多线程下载是为了抢占服务器的更多资源，达到最快的下载速度，但是手机相比较PC的执行效率还是存在一定的差异，如果开启过多线程，CPU会不堪重负，从而影响整个文件的下载速度也有可能出现未响应。
多线程下载原理：
1.首先读取网络文件的长度，然后在本地生成一个与网络文件长度相等的本地文件
2.开启N多个线程下载文件，计算每条线程下载的数据量，公式：int block=文件长度%N==0 ？ 文件长度/N ：文件长度/N+1
3.开启多线程分别从网络不同的位置下载数据，并从本地文件相同的位置写入数据，要计算出每条线程从网络文件的什么位置开始下载数据，到什么位置结束
实例参考：
1.为实现断点下载，需要实时保存每个线程下载的数据长度，这里使用数据库来进行保存
/**
* 创建数据库，目的是实时保存每个线程下载的字节数，有助于实现断点下载
*
*/
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);
}
/**
* 存放断点记录,记录每一条线程的下载进度
*/
@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);
}
}/**
* 操作数据库业务类
* 保存、更新、删除每个线程的当前下载字节数
* 在第一次下载时进行保存，然后对每一次下载进度进行更新，下载完成后删除数据库保存的下载进度
*/
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<Integer, Integer> getData(String path){
SQLiteDatabase db = openHelper.getReadableDatabase();
Cursor cursor = db.rawQuery("select threadid, downlength from filedownlog where downpath=?", new String[]{path});
Map<Integer, Integer> data = new HashMap<Integer, Integer>();
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<Integer, Integer> map){//int threadid, int position
SQLiteDatabase db = openHelper.getWritableDatabase();
db.beginTransaction();
try{
for(Map.Entry<Integer, Integer> 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
* 调用频率非常高,几百毫秒就会调用一次
* @param path
* @param map
*/
public void update(String path, int threadId, int pos){
SQLiteDatabase db = openHelper.getWritableDatabase();
db.execSQL("update filedownlog set downlength=? where downpath=? and threadid=?",
new Object[]{pos, path, threadId});
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();
}
}import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import android.util.Log;
/**
* 下载线程类，用于在文件下载器中开启多个线程
*
*/
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;
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() {
if(downLength < block){//未下载完成
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");
int startPos = block * (threadId - 1) + downLength;//开始位置
int endPos = block * threadId -1;//结束位置
http.setRequestProperty("Range", "bytes=" + startPos + "-"+ endPos);//设置获取实体数据的范围
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 (!downloader.getExit() && (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");
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;
}
}import java.io.File;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import cn.kafei.service.FileService;
import android.content.Context;
import android.util.Log;
/**
* 文件下载器
*
try {
FileDownloader loader = new FileDownloader(context, "http://192.168.1.110:8080/jyt2013.rar",
new File("D:\\文件\\test"), 2);
loader.getFileSize();//得到文件总大小
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;//文件下载进度保存至数据库
/* 停止下载 */
private boolean exit;
/* 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<Integer, Integer> data = new ConcurrentHashMap<Integer, Integer>();
/* 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;
}
/**
* 退出下载
*/
public void exit(){
this.exit = true;
}
public boolean getExit(){
return this.exit;
}
/**
* 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, threadId, pos);
}
/**
* 构建文件下载器
* @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) {
this.fileSize = conn.getContentLength();//根据响应获取文件大小
if (this.fileSize <= 0) throw new RuntimeException("Unkown file size ");
String filename = getFileName(conn);//获取文件名称
this.saveFile = new File(fileSaveDir, filename);//构建保存文件
Map<Integer, Integer> logdata = fileService.getData(downloadUrl);//获取下载记录
if(logdata.size()>0){//如果存在下载记录
for(Map.Entry<Integer, Integer> entry : logdata.entrySet())
