RandomAccessFile类：
此类的实例支持对随机訪问文件的读取和写入。随机訪问文件的行为相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開始读取字节。并随着对字节的读取而前移此文件指针。

假设随机訪问文件以读取/写入模式创建，则输出操作也可用。输出操作从文件指针開始写入字节。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。该文件指针能够通过 getFilePointer 方法读取。并通过 seek 方法设置。

以下有RandomAccessFile实现安卓下的断点下载的demo。

server端能够用tomcat模拟。将被下载的測试文件放入webApp/ROOT文件夹下就可以。
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 + ";" + "開始位置:" + 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;}@Overridepublic 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server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);//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暂时文件，当全然完成下载时再删除。

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 + ";" + "開始位置:" + 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;}@Overridepublic 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server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);//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 + "完成下载了");runningThreadCount--;if (runningThreadCount < 1) {System.out.println("全部的线程都工作完成了。删除暂时记录的文件");for (int i = 0; i < threadCount; i++) {File f = new File(i + ".txt");System.out.println(f.delete());}}}}}}
}

执行结果截图：