0.使用多线程下载会提升文件下载的速度，那么多线程下载文件的过程是：

(1)首先获得下载文件的长度，然后设置本地文件的长度

HttpURLConnection.getContentLength();

RandomAccessFile file = new RandomAccessFile("QQWubiSetup.exe","rwd");

file.setLength(filesize);//设置本地文件的长度

(2)根据文件长度和线程数计算每条线程下载的数据长度和下载位置。

如：文件的长度为6M，线程数为3，那么，每条线程下载的数据长度为2M，每条线程开始下载的位置如下图所示。

例如10M大小，使用3个线程来下载，

线程下载的数据长度 (10%3 == 0 ？ 10/3:10/3+1) ,第1，2个线程下载长度是4M，第三个线程下载长度为2M

下载开始位置：线程id*每条线程下载的数据长度 = ?

下载结束位置：（线程id+1）*每条线程下载的数据长度-1=?

(3)使用Http的Range头字段指定每条线程从文件的什么位置开始下载，下载到什么位置为止，

如：指定从文件的2M位置开始下载，下载到位置(4M-1byte)为止

代码如下：HttpURLConnection.setRequestProperty("Range", "bytes=2097152-4194303");

(4)保存文件，使用RandomAccessFile类指定每条线程从本地文件的什么位置开始写入数据。

RandomAccessFile threadfile = new RandomAccessFile("QQWubiSetup.exe ","rwd");

threadfile.seek(2097152);//从文件的什么位置开始写入数据

1.多线程下载的核心代码示例

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public class MulThreadDownload
{
/**
* 多线程下载
* @param args
*/
public static void main(String[] args)
{
String path = "http://net.hoo.com/QQWubiSetup.exe";
try
{
new MulThreadDownload().download(path, 3);
}
catch (Exception e)
{
e.printStackTrace();
}
}
/**
* 从路径中获取文件名称
* @param path 下载路径
* @return
*/
public static String getFilename(String path)
{
return path.substring(path.lastIndexOf('/')+1);
}
/**
* 下载文件
* @param path 下载路径
* @param threadsize 线程数
*/
public void download(String path, int threadsize) throws Exception
{
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5 * 1000);
//获取要下载的文件的长度
int filelength = conn.getContentLength();
//从路径中获取文件名称
String filename = getFilename(path);
File saveFile = new File(filename);
RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");
//设置本地文件的长度和下载文件相同
accessFile.setLength(filelength);
accessFile.close();
//计算每条线程下载的数据长度
int block = filelength%threadsize==0? filelength/threadsize : filelength/threadsize+1;
for(int threadid=0 ; threadid < threadsize ; threadid++){
new DownloadThread(url, saveFile, block, threadid).start();
}
}

private final class DownloadThread extends Thread
{
private URL url;
private File saveFile;
private int block;//每条线程下载的数据长度
private int threadid;//线程id
public DownloadThread(URL url, File saveFile, int block, int threadid)
{
this.url = url;
this.saveFile = saveFile;
this.block = block;
this.threadid = threadid;
}
@Override
public void run()
{
//计算开始位置公式：线程id*每条线程下载的数据长度= ？
//计算结束位置公式：（线程id +1）*每条线程下载的数据长度-1 =?
int startposition = threadid * block;
int endposition = (threadid + 1 ) * block - 1;
try
{
RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");
//设置从什么位置开始写入数据
accessFile.seek(startposition);
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5 * 1000);
conn.setRequestProperty("Range", "bytes="+ startposition+ "-"+ endposition);
InputStream inStream = conn.getInputStream();
byte[] buffer = new byte[1024];
int len = 0;
while( (len=inStream.read(buffer)) != -1 )
{
accessFile.write(buffer, 0, len);
}
inStream.close();
accessFile.close();
System.out.println("线程id:"+ threadid+ "下载完成");
}
catch (Exception e)
{
e.printStackTrace();
}
}
}
}

2.多线程断点下载功能，这里把断点数据保存到数据库中：注意代码注释的理解

(0)主Activity，关键点使用Handler更新进度条与开启线程下载避免ANR

若不使用Handler却要立即更新进度条数据，可使用：

//resultView.invalidate(); UI线程中立即更新进度条方法

//resultView.postInvalidate(); 非UI线程中立即更新进度条方法

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/**
* 注意这里的设计思路，UI线程与参数保存问题，避免ANR问题，UI控件的显示
* @author kay
*
*/
public class DownloadActivity extends Activity
{
private EditText downloadpathText;
private TextView resultView;
private ProgressBar progressBar;

