnection connection, String filePath, int threadCount, int blockIndex, RandomAccessFile raf) {
        this.connection = connection;
        this.filePath = filePath;
        this.threadCount = threadCount;
        this.blockIndex = blockIndex;
        this.raf = raf;
    }

    @Override
    public Long call() throws Exception {
        long startPos = blockIndex * connection.getContentLength() / threadCount;
        long endPos = (blockIndex + 1) * connection.getContentLength() / threadCount - 1;
        String range = "bytes=" + startPos + "-" + endPos;
        connection.setRequestProperty("Range", range);
        InputStream is = connection.getInputStream();
        byte[] buffer = new byte[1024 * 1024];
        int length;
        long downloadedSize = 0;
        while ((length = is.read(buffer)) != -1) {
            raf.write(buffer, 0, length);
            downloadedSize += length;
        }
        raf.close();
        is.close();
        return downloadedSize;
    }
}