一、多线程下载
多线程下载就是抢占服务器资源
原理：服务器CPU 分配给每条线程的时间片相同，服务器带宽平均分配给每条线程，所以客户端开启的线程越多，就能抢占到更多的服务器资源。
1、设置开启线程数，发送http请求到下载地址，获取下载文件的总长度
然后创建一个长度一致的临时文件，避免下载到一半存储空间不够了，并计算每个线程下载多少数据
2、计算每个线程下载数据的开始和结束位置
再次发送请求，用 Range 头请求开始位置和结束位置的数据
3、将下载到的数据，存放至临时文件中
4、带断点续传的多线程下载
定义一个int变量，记录每条线程下载的数据总长度，然后加上该线程的下载开始位置，得到的结果就是下次下载时，该线程的开始位置，把得到的结果存入缓存文件，当文件下载完成，删除临时进度文件。
public class MultiDownload {
static int ThreadCount = 3;
static int finishedThread = 0;
//确定下载地址
static String filename = "EditPlus.exe";
static String path = "http://192.168.3.45:8080/"+filename;
public static void main(String[] args) {
//1、发送get请求，去获得下载文件的长度
try {
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5000);
conn.setReadTimeout(5000);
if (conn.getResponseCode()==200) {
//如果请求成功，拿到所请求资源文件的长度
int length = conn.getContentLength();
//2、生成一个与原文件同样的大小的临时文件，以免下载一半存储空间不够了
File file = new File(filename);//演示，所以将保存的文件目录放在工程的同目录
//使用RandomAccessFile 生成临时文件，可以用指针定位文件的任意位置，
//而且能够实时写到硬件底层设备，略过缓存，这对下载文件是突然断电等意外是有好处的
RandomAccessFile raf = new RandomAccessFile(file, "rwd");//rwd, 实时写到底层设备
//设置临时文件的大小
raf.setLength(length);
raf.close();
//3、计算出每个线程应该下载多少个字节
int size = length/ThreadCount;//如果有余数，负责最后一部分的线程负责下砸
//开启多线程
for (int threadId = 0; threadId < ThreadCount; threadId++) {
//计算每个线程下载的开始位置和结束位置
int startIndex = threadId*size; // 开始 = 线程id * size
int endIndex = (threadId+1)*size - 1; //结束 = （线程id + 1)*size - 1
//如果是最后一个线程，那么结束位置写死为文件结束位置
if (threadId == ThreadCount - 1) {
endIndex = length - 1;
}
//System.out.println("线程"+threadId+"的下载区间是： "+startIndex+"----"+endIndex);
new DownloadThread(startIndex,endIndex,threadId).start();
}
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}
class DownloadThread extends Thread{
private int startIndex;
private int endIndex;
private int threadId;
public DownloadThread(int startIndex, int endIndex, int threadId) {
super();
this.startIndex = startIndex;
this.endIndex = endIndex;
this.threadId = threadId;
}
@Override
public void run() {
//每个线程再次发送http请求，下载自己对应的那部分数据
try {
File progressFile = new File(threadId+".txt");
//判断进度文件是否存在，如果存在，则接着断点继续下载，如果不存在，则从头下载
if (progressFile.exists()) {
FileInputStream fis = new FileInputStream(progressFile);
BufferedReader br = new BufferedReader(new InputStreamReader(fis));
//从进度文件中度取出上一次下载的总进度，然后与原本的开始进度相加，得到新的开始进度
startIndex += Integer.parseInt(br.readLine());
fis.close();
}
System.out.println("线程"+threadId+"的下载区间是:"+startIndex+"----"+endIndex);
//4、每个线程发送http请求自己的数据
URL url = new URL(MultiDownload.path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5000);
conn.setReadTimeout(5000);
//设置本次http请求所请求的数据的区间
conn.setRequestProperty("Range", "bytes="+startIndex+"-"+endIndex);
//请求部分数据，响应码是206
if (conn.getResponseCode()==206) {
//此时，流里只有ThreadCount分之一的原文件数据
InputStream is = conn.getInputStream();
byte[] b = new byte[1024];
int len = 0;
int total = 0;//total 用于保存断点续传的断点
//拿到临时文件的输出流
File file = new File(MultiDownload.filename);
RandomAccessFile raf = new RandomAccessFile(file, "rwd");
//把文件的写入位置移动至 startIndex
raf.seek(startIndex);
while ((len = is.read(b))!=-1) {
//每次读取流里数据之后，同步把数据写入临时文件
raf.write(b, 0, len);
total += len;
//System.out.println("线程" + threadId + "下载了" + total);
//生成一个一个专门用来记录下载进度的临时文件
RandomAccessFile progressRaf = new RandomAccessFile(progressFile, "rwd");
progressRaf.write((total+"").getBytes());
progressRaf.close();
}
System.out.println("线程"+threadId+"下载完了---------------------");
raf.close();
//当所有的线程下载完之后，将进度文件删除
MultiDownload.finishedThread++;
synchronized (MultiDownload.path) {//所有线程使用同一个锁
if (MultiDownload.finishedThread==MultiDownload.ThreadCount) {
for (int i = 0; i < MultiDownload.ThreadCount; i++) {
File f = new File(i+".txt");
f.delete();
}
MultiDownload.finishedThread=0;