@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

downloadpathText = (EditText) this.findViewById(R.id.downloadpath);
progressBar = (ProgressBar) this.findViewById(R.id.downloadbar);
resultView = (TextView) this.findViewById(R.id.result);
Button button = (Button) this.findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener()
{
@Override
public void onClick(View v)
{
//取得下载路径
String path = downloadpathText.getText().toString();
//判断SDCard是否存在
if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))
{
//下载操作
download(path, Environment.getExternalStorageDirectory());
}
else
{
Toast.makeText(DownloadActivity.this, R.string.sdcarderror, 1).show();
}
}
});
}
//主线程(UI线程)
//业务逻辑正确，但是该程序运行的时候有问题(可能出现ANR问题)
//对于显示控件的界面更新只是由UI线程负责，如果是在非UI线程更新控件的属性值，更新后的显示界面不会反映到屏幕上
/**
* 参数类型：因为启动一个线程还需要使用到上面方法的参数，而主方法启动后很快就会销毁，
* 那么使用Final可以解决参数丢失的问题
* path 注意是Final类型
* savedir 注意是Final类型
*/
private void download(final String path, final File savedir)
{
//这里开启一个线程避免ANR错误
new Thread(new Runnable()
{
@Override
public void run()
{
FileDownloader loader = new FileDownloader(DownloadActivity.this, path, savedir, 3);
//设置进度条的最大刻度为文件的长度
progressBar.setMax(loader.getFileSize());
try
{
loader.download(new DownloadProgressListener()
{
/**
* 注意这里的设计，显示进度条数据需要使用Handler来处理
* 因为非UI线程更新后的数据不能被刷新
*/
@Override
public void onDownloadSize(int size)
{
//实时获知文件已经下载的数据长度
Message msg = new Message();
//设置消息标签
msg.what = 1;
msg.getData().putInt("size", size);
//使用Handler对象发送消息
handler.sendMessage(msg);
}
});
}
catch (Exception e)
{
//发送一个空消息到消息队列
handler.obtainMessage(-1).sendToTarget();
/**
* 或者使用下面的方法发送一个空消息
* Message msg = new Message();
* msg.what = 1;
* handler.sendMessage(msg);
*/
}
}
}).start();
}

/**Handler原理：当Handler被创建时会关联到创建它的当前线程的消息队列，该类用于往消息队列发送消息
*
* 消息队列中的消息由当前线程内部进行处理
*/
private Handler handler = new Handler()
{
//重写Handler里面的handleMessage方法处理消息
@Override
public void handleMessage(Message msg)
{
switch (msg.what)
{
case 1:
//进度条显示
progressBar.setProgress(msg.getData().getInt("size"));
float num = (float)progressBar.getProgress()/(float)progressBar.getMax();
int result = (int)(num*100);
//resultView.invalidate(); UI线程中立即更新进度条方法
//resultView.postInvalidate(); 非UI线程中立即更新进度条方法
resultView.setText(result+ "%");
//判断是否下载成功
if(progressBar.getProgress()==progressBar.getMax())
{
Toast.makeText(DownloadActivity.this, R.string.success, 1).show();
}
break;
case -1:
Toast.makeText(DownloadActivity.this, R.string.error, 1).show();
break;
}
}
};

}

(1)下载类：

注意计算每条线程的下载长度与下载起始位置的方法

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public class DownloadThread extends Thread
{
private static final String TAG = "DownloadThread";
private File saveFile;
private URL downUrl;
private int block;
//下载开始位置
private int threadId = -1;
//下载文件长度
private int downLength;
private boolean finish = false;
private FileDownloader downloader;
public DownloadThread(FileDownloader downloader, URL downUrl, File saveFile, int block, int downLength, int threadId)
{
this.downUrl = downUrl;
this.saveFile = saveFile;
this.block = block;
this.downloader = downloader;
this.threadId = threadId;
this.downLength = downLength;
}

@Override
public void run()
{
//未下载完成
if(downLength < block)
{
try
{
HttpURLConnection http = (HttpURLConnection) downUrl.openConnection();
http.setConnectTimeout(5 * 1000);
http.setRequestMethod("GET");
http.setRequestProperty("Accept", "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");
http.setRequestProperty("Accept-Language", "zh-CN");
http.setRequestProperty("Referer", downUrl.toString());
http.setRequestProperty("Charset", "UTF-8");
//下载开始位置：线程id*每条线程下载的数据长度 = ?
int startPos = block * (threadId - 1) + downLength;
//下载结束位置：（线程id+1）*每条线程下载的数据长度-1=?
int endPos = block * threadId -1;
//设置获取实体数据的范围
http.setRequestProperty("Range", "bytes=" + startPos + "-"+ endPos);
http.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");
http.setRequestProperty("Connection", "Keep-Alive");

