java基础之IO流--使用RandomAccessFile实现数据的分段写入

我分段写入的是一张大图片

简略了对流的关闭的操作

package newFeatures8;import java.io.BufferedReader;import java.io.File;import java.io.FileInputStream;import java.io.InputStream;import java.io.InputStreamReader;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.URL;public class MuchThreadDown {private static int threadCount = 3;// 开启3个线程private static int blockSize = 0;// 每个线程下载的大小private static int runningTrheadCount = 0;// 当前运行的线程数private static String path = "http://img.ivsky.com/img/tupian/pre/201707/29/shuiguo_yuebing-001.jpg";public static void main(String[] args) {try {// 1.请求url地址获取服务端资源的大小URL url = new URL(path);HttpURLConnection openConnection = (HttpURLConnection) url.openConnection();openConnection.setRequestMethod("GET");openConnection.setConnectTimeout(10 * 1000);int code = openConnection.getResponseCode();if (code == 200) {// 获取资源的大小//如果该资源的大小未知或是其字节比Integer.MAX_VALUE(非常非常接近2G)还大,则返回-1int filelength = openConnection.getContentLength();String fileName = getFileName(path);// 2.在本地创建一个与服务端资源同样大小的一个文件（占位）RandomAccessFile randomAccessFile = new RandomAccessFile(new File(fileName), "rw");randomAccessFile.setLength(filelength);// 设置随机访问文件的大小,本地的占位文件randomAccessFile.close();// 3.要分配每个线程下载文件的开始位置和结束位置。开始分段blockSize = filelength / threadCount;// 计算出每个线程理论下载大小for (int threadId = 0; threadId < threadCount; threadId++) {int startIndex = threadId * blockSize;// 计算每个线程下载的开始位置int endIndex = (threadId + 1) * blockSize - 1;// 计算每个线程下载的结束位置// 如果是最后一个线程，结束位置需要单独计算if (threadId == threadCount - 1) {endIndex = filelength - 1;}// 4.开启线程去执行下载new Thread(new DownloadThread(threadId, startIndex, endIndex)).start();}}} catch (Exception e) {e.printStackTrace();}}public static class DownloadThread implements Runnable{private int threadId;private int startIndex;private int endIndex;private int lastPostion;public DownloadThread(int threadId, int startIndex, int endIndex) {this.threadId = threadId;this.startIndex = startIndex;this.endIndex = endIndex;}@Overridepublic void run() {synchronized (DownloadThread.class) {//静态类的锁是当前类所属的字节码文件对象runningTrheadCount = runningTrheadCount + 1;// 开启一线程，线程数加1}// 分段请求网络连接，分段保存文件到本地try {URL url = new URL(path);HttpURLConnection openConnection = (HttpURLConnection) url.openConnection();openConnection.setRequestMethod("GET");openConnection.setConnectTimeout(10 * 1000);System.out.println("理论上下载：  线程：" + threadId + "，开始位置：" + startIndex + ";结束位置:" + endIndex);// 读取上次下载结束的位置,本次从这个位置开始直接下载。File file2 = new File(threadId + ".jpg");if (file2.exists()) {BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(new FileInputStream(file2)));String lastPostion_str = bufferedReader.readLine();lastPostion = Integer.parseInt(lastPostion_str);// 读取文件获取上次下载的位置bufferedReader.close();} else {lastPostion = startIndex;}// 设置分段下载的头信息。 Range:做分段数据请求用的。// bytes:0-500:请求服务器资源中0-500之间的字节信息openConnection.setRequestProperty("Range", "bytes:" + lastPostion + "-" + endIndex);System.out.println("实际下载：  线程：" + threadId + "，开始位置：" + lastPostion + ";结束位置:" + endIndex);if (openConnection.getResponseCode() == 200) {// 200：请求全部资源成功，// 206代表部分资源请求成功InputStream inputStream = openConnection.getInputStream();// 请求成功将流写入本地文件中，已经创建的占位那个文件中RandomAccessFile randomAccessFile = new RandomAccessFile(new File(getFileName(path)), "rw");randomAccessFile.seek(lastPostion);// 设置随机文件从哪个位置开始写。// 将流中的数据写入文件byte[] buffer = new byte[1024];//定义一个1k的缓冲字节数组int length = -1;int total = 0;// 记录本次线程下载的总大小while ((length = inputStream.read(buffer)) != -1) {//开始写入到本地文件randomAccessFile.write(buffer, 0, length);total += length;// 去保存当前线程下载的位置，保存到文件中int currentThreadPostion = lastPostion + total;// 计算出当前线程本次下载的位置// 创建临时的    随机文件保存当前线程下载的位置File file = new File(threadId + ".jpg");RandomAccessFile accessfile = new RandomAccessFile(file, "rwd");accessfile.write(String.valueOf(currentThreadPostion).getBytes());accessfile.close();}// 关闭相关的流信息inputStream.close();randomAccessFile.close();System.out.println("线程：" + threadId + "，下载完毕");// 当所有线程下载结束，删除存放下载位置的文件。synchronized (DownloadThread.class) {runningTrheadCount--;// 标志着一个线程下载结束。if (runningTrheadCount == 0) {System.out.println("所有线程下载完成");for (int threadId = 0; threadId < threadCount; threadId++) {File file = new File(threadId + ".jpg");System.out.println(file.getAbsolutePath());file.delete();}System.out.println("合并完成!");}}}} catch (Exception e) {e.printStackTrace();}}}public static String getFileName(String url) {return url.substring(url.lastIndexOf("/") + 1);}}