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()); } } } } } } }