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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());
}
}
}
}
}
}
}