data.put(entry.getKey(), entry.getValue());//把各条线程已经下载的数据长度放入data中
}
if(this.data.size()==this.threads.length){//下面计算所有线程已经下载的数据总长度
for (int i = 0; i < this.threads.length; i++) {
this.downloadSize += this.data.get(i+1);
}
print("已经下载的长度"+ this.downloadSize);
}
//计算每条线程下载的数据长度
this.block = (this.fileSize % this.threads.length)==0? this.fileSize / this.threads.length : this.fileSize / this.threads.length + 1;
}else{
throw new RuntimeException("server no response ");
}
} catch (Exception e) {
print(e.toString());
throw new RuntimeException("don't connection this url");
}
}
/**
* 获取文件名
*/
private String getFileName(HttpURLConnection conn) {
String filename = this.downloadUrl.substring(this.downloadUrl.lastIndexOf('/') + 1);
if(filename==null || "".equals(filename.trim())){//如果获取不到文件名称
for (int i = 0;; i++) {
String mine = conn.getHeaderField(i);
if (mine == null) break;
if("content-disposition".equals(conn.getHeaderFieldKey(i).toLowerCase())){
Matcher m = Pattern.compile(".*filename=(.*)").matcher(mine.toLowerCase());
if(m.find()) return m.group(1);
}
}
filename = UUID.randomUUID()+ ".tmp";//默认取一个文件名
}
return filename;
}
/**
* 开始下载文件
* @param listener 监听下载数量的变化,如果不需要了解实时下载的数量,可以设置为null
* @return 已下载文件大小
* @throws Exception
*/
public int download(DownloadProgressListener listener) throws Exception{
try {
RandomAccessFile randOut = new RandomAccessFile(this.saveFile, "rw");
if(this.fileSize>0) randOut.setLength(this.fileSize);
randOut.close();
URL url = new URL(this.downloadUrl);
if(this.data.size() != this.threads.length){//如果原先未曾下载或者原先的下载线程数与现在的线程数不一致
this.data.clear();
for (int i = 0; i < this.threads.length; i++) {
this.data.put(i+1, 0);//初始化每条线程已经下载的数据长度为0
}
this.downloadSize = 0;
}
for (int i = 0; i < this.threads.length; i++) {//开启线程进行下载
int downLength = this.data.get(i+1);
if(downLength < this.block && this.downloadSize<this.fileSize){//判断线程是否已经完成下载,否则继续下载
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();
}else{
this.threads[i] = null;
}
}
fileService.delete(this.downloadUrl);//如果存在下载记录，删除它们，然后重新添加
fileService.save(this.downloadUrl, this.data);
boolean notFinish = true;//下载未完成
while (notFinish) {// 循环判断所有线程是否完成下载
Thread.sleep(900);
notFinish = false;//假定全部线程下载完成
for (int i = 0; i < this.threads.length; i++){
if (this.threads[i] != null && !this.threads[i].isFinish()) {//如果发现线程未完成下载
notFinish = true;//设置标志为下载没有完成
if(this.threads[i].getDownLength() == -1){//如果下载失败,再重新下载
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();
}
}
}
if(listener!=null) listener.onDownloadSize(this.downloadSize);//通知目前已经下载完成的数据长度
}
if(downloadSize == this.fileSize) fileService.delete(this.downloadUrl);//下载完成删除记录
} catch (Exception e) {
print(e.toString());
throw new Exception("file download error");
}
return this.downloadSize;
}
/**
* 获取Http响应头字段
* @param http
* @return
*/
public static Map<String, String> getHttpResponseHeader(HttpURLConnection http) {
Map<String, String> header = new LinkedHashMap<String, String>();
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<String, String> header = getHttpResponseHeader(http);
for(Map.Entry<String, String> entry : header.entrySet()){
String key = entry.getKey()!=null ? entry.getKey()+ ":" : "";
print(key+ entry.getValue());
}
}
private static void print(String msg){
Log.i(TAG, msg);
}
}/**
* 文件下载进度接口监听器
*/
public interface DownloadProgressListener {
public void onDownloadSize(int size);
}/**
* 执行次序：点击开始按钮==》dowload方法==》DownloadTask（开启子线程）==》UIHander（重绘屏幕显示进度）
*
*/
public class MainActivity extends Activity {
private EditText pathEdit;// 下载路径
private TextView resultView;// 下载进度显示