InputStream inStream = http.getInputStream();
byte[] buffer = new byte[1024];
int offset = 0;
print("Thread " + this.threadId + " start download from position "+ startPos);
RandomAccessFile threadfile = new RandomAccessFile(this.saveFile, "rwd");
threadfile.seek(startPos);
while ((offset = inStream.read(buffer, 0, 1024)) != -1)
{
threadfile.write(buffer, 0, offset);
downLength += offset;
downloader.update(this.threadId, downLength);
downloader.append(offset);
}
threadfile.close();
inStream.close();
print("Thread " + this.threadId + " download finish");
//标记是否完成
this.finish = true;
}
catch (Exception e)
{
this.downLength = -1;
print("Thread "+ this.threadId+ ":"+ e);
}
}
}

private static void print(String msg)
{
Log.i(TAG, msg);
}

/**
* 下载是否完成
* @return
*/
public boolean isFinish()
{
return finish;
}

/**
* 已经下载的内容大小
* @return 如果返回值为-1,代表下载失败
*/
public long getDownLength()
{
return downLength;
}
}

文件下载器，使用

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/**
* 文件下载器,使用这个类的方法如下示例：
* FileDownloader loader = new FileDownloader(context, "http://browse.babasport.com/ejb3/ActivePort.exe",
* new File("D:\\androidsoft\\test"), 2);
* loader.getFileSize();//得到文件总大小
* try {
* loader.download(new DownloadProgressListener(){
* public void onDownloadSize(int size) {
* print("已经下载："+ size);
* }
* });
* }
* catch (Exception e)
* {
* e.printStackTrace();
* }
*/
public class FileDownloader
{
private static final String TAG = "FileDownloader";
private Context context;
private FileService fileService;
//已下载文件长度
private int downloadSize = 0;
//原始文件长度
private int fileSize = 0;
///线程数
private DownloadThread[] threads;
//本地保存文件
private File saveFile;
//缓存各线程下载的长度
private Map data = new ConcurrentHashMap();
//每条线程下载的长度
private int block;
//下载路径
private String downloadUrl;
//获取线程数
public int getThreadSize()
{
return threads.length;
}
/**
* 获取文件大小
* @return
*/
public int getFileSize()
{
return fileSize;
}
/**
* 累计已下载大小
* @param size
*/
protected synchronized void append(int size)
{
downloadSize += size;
}
/**
* 更新指定线程最后下载的位置
* @param threadId 线程id
* @param pos 最后下载的位置
*/
protected synchronized void update(int threadId, int pos)
{
this.data.put(threadId, pos);
this.fileService.update(this.downloadUrl, this.data);
}
/**
* 文件下载构造器
* @param downloadUrl 下载路径
* @param fileSaveDir 文件保存目录
* @param threadNum 下载线程数
*/
public FileDownloader(Context context, String downloadUrl, File fileSaveDir, int threadNum)
{
try
{
this.context = context;
this.downloadUrl = downloadUrl;
fileService = new FileService(this.context);
URL url = new URL(this.downloadUrl);
if(!fileSaveDir.exists()) fileSaveDir.mkdirs();
this.threads = new DownloadThread[threadNum];
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-flash, application/xaml+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("Referer", downloadUrl);
conn.setRequestProperty("Charset", "UTF-8");
conn.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");
conn.setRequestProperty("Connection", "Keep-Alive");
conn.connect();
printResponseHeader(conn);
if (conn.getResponseCode()==200)
{
//根据响应获取文件大小
this.fileSize = conn.getContentLength();
if (this.fileSize <= 0) throw new RuntimeException("Unkown file size ");
//获取文件名称
String filename = getFileName(conn);
//构建保存文件
this.saveFile = new File(fileSaveDir, filename);
//获取下载记录
Map logdata = fileService.getData(downloadUrl);
//如果存在下载记录
if(logdata.size()>0)
{
//把各条线程已经下载的数据长度放入data中
for(Map.Entry entry : logdata.entrySet())
data.put(entry.getKey(), entry.getValue());
}
//下面计算所有线程已经下载的数据长度
if(this.data.size()==this.threads.length)
{
for (int i = 0; i 0) randOut.setLength(this.fileSize);
randOut.close();
URL url = new URL(this.downloadUrl);
if(this.data.size() != this.threads.length)
{
this.data.clear();
for (int i = 0; i < this.threads.length; i++)
{
//初始化每条线程已经下载的数据长度为0
this.data.put(i+1, 0);
}
}
//开启线程进行下载
for (int i = 0; i < this.threads.length; i++)
{
int downLength = this.data.get(i+1);
//判断线程是否已经完成下载,否则继续下载
if(downLength < this.block && this.downloadSize<this.fileSize)
{
this.threads[i] = new DownloadThread(this, url, this.saveFile, this.block, this.data.get(i+1), i+1);
this.threads[i].setPriority(7);//可删除这条
this.threads[i].start();
}
else
{
this.threads[i] = null;
}
}
this.fileService.save(this.downloadUrl, this.data);
//下载未完成
boolean notFinish = true;
// 循环判断所有线程是否完成下载
while (notFinish)
{
Thread.sleep(900);
//假定全部线程下载完成
notFinish = false;
for (int i = 0; i < this.threads.length; i++)
{
//如果发现线程未完成下载
if (this.threads[i] != null && !this.threads[i].isFinish())
{
//设置标志为下载没有完成
notFinish = true;
//如果下载失败,再重新下载
if(this.threads[i].getDownLength() == -1)
{
this.threads[i] = new DownloadThread(this, url, this.saveFile, this.block, this.data.get(i+1), i+1);
this.threads[i].setPriority(7);
this.threads[i].start();
}
}
}
//通知目前已经下载完成的数据长度
if(listener!=null) listener.onDownloadSize(this.downloadSize);
}
//删除数据库中下载信息
fileService.delete(this.downloadUrl);
}
catch (Exception e)
{
print(e.toString());
throw new Exception("file download fail");
}
return this.downloadSize;
}

