Java之多线程断点下载的实现

RandomAccessFile类：
此类的实例支持对随机访问文件的读取和写入。随机访问文件的行为类似存储在文件系统中的一个大型 byte 数组。存在指向该隐含数组，光标或索引，称为文件指针；输入操作从文件指针开始读取字节，并随着对字节的读取而前移此文件指针。如果随机访问文件以读取/写入模式创建，则输出操作也可用；输出操作从文件指针开始写入字节，并随着对字节的写入而前移此文件指针。写入隐含数组的当前末尾之后的输出操作导致该数组扩展。该文件指针可以通过 getFilePointer 方法读取，并通过 seek 方法设置。

下面有RandomAccessFile实现安卓下的断点下载的demo。
服务器端可以用tomcat模拟，将被下载的测试文件放入webApp/ROOT目录下即可。
先给出java借助HttpURLConnection类实现的多线程下载代码：

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";
        URL url = new URL(path); 
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(5000);//超时时间
        int code = conn.getResponseCode();
        System.out.println(code);

        if(code / 100 == 2){
            int size = conn.getContentLength();//获取资源文件的长度
            System.out.println("请求资源大小:" + size);
            blockSize = size / threadCount;//将资源文件分为多少块，没一块的大小

            runningThreadCount = threadCount;
            long startIndex = 0;
            long endIndex = 0;
            //开启若干个子线程去实现多线程的下载
            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("开启线程:" + i + ";" + "开始位置:" + startIndex + ":" + "结束位置:" + 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);//设置服务器上的文件的读取位置

                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 + "个文件的开始位置:" + String.valueOf(startIndex));
                    int len = 0;
                    byte[] buffer = new byte[1024];
                    while ((len = is.read(buffer)) != -1){
                        raf.write(buffer, 0, len);//写文件
                    }
                    raf.close();
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

断点下载的原理就是将上次文件下载的位置保存为临时文件，当完全下载完毕时再删除。

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";
        URL url = new URL(path); 
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(5000);//超时时间
        int code = conn.getResponseCode();
        System.out.println(code);

        if(code / 100 == 2){
            int size = conn.getContentLength();//获取资源文件的长度
            System.out.println("请求资源大小:" + size);
            blockSize = size / threadCount;//将资源文件分为多少块，没一块的大小

            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("开启线程:" + i + ";" + "开始位置:" + startIndex + ":" + "结束位置:" + 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");
                //接着上次的文件继续下载
                if(positionFile.exists() && positionFile.length() > 0){
                    FileInputStream fis = new FileInputStream(positionFile);
                    BufferedReader reader = new BufferedReader(new InputStreamReader(fis));
                    //获取当前线程上次下载的总大小是多少
                    String lasttotalstr = reader.readLine();
                    int lastTotal = Integer.valueOf(lasttotalstr);
                    System.out.println("上次线程下载的总大小:" + lastTotal);
                    startIndex += lastTotal;
                    total += lastTotal;
                    fis.close();
                }
                conn.setReadTimeout(5000);
                conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);//设置服务器上的文件的读取位置

                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 + "个文件的开始位置:" + 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);//写文件
                        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("线程" + threadId + "下载完毕了");
                    runningThreadCount--;
                    if (runningThreadCount < 1) {
                        System.out.println("所有的线程都工作完毕了。删除临时记录的文件");
                        for (int i = 0; i < threadCount; i++) {
                            File f = new File(i + ".txt");
                            System.out.println(f.delete());
                        }
                    }
                }
            }
        }
    }
}

运行结果截图：