大家好，又见面了，我是全栈君。
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：
代码语言：javascript复制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;
}
@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。
代码语言：javascript复制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;
}
@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：
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