/**
* 获取Http响应头字段
* @param http
* @return
*/
public static Map getHttpResponseHeader(HttpURLConnection http) {
Map header = new LinkedHashMap();
for (int i = 0;; i++) {
String mine = http.getHeaderField(i);
if (mine == null) break;
header.put(http.getHeaderFieldKey(i), mine);
}
return header;
}
/**
* 打印Http头字段
* @param http
*/
public static void printResponseHeader(HttpURLConnection http)
{
Map header = getHttpResponseHeader(http);
for(Map.Entry entry : header.entrySet())
{
String key = entry.getKey()!=null ? entry.getKey()+ ":" : "";
print(key+ entry.getValue());
}
}
private static void print(String msg)
{
Log.i(TAG, msg);
}
}

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public interface DownloadProgressListener
{
public void onDownloadSize(int size);
}

(2)文件操作，断点数据库存储

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public class DBOpenHelper extends SQLiteOpenHelper
{
private static final String DBNAME = "itcast.db";
private static final int VERSION = 1;

public DBOpenHelper(Context context)
{
super(context, DBNAME, null, VERSION);
}

@Override
public void onCreate(SQLiteDatabase db)
{
db.execSQL("CREATE TABLE IF NOT EXISTS filedownlog (id integer primary key autoincrement, downpath varchar(100), threadid INTEGER, downlength INTEGER)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
{
db.execSQL("DROP TABLE IF EXISTS filedownlog");
onCreate(db);
}
}

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/**
* 文件下载业务bean
*/
public class FileService
{
private DBOpenHelper openHelper;
public FileService(Context context)
{
openHelper = new DBOpenHelper(context);
}
/**
* 获取每条线程已经下载的文件长度
* @param path
* @return
*/
public Map getData(String path)
{
SQLiteDatabase db = openHelper.getReadableDatabase();
Cursor cursor = db.rawQuery("select threadid, downlength from filedownlog where downpath=?", new String[]{path});
Map data = new HashMap();
while(cursor.moveToNext())
{
data.put(cursor.getInt(0), cursor.getInt(1));
}
cursor.close();
db.close();
return data;
}
/**
* 保存每条线程已经下载的文件长度
* @param path
* @param map
*/
public void save(String path, Map map)
{//int threadid, int position
SQLiteDatabase db = openHelper.getWritableDatabase();
db.beginTransaction();
try
{
for(Map.Entry entry : map.entrySet())
{
db.execSQL("insert into filedownlog(downpath, threadid, downlength) values(?,?,?)",
new Object[]{path, entry.getKey(), entry.getValue()});
}
db.setTransactionSuccessful();
}
finally
{
db.endTransaction();
}
db.close();
}
/**
* 实时更新每条线程已经下载的文件长度
* @param path
* @param map
*/
public void update(String path, Map map)
{
SQLiteDatabase db = openHelper.getWritableDatabase();
db.beginTransaction();
try{
for(Map.Entry entry : map.entrySet()){
db.execSQL("update filedownlog set downlength=? where downpath=? and threadid=?",
new Object[]{entry.getValue(), path, entry.getKey()});
}
db.setTransactionSuccessful();
}finally{
db.endTransaction();
}
db.close();
}
/**
* 当文件下载完成后，删除对应的下载记录
* @param path
*/
public void delete(String path)
{
SQLiteDatabase db = openHelper.getWritableDatabase();
db.execSQL("delete from filedownlog where downpath=?", new Object[]{path});
db.close();
}
}