基本原理：利用URLConnection获取要下载文件的长度、头部等相关信息，并设置响应的头部信息。并且通过URLConnection获取输入流，将文件分成指定的块，每一块单独开辟一个线程完成数据的读取、写入。通过输入流读取下载文件的信息，然后将读取的信息用RandomAccessFile随机写入到本地文件中。同时，每个线程写入的数据都文件指针也就是写入数据的长度，需要保存在一个临时文件中。这样当本次下载没有完成的时候，下次下载的时候就从这个文件中读取上一次下载的文件长度，然后继续接着上一次的位置开始下载。并且将本次下载的长度写入到这个文件中。
一、下载文件信息类、实体
封装即将下载资源的信息
DownloadInfo.java
package com.hoo.entity;
/**
* <b>function:</b> 下载文件信息类
* @author hoojo
* @createDate 2011-9-21 下午05:14:58
* @file DownloadInfo.java
* @package com.hoo.entity
* @project MultiThreadDownLoad
* @blog http://blog.csdn.net/IBM_hoojo
* @email hoojo_@126.com
* @version 1.0
*/
public class DownloadInfo {
//下载文件url
private String url;
//下载文件名称
private String fileName;
//下载文件路径
private String filePath;
//分成多少段下载， 每一段用一个线程完成下载
private int splitter;
//下载文件默认保存路径
private final static String FILE_PATH = "C:/temp";
//默认分块数、线程数
private final static int SPLITTER_NUM = 5;
public DownloadInfo() {
super();
}
/**
* @param url 下载地址
*/
public DownloadInfo(String url) {
this(url, null, null, SPLITTER_NUM);
}
/**
* @param url 下载地址url
* @param splitter 分成多少段或是多少个线程下载
*/
public DownloadInfo(String url, int splitter) {
this(url, null, null, splitter);
}
/***
* @param url 下载地址
* @param fileName 文件名称
* @param filePath 文件保存路径
* @param splitter 分成多少段或是多少个线程下载
*/
public DownloadInfo(String url, String fileName, String filePath, int splitter) {
super();
if (url == null || "".equals(url)) {
throw new RuntimeException("url is not null!");
}
this.url = url;
this.fileName = (fileName == null || "".equals(fileName)) ? getFileName(url) : fileName;
this.filePath = (filePath == null || "".equals(filePath)) ? FILE_PATH : filePath;
this.splitter = (splitter < 1) ? SPLITTER_NUM : splitter;
}
/**
* <b>function:</b> 通过url获得文件名称
* @author hoojo
* @createDate 2011-9-30 下午05:00:00
* @param url
* @return
*/
private String getFileName(String url) {
return url.substring(url.lastIndexOf("/") + 1, url.length());
}
public String getUrl() {
return url;
}
public void setUrl(String url) {
if (url == null || "".equals(url)) {
throw new RuntimeException("url is not null!");
}
this.url = url;
}
public String getFileName() {
return fileName;
}
public void setFileName(String fileName) {
this.fileName = (fileName == null || "".equals(fileName)) ? getFileName(url) : fileName;
}
public String getFilePath() {
return filePath;
}
public void setFilePath(String filePath) {
this.filePath = (filePath == null || "".equals(filePath)) ? FILE_PATH : filePath;
}
public int getSplitter() {
return splitter;
}
public void setSplitter(int splitter) {
this.splitter = (splitter < 1) ? SPLITTER_NUM : splitter;
}
@Override
public String toString() {
return this.url + "#" + this.fileName + "#" + this.filePath + "#" + this.splitter;
}
}
二、随机写入一段文件
这个类主要是完成向本地的指定文件指针出开始写入文件，并返回当前写入文件的长度（文件指针）。这个类将被线程调用，文件被分成对应的块后，将被线程调用。每个线程都将会调用这个类完成文件的随机写入。
SaveItemFile.java
package com.hoo.download;
import java.io.IOException;
import java.io.RandomAccessFile;
/**
* <b>function:</b> 写入文件、保存文件
* @author hoojo
* @createDate 2011-9-21 下午05:44:02
* @file SaveItemFile.java
* @package com.hoo.download
* @project MultiThreadDownLoad
* @blog http://blog.csdn.net/IBM_hoojo
* @email hoojo_@126.com
* @version 1.0
*/
public class SaveItemFile {