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
二、Android手机版带断点续传的多线程下载
Android手机版的带断点续传的多线程下载逻辑与PC版的几乎一样，只不过在Android手机中耗时操作不能放在主线程，网络下载属于耗时操作，所以多线程下载要在Android中开启一个子线程执行。并使用消息队列机制刷新文本进度条。
public class MainActivity extends Activity {
static int ThreadCount = 3;
static int FinishedThread = 0;
int currentProgess;
static String Filename = "QQPlayer.exe";
static String Path = "http://192.168.3.45:8080/"+Filename;
static MainActivity ma;
static ProgressBar pb;
static TextView tv;
static Handler handler = new Handler(){
public void handleMessage(android.os.Message msg){
tv.setText((long)pb.getProgress()*100 /pb.getMax() +"%");
};
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ma = this;
pb = (ProgressBar) findViewById(R.id.pb);
tv = (TextView) findViewById(R.id.tv);
}
public void download(View v){
Thread t = new Thread(){
@Override
public void run() {
//发送http请求获取文件的长度，创建临时文件
try {
URL url= new URL(Path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5000);
conn.setReadTimeout(5000);
if (conn.getResponseCode()==200) {
int length = conn.getContentLength();
//设置进度条的最大值就是原文件的总长度
pb.setMax(length);
//生成一个与原文件相同大小的临时文件
File file = new File(Environment.getExternalStorageDirectory(),Filename);
RandomAccessFile raf = new RandomAccessFile(file, "rwd");
raf.setLength(length);
raf.close();
//计算每个线程需要下载的数据大小
int size = length/ThreadCount;
//开启多线程
for (int threadId = 0; threadId < ThreadCount; threadId++) {
int startIndex = threadId*size;
int endIndex = (threadId + 1)*size - 1;
if (threadId==ThreadCount - 1) {
endIndex = length - 1;
}
new DownloadThread(startIndex, endIndex, threadId).start();
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
};
t.start();
}
class DownloadThread extends Thread{
private int startIndex;
private int endIndex;
private int threadId;
public DownloadThread(int startIndex, int endIndex, int threadId) {
super();
this.startIndex = startIndex;
this.endIndex = endIndex;
this.threadId = threadId;
}
@Override
public void run() {
// 每个线程发送http请求自己的数据
try{
//先判断是不是断点续传
File progessFile = new File(Environment.getExternalStorageDirectory(),threadId+".txt");
if (progessFile.exists()) {
FileReader fr = new FileReader(progessFile);
BufferedReader br = new BufferedReader(fr);
int lastProgess = Integer.parseInt(br.readLine());
startIndex += lastProgess;
//把上次下载的进度显示至进度条
currentProgess +=lastProgess;
pb.setProgress(currentProgess);
//发消息，让主线程刷新文本进度
handler.sendEmptyMessage(1);
br.close();
fr.close();
}
URL url = new URL(Path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5000);
conn.setReadTimeout(5000);
conn.setRequestProperty("Range", "bytes="+startIndex+"-"+endIndex);
if (conn.getResponseCode()==206) {
InputStream is = conn.getInputStream();
byte[] buffer = new byte[1024];
int len = 0;
int total = 0;
File file = new File(Environment.getExternalStorageDirectory(),Filename);
RandomAccessFile raf = new RandomAccessFile(file, "rwd");
raf.seek(startIndex);
while ((len = is.read(buffer))!= -1) {
raf.write(buffer, 0, len);
total += len;
//每次读取流里数据之后，把本次读取的数据的长度显示至进度条
currentProgess += len;
pb.setProgress(currentProgess);
//发消息，让主线程刷新文本进度
handler.sendEmptyMessage(1);
//生成临时文件保存下载进度，用于断点续传，在所有线程现在完毕后删除临时文件
RandomAccessFile progressRaf = new RandomAccessFile(progessFile, "rwd");
progressRaf.write((total+"").getBytes());
progressRaf.close();
}
raf.close();
System.out.println("线程"+threadId+"下载完了");
//当所有线程都下在完了之后，删除临时进度文件
FinishedThread++;
synchronized (Path) {
if (FinishedThread==ThreadCount) {
for (int i = 0; i < ThreadCount; i++) {
File f = new File(Environment.getExternalStorageDirectory(),i+".txt");
f.delete();
}
FinishedThread=0;
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}