private Button downloadButton;// 下载按钮
private Button stopButton;//停止按钮
private ProgressBar progressBar;// 进度条
private Handler handler=new UIHander();//handler的作用是用于往创建Hander对象所在的线程所绑定的消息队列发送消息
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
pathEdit = (EditText) this.findViewById(R.id.path);
resultView = (TextView) this.findViewById(R.id.resultView);
downloadButton = (Button) this.findViewById(R.id.downloadbutton);
stopButton = (Button) this.findViewById(R.id.stopbutton);
progressBar = (ProgressBar) this.findViewById(R.id.progressBar);
ButtonClickListener listener=new ButtonClickListener();
stopButton.setOnClickListener(listener);
downloadButton.setOnClickListener(listener);
}
/**
* 开始和停止按钮监听器
*
*/
private final class ButtonClickListener implements View.OnClickListener {
public void onClick(View v) {
switch (v.getId()) {
case R.id.downloadbutton://如果是点击开始按钮就执行下载操作
String path = pathEdit.getText().toString();
if (Environment.getExternalStorageState().equals(
Environment.MEDIA_MOUNTED)) {
File saveDir = Environment.getExternalStorageDirectory();
dowload(path, saveDir);//开始下载
} else {
Toast.makeText(getApplicationContext(), R.string.sdcarderror, 1)
.show();
}
downloadButton.setEnabled(false);
stopButton.setEnabled(true);
break;
case R.id.stopbutton://如果是点击停止按钮，就执行停止下载操作
exit();//停止下载
downloadButton.setEnabled(true);
stopButton.setEnabled(false);
break;
}
}
/*
由于用户的输入事件(点击button, 触摸屏幕....)是由主线程负责处理的，如果主线程处于工作状态，
此时用户产生的输入事件如果没能在5秒内得到处理，系统就会报“应用无响应”错误。
所以在主线程里不能执行一件比较耗时的工作，否则会因主线程阻塞而无法处理用户的输入事件，
导致“应用无响应”错误的出现。耗时的工作应该在子线程里执行。
*/
private DownloadTask task;
/**
* 退出下载
*/
public void exit(){
if(task!=null)task.exit();
}
private void dowload(String path, File saveDir) {
task=new DownloadTask(path,saveDir);//开始下载任务线程类
new Thread(task).start();//开启子线程进行下载
}
/**
* 下载任务线程，主要负责任务的下载
* 把耗时的工作交给子线程
* UI控件画面的重绘（更新）是由主线程负责处理的，
* 如果在子线程中更新UI控件的值，更新后的值不会重绘到屏幕上
* 一定要在主线程里更新UI控件的值，这样才能在屏幕上显示出来
* 不能在子线程中更新UI控件的值
*/
private final class DownloadTask implements Runnable{
private String path;//下载路径
private File saveDir;//存放位置
private FileDownloader loader;//文件下载器
public DownloadTask(String path, File saveDir) {
this.path=path;
this.saveDir=saveDir;
}
/**
* 退出下载
*/
public void exit(){
if(loader!=null)loader.exit();
}
public void run() {
try {
loader = new FileDownloader(
getApplicationContext(), path, saveDir, 2);//2表示开启的下载线程数，因为手机的执行效率有限，不建议开启过多线程
progressBar.setMax(loader.getFileSize());// 得到文件总大小,设置为进度条的最大刻度
//监听下载文件大小变化
loader.download(new DownloadProgressListener() {
public void onDownloadSize(int size) {
Message msg=new Message();//消息对象
msg.what=1;//定义消息标识ID为1
msg.getData().putInt("size", size);
handler.sendMessage(msg);//将消息发送出去
}
});
} catch (Exception e) {
e.printStackTrace();
handler.sendMessage(handler.obtainMessage(-1));//-1表示定义的消息标识ID
}
}
}
}
/**
* 在下载过程中，实现不断的重绘UI进度的值
* 如果把显示进度放在子线程中，重绘不起作用
* 因为重绘UI只能在主线程中进行
*/
private final class UIHander extends Handler{
public void handleMessage(Message msg){
switch (msg.what) {
case 1://如果是1，则表示重绘UI界面的下载进度
int size=msg.getData().getInt("size");//从消息队列中取出当前已下载的文件大小，作为进度条的当前刻度
progressBar.setProgress(size);//进度条的当前刻度
float num=(float)progressBar.getProgress()/(float)progressBar.getMax();//计算当前下载进度
int result=(int)(num*100);
resultView.setText(result+"%");
//如果下载的进度等于进度条的最大刻度。说明下载完成，给出提示
if(progressBar.getProgress()==progressBar.getMax()){
Toast.makeText(getApplicationContext(), R.string.success, 1).show();
}
break;
case -1://如果下载出错，给出提示
Toast.makeText(getApplicationContext(), R.string.error, 1).show();
break;
}
}
}
}界面如下图所示：
源码下载：android多线程断点下载