//存储文件
private RandomAccessFile itemFile;
public SaveItemFile() throws IOException {
this("", 0);
}
/**
* @param name 文件路径、名称
* @param pos 写入点位置 position
* @throws IOException
*/
public SaveItemFile(String name, long pos) throws IOException {
itemFile = new RandomAccessFile(name, "rw");
//在指定的pos位置开始写入数据
itemFile.seek(pos);
}
/**
* <b>function:</b> 同步方法写入文件
* @author hoojo
* @createDate 2011-9-26 下午12:21:22
* @param buff 缓冲数组
* @param start 起始位置
* @param length 长度
* @return
*/
public synchronized int write(byte[] buff, int start, int length) {
int i = -1;
try {
itemFile.write(buff, start, length);
i = length;
} catch (IOException e) {
e.printStackTrace();
}
return i;
}
public void close() throws IOException {
if (itemFile != null) {
itemFile.close();
}
}
}
三、单个线程下载文件
这个类主要是完成单个线程的文件下载，将通过URLConnection读取指定url的资源信息。然后用InputStream读取文件内容，然后调用调用SaveItemFile类，向本地写入当前要读取的块的内容。
DownloadFile.java
package com.hoo.download;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import com.hoo.util.LogUtils;
/**
* <b>function:</b> 单线程下载文件
* @author hoojo
* @createDate 2011-9-22 下午02:55:10
* @file DownloadFile.java
* @package com.hoo.download
* @project MultiThreadDownLoad
* @blog http://blog.csdn.net/IBM_hoojo
* @email hoojo_@126.com
* @version 1.0
*/
public class DownloadFile extends Thread {
//下载文件url
private String url;
//下载文件起始位置
private long startPos;
//下载文件结束位置
private long endPos;
//线程id
private int threadId;
//下载是否完成
private boolean isDownloadOver = false;
private SaveItemFile itemFile;
private static final int BUFF_LENGTH = 1024 * 8;
/**
* @param url 下载文件url
* @param name 文件名称
* @param startPos 下载文件起点
* @param endPos 下载文件结束点
* @param threadId 线程id
* @throws IOException
*/
public DownloadFile(String url, String name, long startPos, long endPos, int threadId) throws IOException {
super();
this.url = url;
this.startPos = startPos;
this.endPos = endPos;
this.threadId = threadId;
//分块下载写入文件内容
this.itemFile = new SaveItemFile(name, startPos);
}
@Override
public void run() {
while (endPos > startPos && !isDownloadOver) {
try {
URL url = new URL(this.url);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
// 设置连接超时时间为10000ms
conn.setConnectTimeout(10000);
// 设置读取数据超时时间为10000ms
conn.setReadTimeout(10000);
setHeader(conn);
String property = "bytes=" + startPos + "-";
conn.setRequestProperty("RANGE", property);
//输出log信息
LogUtils.log("开始 " + threadId + "：" + property + endPos);
//printHeader(conn);
//获取文件输入流，读取文件内容
InputStream is = conn.getInputStream();
byte[] buff = new byte[BUFF_LENGTH];
int length = -1;
LogUtils.log("#start#Thread: " + threadId + ", startPos: " + startPos + ", endPos: " + endPos);
while ((length = is.read(buff)) > 0 && startPos < endPos && !isDownloadOver) {
//写入文件内容，返回最后写入的长度
startPos += itemFile.write(buff, 0, length);
}
LogUtils.log("#over#Thread: " + threadId + ", startPos: " + startPos + ", endPos: " + endPos);
LogUtils.log("Thread " + threadId + " is execute over!");
this.isDownloadOver = true;
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if (itemFile != null) {
itemFile.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
if (endPos < startPos && !isDownloadOver) {
