斷點(diǎn)續(xù)傳原理,其實(shí)是利用了臨時(shí)文件保存了下載進(jìn)度,當(dāng)再次下載的時(shí)候,先讀取臨時(shí)文件里面的進(jìn)度,再下載

public class MultiThreadDown {
    static int ThreadCount = 4;
    static String path = "http://192.168.1.103:8080/android/pdf.exe";
    static int finishedThread = 0;
    public static void main(String[] args) throws Exception {
        URL url = new URL(path);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setReadTimeout(5000);
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(5000);
        if (conn.getResponseCode() == 200) {
            // 1.先獲取請(qǐng)求資源的大小
            int length = conn.getContentLength();
            File file = new File("pdf.exe");
            // 生成臨時(shí)文件
            RandomAccessFile raf = new RandomAccessFile(file, "rwd");
            // 設(shè)置臨時(shí)文件的大小
            raf.setLength(length);
            raf.close();
            // 計(jì)算每個(gè)線程應(yīng)該要下載多少個(gè)字節(jié)
            int size = length / ThreadCount;
            for (int i = 0; i < ThreadCount; i++) {
                // 計(jì)算線程下載的開(kāi)始位置和結(jié)束位置
                int startIndex = i * size;
                int endIndex = (i + 1) * size - 1;
                // 如果是最后一個(gè)線程,那么結(jié)束位置寫(xiě)死
                if (i == (ThreadCount - 1)) {
                    endIndex = length - 1;
                }
                new DownThread(startIndex, endIndex, i).start();
            }
        }
    }
}
class DownThread extends Thread {
    int startIndex;
    int endIndex;
    int threadId;
    public DownThread(int startIndex, int endIndex, int threadId) {
        super();
        this.startIndex = startIndex;
        this.endIndex = endIndex;
        this.threadId = threadId;
    }
    @Override
    public void run() {
        // 再次發(fā)送HTTP請(qǐng)求,下載源文件
        try {
            // 生成一個(gè)專門(mén)用來(lái)記錄下載進(jìn)度的臨時(shí)文件
            File progressFile = new File(threadId + ".txt");
            if (progressFile.exists()) {
                FileInputStream fis = new FileInputStream(progressFile);
                BufferedReader br = new BufferedReader(new InputStreamReader(
                        fis));
                // 從進(jìn)度臨時(shí)文件中讀取出上一次下載的總進(jìn)度,然后與原本的開(kāi)始位置相加,得到新的開(kāi)始位置
                startIndex += Integer.parseInt(br.readLine());
                fis.close();
            }
            URL url = new URL(MultiThreadDown.path);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(5000);
            conn.setRequestMethod("GET");
            conn.setConnectTimeout(5000);
            System.out.println("線程threadId" + threadId + "下載區(qū)間:" + startIndex
                    + "-" + endIndex);
            // 設(shè)置請(qǐng)求的數(shù)據(jù)的區(qū)間
            conn.setRequestProperty("Range", "bytes=" + startIndex + "-"
                    + endIndex);
            // 請(qǐng)求部分?jǐn)?shù)據(jù),響應(yīng)碼為206
            if (conn.getResponseCode() == 206) {
                // 此時(shí)只有 1/threadcount數(shù)據(jù)
                InputStream is = conn.getInputStream();
                byte[] b = new byte[1024];
                int len = 0;
                long total = 0;
                File file = new File("pdf.exe");
                // 這樣可以保證數(shù)據(jù)同步寫(xiě)入硬盤(pán)中,防止停電等原因da
                RandomAccessFile raf = new RandomAccessFile(file, "rwd");
                // 把文件的寫(xiě)入位置移動(dòng)至startIndex
                raf.seek(startIndex);
                while ((len = is.read(b)) != -1) {
                    // 每次讀取流里的數(shù)據(jù)寫(xiě)入臨時(shí)文件
                    raf.write(b, 0, len);
                    total += len;
                    RandomAccessFile progressRaf = new RandomAccessFile(
                            progressFile, "rwd");
                    progressRaf.write((total + "").getBytes());
                    progressRaf.close();
                }
                raf.close();
                progressFile.delete(); // 下載完成后,將臨時(shí)文件刪除
                ++MultiThreadDown.finishedThread;
                synchronized (MultiThreadDown.path) {
                    if (MultiThreadDown.finishedThread == MultiThreadDown.ThreadCount) {
                        for (int i = 0; i < MultiThreadDown.ThreadCount; i++) {
                            File f = new File(i + ".txt");
                            f.delete();
                        }
                        MultiThreadDown.finishedThread = 0;
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}