Java之多線程斷點下載的實現

mxf8 10年前發布 | 4K 次閱讀 Java

RandomAccessFile類：
此類的實例支持對隨機訪問文件的讀取和寫入。隨機訪問文件的行為類似存儲在文件系統中的一個大型 byte 數組。存在指向該隱含數組，光標或索引，稱為文件指針；輸入操作從文件指針開始讀取字節，并隨著對字節的讀取而前移此文件指針。如果隨機訪問文件以讀取/ 寫入模式創建，則輸出操作也可用；輸出操作從文件指針開始寫入字節，并隨著對字節的寫入而前移此文件指針。寫入隱含數組的當前末尾之后的輸出操作導致該數組擴展。該文件指針可以通過 getFilePointer 方法讀取，并通過 seek 方法設置。

下面有RandomAccessFile實現安卓下的斷點下載的demo。
服務器端可以用tomcat模擬，將被下載的測試文件放入webApp/ROOT目錄下即可。
先給出java借助HttpURLConnection類實現的多線程下載代碼：

public class MultiThread {
    private static int threadCount = 3;
    private static long blockSize;
    private static int runningThreadCount;
    public static void main(String[] args) throws Exception {
        String path = "http://10.0.67.172/test.exe&quot;;
        URL url = new URL(path); 
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(5000);//超時時間
        int code = conn.getResponseCode();
        System.out.println(code);

    if(code / 100 == 2){
        int size = conn.getContentLength();//獲取資源文件的長度
        System.out.println("請求資源大小:" + size);
        blockSize = size / threadCount;//將資源文件分為多少塊，沒一塊的大小

        runningThreadCount = threadCount;
        long startIndex = 0;
        long endIndex = 0;
        //開啟若干個子線程去實現多線程的下載
        for(int i = 0; i < threadCount; i++){
            startIndex = i * blockSize;
            endIndex = (i + 1) * blockSize - 1;
            if(i == threadCount-1){
                endIndex = size - 1;
            }
            System.out.println("開啟線程:" + i + ";" + "開始位置:" + startIndex + ":" + "結束位置:" + endIndex);
            new DownThread(path, startIndex, endIndex, i).start();
        }
    }
}

private static class DownThread extends Thread{
    private String path;
    private long startIndex;
    private long endIndex;
    private int threadId;

    public DownThread(String path, long startIndex, long endIndex, int threadId) {
        super();
        this.path = path;
        this.startIndex = startIndex;
        this.endIndex = endIndex;
        this.threadId = threadId;
    }

    @Override
    public void run() {
        try {
            URL url = new URL(path);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            conn.setReadTimeout(5000);
            conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);//設置服務器上的文件的讀取位置

            int code = conn.getResponseCode();
            if(code / 100 == 2){
                InputStream is = conn.getInputStream();
                File file = new File("temp.exe");
                RandomAccessFile raf = new RandomAccessFile(file, "rw");
                raf.seek(startIndex);
                System.out.println("第" + threadId + "個文件的開始位置:" + String.valueOf(startIndex));
                int len = 0;
                byte[] buffer = new byte[1024];
                while ((len = is.read(buffer)) != -1){
                    raf.write(buffer, 0, len);//寫文件
                }
                raf.close();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

}</pre> 


 斷點下載的原理就是將上次文件下載的位置保存為臨時文件，當完全下載完畢時再刪除。 

public class MultiThread {
    private static int threadCount = 3;
    private static long blockSize;
    private static int runningThreadCount;
    public static void main(String[] args) throws Exception {
        String path = "http://10.0.67.172/test.rar&quot;;
        URL url = new URL(path); 
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(5000);//超時時間
        int code = conn.getResponseCode();
        System.out.println(code);

    if(code / 100 == 2){
        int size = conn.getContentLength();//獲取資源文件的長度
        System.out.println("請求資源大小:" + size);
        blockSize = size / threadCount;//將資源文件分為多少塊，沒一塊的大小

        runningThreadCount = threadCount;
        long startIndex = 0;
        long endIndex = 0;
        for(int i = 0; i < threadCount; i++){
            startIndex = i * blockSize;
            endIndex = (i + 1) * blockSize - 1;
            if(i == threadCount-1){
                endIndex = size - 1;
            }
            System.out.println("開啟線程:" + i + ";" + "開始位置:" + startIndex + ":" + "結束位置:" + endIndex);
            new DownThread(path, startIndex, endIndex, i).start();
        }
    }
}

private static class DownThread extends Thread{
    private String path;
    private long startIndex;
    private long endIndex;
    private int threadId;

    public DownThread(String path, long startIndex, long endIndex, int threadId) {
        super();
        this.path = path;
        this.startIndex = startIndex;
        this.endIndex = endIndex;
        this.threadId = threadId;
    }

    @Override
    public void run() {
        int total = 0;
        try {
            File positionFile = new File(threadId + ".txt");

            URL url = new URL(path);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            //接著上次的文件繼續下載
            if(positionFile.exists() && positionFile.length() > 0){
                FileInputStream fis = new FileInputStream(positionFile);
                BufferedReader reader = new BufferedReader(new InputStreamReader(fis));
                //獲取當前線程上次下載的總大小是多少
                String lasttotalstr = reader.readLine();
                int lastTotal = Integer.valueOf(lasttotalstr);
                System.out.println("上次線程下載的總大小:" + lastTotal);
                startIndex += lastTotal;
                total += lastTotal;
                fis.close();
            }
            conn.setReadTimeout(5000);
            conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);//設置服務器上的文件的讀取位置

            int code = conn.getResponseCode();
            if(code / 100 == 2){
                InputStream is = conn.getInputStream();
                File file = new File("temp.rar");
                RandomAccessFile raf = new RandomAccessFile(file, "rw");
                raf.seek(startIndex);
                System.out.println("第" + threadId + "個文件的開始位置:" + String.valueOf(startIndex));
                int len = 0;
                byte[] buffer = new byte[1024];
                while ((len = is.read(buffer)) != -1){
                    RandomAccessFile rf = new RandomAccessFile(positionFile, "rwd");
                    raf.write(buffer, 0, len);//寫文件
                    total += len;
                    rf.write(String.valueOf(total).getBytes());
                    rf.close();
                }
                is.close();
                raf.close();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally{
            synchronized (DownThread.class) {
                System.out.println("線程" + threadId + "下載完畢了");
                runningThreadCount--;
                if (runningThreadCount < 1) {
                    System.out.println("所有的線程都工作完畢了。刪除臨時記錄的文件");
                    for (int i = 0; i < threadCount; i++) {
                        File f = new File(i + ".txt");
                        System.out.println(f.delete());
                    }
                }
            }
        }
    }
}

}</pre>

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