LogUtils.log("Thread " + threadId + " startPos > endPos, not need download file !");
this.isDownloadOver = true;
}
if (endPos == startPos && !isDownloadOver) {
LogUtils.log("Thread " + threadId + " startPos = endPos, not need download file !");
this.isDownloadOver = true;
}
}
/**
* <b>function:</b> 打印下载文件头部信息
* @author hoojo
* @createDate 2011-9-22 下午05:44:35
* @param conn HttpURLConnection
*/
public static void printHeader(URLConnection conn) {
int i = 1;
while (true) {
String header = conn.getHeaderFieldKey(i);
i++;
if (header != null) {
LogUtils.info(header + ":" + conn.getHeaderField(i));
} else {
break;
}
}
}
/**
* <b>function:</b> 设置URLConnection的头部信息，伪装请求信息
* @author hoojo
* @createDate 2011-9-28 下午05:29:43
* @param con
*/
public static void setHeader(URLConnection conn) {
conn.setRequestProperty("User-Agent", "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.3) Gecko/2008092510 Ubuntu/8.04 (hardy) Firefox/3.0.3");
conn.setRequestProperty("Accept-Language", "en-us,en;q=0.7,zh-cn;q=0.3");
conn.setRequestProperty("Accept-Encoding", "utf-8");
conn.setRequestProperty("Accept-Charset", "ISO-8859-1,utf-8;q=0.7,*;q=0.7");
conn.setRequestProperty("Keep-Alive", "300");
conn.setRequestProperty("connnection", "keep-alive");
conn.setRequestProperty("If-Modified-Since", "Fri, 02 Jan 2009 17:00:05 GMT");
conn.setRequestProperty("If-None-Match", "\"1261d8-4290-df64d224\"");
conn.setRequestProperty("Cache-conntrol", "max-age=0");
conn.setRequestProperty("Referer", "http://www.baidu.com");
}
public boolean isDownloadOver() {
return isDownloadOver;
}
public long getStartPos() {
return startPos;
}
public long getEndPos() {
return endPos;
}
}
四、分段多线程写入文件内容
这个类主要是完成读取指定url资源的内容，获取该资源的长度。然后将该资源分成指定的块数，将每块的起始下载位置、结束下载位置，分别保存在一个数组中。每块都单独开辟一个独立线程开始下载。在开始下载之前，需要创建一个临时文件，写入当前下载线程的开始下载指针位置和结束下载指针位置。
BatchDownloadFile.java
package com.hoo.download;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import com.hoo.entity.DownloadInfo;
import com.hoo.util.LogUtils;
/**
* <b>function:</b> 分批量下载文件
* @author hoojo
* @createDate 2011-9-22 下午05:51:54
* @file BatchDownloadFile.java
* @package com.hoo.download
* @project MultiThreadDownLoad
* @blog http://blog.csdn.net/IBM_hoojo
* @email hoojo_@126.com
* @version 1.0
*/
public class BatchDownloadFile implements Runnable {
//下载文件信息
private DownloadInfo downloadInfo;
//一组开始下载位置
private long[] startPos;
//一组结束下载位置
private long[] endPos;
//休眠时间
private static final int SLEEP_SECONDS = 500;
//子线程下载
private DownloadFile[] fileItem;
//文件长度
private int length;
//是否第一个文件
private boolean first = true;
//是否停止下载
private boolean stop = false;
//临时文件信息
private File tempFile;
public BatchDownloadFile(DownloadInfo downloadInfo) {
this.downloadInfo = downloadInfo;
String tempPath = this.downloadInfo.getFilePath() + File.separator + downloadInfo.getFileName() + ".position";
tempFile = new File(tempPath);
//如果存在读入点位置的文件
if (tempFile.exists()) {
first = false;
//就直接读取内容
try {
readPosInfo();
} catch (IOException e) {
e.printStackTrace();
}
} else {
//数组的长度就要分成多少段的数量
startPos = new long[downloadInfo.getSplitter()];
endPos = new long[downloadInfo.getSplitter()];
}
}
@Override
public void run() {
//首次下载，获取下载文件长度
if (first) {
length = this.getFileSize();//获取文件长度
if (length == -1) {
