随机访问类RandomAccessFile-多线程下载

随机访问类RandomAccessFile

输入流FileInputStream和输出流 FileOutputStream，实现的是对磁盘文件的顺序读写，而且读写要分别创建不同对象。

相比之下RandomAccessFile类则可对文件实现随机读写操作。
RandomAccessFile对象的文件位置指针遵循下面的规律：
·新建RandomAccessFile对象的文件位置指针位于文件的开头处；
·每次读写操作之后，文件位置的指针都相应后移到读写的字节数；
·可以通过getFilePointer方法来获得文件位置指针的位置，通过seek方法来设置文件指针的位置。
如果某个文件有30个字节，读取数据过程中，从20-30读取，用skip( )//跳过方法，但在读取的过程中，前面的字节都被删除掉了，

如果用户有这样的需求，先读取10-20字节，之后再读1-10之间的数，再20-30之间，

多线程下载

public  void threadDownLoad(String path,int threadSize) throws Exception{
        URL url = new URL(path);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");
        if(connection.getResponseCode() != 200){
            throw new RuntimeException("ResponseCode != 200");
        }
        //要下载的总大小
        int dataLength = connection.getContentLength();
        connection.disconnect();
        //线程数
        threadSize = threadSize == 0 ? 5 : threadSize;
        threadSize = dataLength % threadSize == 0 ? threadSize : threadSize+1;
        //每个线程块要下载的数量
        int blockSize = dataLength / threadSize;
        String newFileName = rename(path);
        System.out.println("分"+threadSize+"个线程开始下载");
        for(int i=0;i<threadSize;i++){
            RandomAccessFile accessFile = new RandomAccessFile(newFileName, "rw");
            int startSize =blockSize * i; 
            accessFile.seek(startSize);
            Thread thread = new Thread(new DownLoad(startSize, path, blockSize, accessFile,i+1));
            thread.start();
        }
    }

定义的线程下载类

private class DownLoad implements Runnable{
        private String path;
        private RandomAccessFile accessFile;
        private int startSize;
        private int blockSize;
        private int threadName;
        public DownLoad(int startSize,String path,int blockSize,RandomAccessFile accessFile,int threadName) {
            this.path = path;
            this.accessFile = accessFile;
            this.startSize = startSize;
            this.blockSize = blockSize;
            this.threadName = threadName;
        }
        @Override
        public void run() {
            try {
                System.out.println("线程"+threadName+"开始下载.....");
                URL url = new URL(path);
                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                connection.setRequestMethod("GET");
                connection.setReadTimeout(6*1000);
                connection.setRequestProperty("Range", "bytes=" + startSize + "-");
                if(connection.getResponseCode()!=206){
                    throw new RuntimeException("ResponseCode != 206");
                }
                InputStream inputStream = connection.getInputStream();
                inputStream = new BufferedInputStream(inputStream);
                byte[] buffer = new byte[1024];
                int len = -1;
                int length = 0;
                while(length < blockSize && ((len = inputStream.read(buffer)) != -1)){
                    accessFile.write(buffer, 0, len);
                    length += len;
                }
                accessFile.close();
                inputStream.close();
                connection.disconnect();
                System.out.println("线程"+threadName+"下载成功!");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }