实现原理:
用httpConnection.setRequestProperty("RANGE", "bytes= xxx-xxx");获取指定的数据块
用RandomAccessFile实现文件随机访问,写入指定数据块到文件.
关键事项:获取远程文件大小,根据文件大小确定下载线程个数(可固定线程数,也可固定每个线程下载数据块大小,用户自己决定).
package org.hundred.multithreads;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
/**
* 文件下载管理类
*/
public class DownLoadManager {
/**
* 每个线程下载的字节数
*/
static final long unitSize = 1000 * 1024;
/**
* 启动多个线程下载文件
*/
public void doDownload(String remoteFileUrl)
throws IOException {
String fileName = new URL(remoteFileUrl).getFile();
fileName = fileName.substring(fileName.lastIndexOf("/")+1,fileName.length()).replace("%20", " ");
long fileSize = this.getRemoteFileSize(remoteFileUrl);
if(fileSize == 0){
return;
}
this.createFile(fileName, fileSize);
long threadCount = fileSize / unitSize;
System.out.println("共启动 "
+ (fileSize % unitSize == 0 ? threadCount : threadCount + 1)
+ " 个线程");
long offset = 0;
if (fileSize <= unitSize) {// 如果远程文件尺寸小于等于unitSize
DownloadThread downloadThread = new DownloadThread(remoteFileUrl,
fileName, offset, fileSize);
downloadThread.start();
} else {// 如果远程文件尺寸大于unitSize
for (int i = 1; i <= threadCount; i++) {
DownloadThread downloadThread = new DownloadThread(
remoteFileUrl, fileName, offset, unitSize);
downloadThread.start();
offset = offset + unitSize;
}
if (fileSize % unitSize != 0) {// 如果不能整除，则需要再创建一个线程下载剩余字节
DownloadThread downloadThread = new DownloadThread(
remoteFileUrl, fileName, offset, fileSize
- unitSize * threadCount);
downloadThread.start();
}
}
}
/**
* 获取远程文件尺寸
*/
private long getRemoteFileSize(String remoteFileUrl) throws IOException {
long fileSize = 0;
HttpURLConnection httpConnection = (HttpURLConnection) new URL(
remoteFileUrl).openConnection();
httpConnection.setRequestMethod("HEAD");
int responseCode = httpConnection.getResponseCode();
if(responseCode >= 400){
System.out.println("Web服务器响应错误!");
return 0;
}
String sHeader;
for(int i=1;;i++){
sHeader = httpConnection.getHeaderFieldKey(i);
if(sHeader != null && sHeader.equals("Content-Length")){
System.out.println("文件大小ContentLength:"+httpConnection.getContentLength());
fileSize = Long.parseLong(httpConnection.getHeaderField(sHeader));
break;
}
}
return fileSize;
}
/**
* 创建指定大小的文件
*/
private void createFile(String fileName, long fileSize) throws IOException {
File newFile = new File(fileName);
RandomAccessFile raf = new RandomAccessFile(newFile, "rw");
raf.setLength(fileSize);
raf.close();
}
}
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
package org.angus.multithreads;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
/**
* 负责文件下载的类
*/
public class DownloadThread extends Thread {
/**
* 待下载的文件
*/
private String url = null;
/**
* 本地文件名
*/
private String fileName = null;
/**
* 偏移量
*/
private long offset = 0;
/**
* 分配给本线程的下载字节数
*/
private long length = 0;
/**
* @param url 下载文件地址
* @param fileName 另存文件名
* @param offset 本线程下载偏移量
* @param length 本线程下载长度
*
* @author Angus.wang
* */
public DownloadThread(String url, String file, long offset, long length) {
this.url = url;
this.fileName = file;
this.offset = offset;
this.length = length;
System.out.println("偏移量=" + offset + ";字节数=" + length);
}
public void run() {
try {
HttpURLConnection httpConnection = (HttpURLConnection) new URL(
this.url).openConnection();
httpConnection.setRequestMethod("GET");
httpConnection.setRequestProperty("RANGE", "bytes=" + this.offset
+ "-" + (this.offset + this.length - 1));
System.out.println("RANGE bytes=" + this.offset + "-" + (this.offset + this.length - 1));
BufferedInputStream bis = new BufferedInputStream(httpConnection
.getInputStream());
byte[] buff = new byte[1024];
int bytesRead;
File newFile = new File(fileName);
RandomAccessFile raf = new RandomAccessFile(newFile, "rw");
while ((bytesRead = bis.read(buff, 0, buff.length)) != -1) {
raf.seek(this.offset);
raf.write(buff, 0, bytesRead);
this.offset = this.offset + bytesRead;
}
raf.close();
bis.close();
} catch (IOException ioe) {
ioe.printStackTrace();
}
}
}
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
package org.angus.multithreads;
/**
* 多线程文件下载测试
* */
public class FileDownloadTest {
/**
* @param args
*/
public static void main(String[] args) {
try{
String remoteFileUrl = "http://dl.maxthon.cn/cn/mx2/mx_2.5.1.4751cn.exe";
DownLoadManager downLoadManager = new DownLoadManager();
downLoadManager.doDownload(remoteFileUrl);
}catch(Exception e){
e.printStackTrace();
}
}
}
时间： 2024-10-19 22:28:09