LogUtils.log("file length is know!");
stop = true;
} else if (length == -2) {
LogUtils.log("read file length is error!");
stop = true;
} else if (length > 0) {
/**
* eg
* start: 1, 3, 5, 7, 9
* end: 3, 5, 7, 9, length
*/
for (int i = 0, len = startPos.length; i < len; i++) {
int size = i * (length / len);
startPos[i] = size;
//设置最后一个结束点的位置
if (i == len - 1) {
endPos[i] = length;
} else {
size = (i + 1) * (length / len);
endPos[i] = size;
}
LogUtils.log("start-end Position[" + i + "]: " + startPos[i] + "-" + endPos[i]);
}
} else {
LogUtils.log("get file length is error, download is stop!");
stop = true;
}
}
//子线程开始下载
if (!stop) {
//创建单线程下载对象数组
fileItem = new DownloadFile[startPos.length];//startPos.length = downloadInfo.getSplitter()
for (int i = 0; i < startPos.length; i++) {
try {
//创建指定个数单线程下载对象，每个线程独立完成指定块内容的下载
fileItem[i] = new DownloadFile(
downloadInfo.getUrl(),
this.downloadInfo.getFilePath() + File.separator + downloadInfo.getFileName(),
startPos[i], endPos[i], i
);
fileItem[i].start();//启动线程，开始下载
LogUtils.log("Thread: " + i + ", startPos: " + startPos[i] + ", endPos: " + endPos[i]);
} catch (IOException e) {
e.printStackTrace();
}
}
//循环写入下载文件长度信息
while (!stop) {
try {
writePosInfo();
LogUtils.log("downloading……");
Thread.sleep(SLEEP_SECONDS);
stop = true;
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int i = 0; i < startPos.length; i++) {
if (!fileItem[i].isDownloadOver()) {
stop = false;
break;
}
}
}
LogUtils.info("Download task is finished!");
}
}
/**
* 将写入点数据保存在临时文件中
* @author hoojo
* @createDate 2011-9-23 下午05:25:37
* @throws IOException
*/
private void writePosInfo() throws IOException {
DataOutputStream dos = new DataOutputStream(new FileOutputStream(tempFile));
dos.writeInt(startPos.length);
for (int i = 0; i < startPos.length; i++) {
dos.writeLong(fileItem[i].getStartPos());
dos.writeLong(fileItem[i].getEndPos());
//LogUtils.info("[" + fileItem[i].getStartPos() + "#" + fileItem[i].getEndPos() + "]");
}
dos.close();
}
/**
* <b>function:</b>读取写入点的位置信息
* @author hoojo
* @createDate 2011-9-23 下午05:30:29
* @throws IOException
*/
private void readPosInfo() throws IOException {
DataInputStream dis = new DataInputStream(new FileInputStream(tempFile));
int startPosLength = dis.readInt();
startPos = new long[startPosLength];
endPos = new long[startPosLength];
for (int i = 0; i < startPosLength; i++) {
startPos[i] = dis.readLong();
endPos[i] = dis.readLong();
}
dis.close();
}
/**
* <b>function:</b> 获取下载文件的长度
* @author hoojo
* @createDate 2011-9-26 下午12:15:08
* @return
*/
private int getFileSize() {
int fileLength = -1;
try {
URL url = new URL(this.downloadInfo.getUrl());
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
DownloadFile.setHeader(conn);
int stateCode = conn.getResponseCode();
//判断http status是否为HTTP/1.1 206 Partial Content或者200 OK
if (stateCode != HttpURLConnection.HTTP_OK && stateCode != HttpURLConnection.HTTP_PARTIAL) {
LogUtils.log("Error Code: " + stateCode);
return -2;
} else if (stateCode >= 400) {
LogUtils.log("Error Code: " + stateCode);
return -2;
} else {
//获取长度
fileLength = conn.getContentLength();
LogUtils.log("FileLength: " + fileLength);
}
//读取文件长度
/*for (int i = 1; ; i++) {
