java实现一个多线程下载工具类

URL对象中的前面几个方法都非常容易理解，而该对象提供的openStream()方法可以就读该URL资源的InputStream，通过该方法可以非常方便地读取远程资源--甚至实现多线程下载。

import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;


public class DownUtil {
//定义下载资源的路径
private String path;
//指定所下载的文件的保存位置
private String targetFile;
//定义需要使用多少个线程下载资源
private int threadNum;
//定义下载的线程对象
private DownThread[] threads;
//定义下载的文件的总大小
private int fileSize;


public DownUtil(String path,String targetFile,int threadNum){
this.path=path;
this.threadNum =threadNum;
//初始化threads数组
threads =new DownThread(threadNum);
this.targetFile=targetFile;
}

public void download() throws Exception{
URL url =new URL(path);
HttpURLConnection conn =(HttpURLConnection) url.openConnection();
conn.setConnectTimeout(5*1000);
conn.setRequestMethod("GET");
conn.setRequestProperty("Accept", 
"image/gif,image/jpeg,image/pjpeg,image/pjpeg"
+"application/x-shockwave-flish,application/xam+xml"
+"application/vnd.ms-xpsdocument,application/x-ms-xbap,"
+"application/x-ms-application,application/vnd.ms-excel,"
+"application/vnd.ms-powerpoint,application/msword,*/*");
conn.setRequestProperty("Accept-Language", "zh-CN");
conn.setRequestProperty("Charset", "UTF-8");
conn.setRequestProperty("Connection", "Keep-Alive");
//得到文件大小
fileSize = conn.getContentLength();
conn.disconnect();
int currentPartSize = fileSize / threadNum+1;
RandomAccessFile file = new RandomAccessFile(targetFile,"rw");
//设置本地文件的大小
file.setLength(fileSize);
file.close();
for (int i=0;i<threadNum;i++){
//计算每个线程下载的开始位置
int startPos = i*currentPartSize;
//每个线程使用一个RandomAccessFile进行下载
RandomAccessFile currentPart = new RandomAccessFile(targetFile,"rw");
//定位该线程的下载位置
currentPart.seek(startPos);
//创建下载线程
threads[i] =new DownThread(startPos,currentPartSize,currentPart);
//启动下载线程
threads[i].start();
}
}

//获取下载的完成百分比
public double getCompleteRate(){
//统计多个线程已经下载的总大小
int sumSize =0;
for (int i=0;i<threadNum;i++){
sumSize+=threads[i].length;
}
//返回一已经完成的百分比
return sumSize*1.0/fileSize;
}
private class DownThread extends Thread{
//当前线程的下载位置
private int startPos;
//定义当前线程负责下载的文件大小
private int currentPartSize;
//当前线程需要下载的文件块
private RandomAccessFile currentPart;
//定义该县城已下载的字节数
public int length;
public DownThread(int startPos,int currentPartSize,RandomAccessFile currentPart){
this.startPos=startPos;
this.currentPartSize=currentPartSize;
this.currentPart=currentPart;

}
public void run(){
try{
URL url =new URL(path);
HttpURLConnection conn=(HttpURLConnection)url.openConnection();
conn.setConnectTimeout(5*1000);
conn.setRequestMethod("GET");
conn.setRequestProperty("Accept", 
"image/gif,image/jpeg,image/pjpeg,image/pjpeg"
+"application/x-shockwave-flish,application/xam+xml"
+"application/vnd.ms-xpsdocument,application/x-ms-xbap,"
+"application/x-ms-application,application/vnd.ms-excel,"
+"application/vnd.ms-powerpoint,application/msword,*/*");
conn.setRequestProperty("Accept-Language", "zh-CN");
conn.setRequestProperty("Charset", "UTF-8");
InputStream inStream = conn.getInputStream();
//跳过startPos个字节，表明该线程只下载自己负责的那部分文件
inStream.skip(this.startPos);
byte[] buffer=new byte[1024];
int hasRead =0;
//读取网络数据，并写入本地文件
while(length<currentPartSize&&(hasRead = inStream.read(buffer))!=-1){
currentPart.write(buffer,0,hasRead);
//累计该线程下载的总大小
length+=hasRead;
}
currentPart.close();
inStream.close();
}
catch(Exception e){
e.printStackTrace();
}
}
}