String header = conn.getHeaderFieldKey(i);
if (header != null) {
if ("Content-Length".equals(header)) {
fileLength = Integer.parseInt(conn.getHeaderField(i));
break;
}
} else {
break;
}
}
*/
DownloadFile.printHeader(conn);
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return fileLength;
}
}
五、工具类、测试类
日志工具类
LogUtils.java
package com.hoo.util;
/**
* <b>function:</b> 日志工具类
* @author hoojo
* @createDate 2011-9-21 下午05:21:27
* @file LogUtils.java
* @package com.hoo.util
* @project MultiThreadDownLoad
* @blog http://blog.csdn.net/IBM_hoojo
* @email hoojo_@126.com
* @version 1.0
*/
public abstract class LogUtils {
public static void log(Object message) {
System.err.println(message);
}
public static void log(String message) {
System.err.println(message);
}
public static void log(int message) {
System.err.println(message);
}
public static void info(Object message) {
System.out.println(message);
}
public static void info(String message) {
System.out.println(message);
}
public static void info(int message) {
System.out.println(message);
}
}
下载工具类
DownloadUtils.java
package com.hoo.util;
import com.hoo.download.BatchDownloadFile;
import com.hoo.entity.DownloadInfo;
/**
* <b>function:</b> 分块多线程下载工具类
* @author hoojo
* @createDate 2011-9-28 下午05:22:18
* @file DownloadUtils.java
* @package com.hoo.util
* @project MultiThreadDownLoad
* @blog http://blog.csdn.net/IBM_hoojo
* @email hoojo_@126.com
* @version 1.0
*/
public abstract class DownloadUtils {
public static void download(String url) {
DownloadInfo bean = new DownloadInfo(url);
LogUtils.info(bean);
BatchDownloadFile down = new BatchDownloadFile(bean);
new Thread(down).start();
}
public static void download(String url, int threadNum) {
DownloadInfo bean = new DownloadInfo(url, threadNum);
LogUtils.info(bean);
BatchDownloadFile down = new BatchDownloadFile(bean);
new Thread(down).start();
}
public static void download(String url, String fileName, String filePath, int threadNum) {
DownloadInfo bean = new DownloadInfo(url, fileName, filePath, threadNum);
LogUtils.info(bean);
BatchDownloadFile down = new BatchDownloadFile(bean);
new Thread(down).start();
}
}
下载测试类
TestDownloadMain.java
package com.hoo.test;
import com.hoo.util.DownloadUtils;
/**
* <b>function:</b> 下载测试
* @author hoojo
* @createDate 2011-9-23 下午05:49:46
* @file TestDownloadMain.java
* @package com.hoo.download
* @project MultiThreadDownLoad
* @blog http://blog.csdn.net/IBM_hoojo
* @email hoojo_@126.com
* @version 1.0
*/
public class TestDownloadMain {
public static void main(String[] args) {
/*DownloadInfo bean = new DownloadInfo("http://i7.meishichina.com/Health/UploadFiles/201109/2011092116224363.jpg");
System.out.println(bean);
BatchDownloadFile down = new BatchDownloadFile(bean);
new Thread(down).start();*/
//DownloadUtils.download("http://i7.meishichina.com/Health/UploadFiles/201109/2011092116224363.jpg");
DownloadUtils.download("http://mp3.baidu.com/j?j=2&url=http%3A%2F%2Fzhangmenshiting2.baidu.com%2Fdata%2Fmusic%2F1669425%2F%25E9%2599%25B7%25E5%2585%25A5%25E7%2588%25B1%25E9%2587%258C%25E9%259D%25A2.mp3%3Fxcode%3D2ff36fb70737c816553396c56deab3f1", "aa.mp3", "c:/temp", 5);
}
}
多线程下载主要在第三部和第四部，其他的地方还是很好理解。源码中提供相应的注释了，便于理解。
——摘自博客园hoojo