多線程下載是爲了搶佔服務器的更多資源，達到最快的下載速度，但是手機相比較PC的執行效率還是存在一定的差異，如果開啓過多線程，CPU會不堪重負，從而影響整個文件的下載速度也有可能出現未響應。
多線程下載原理：
1.首先讀取網絡文件的長度，然後在本地生成一個與網絡文件長度相等的本地文件
2.開啓N多個線程下載文件，計算每條線程下載的數據量，公式：int block=文件長度%N==0 ？ 文件長度/N ：文件長度/N+1
3.開啓多線程分別從網絡不同的位置下載數據，並從本地文件相同的位置寫入數據，要計算出每條線程從網絡文件的什麼位置開始下載數據，到什麼位置結束
實例參考：
1.爲實現斷點下載，需要實時保存每個線程下載的數據長度，這裏使用數據庫來進行保存
/**
* 創建數據庫，目的是實時保存每個線程下載的字節數，有助於實現斷點下載
*
*/
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);
}
/**
* 存放斷點記錄,記錄每一條線程的下載進度
*/
@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);
}
}/**
* 操作數據庫業務類
* 保存、更新、刪除每個線程的當前下載字節數
* 在第一次下載時進行保存，然後對每一次下載進度進行更新，下載完成後刪除數據庫保存的下載進度
*/
public class FileService {
private DBOpenHelper openHelper;
public FileService(Context context) {
openHelper = new DBOpenHelper(context);
}
/**
* 獲取每條線程已經下載的文件長度
* @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;
}
/**
* 保存每條線程已經下載的文件長度
* @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();
}
/**
* 實時更新每條線程已經下載的文件長度
* 調用頻率非常高,幾百毫秒就會調用一次
* @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();
}
/**
* 當文件下載完成後，刪除對應的下載記錄
* @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;
/**
* 下載線程類，用於在文件下載器中開啓多個線程
*
*/
public class DownloadThread extends Thread {
private static final String TAG = "DownloadThread";
private File saveFile;
private URL downUrl;
private int block;
/* 下載開始位置 */
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){//未下載完成
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;//開始位置
int endPos = block * threadId -1;//結束位置
http.setRequestProperty("Range", "bytes=" + startPos + "-"+ endPos);//設置獲取實體數據的範圍
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);
}
/**
* 下載是否完成
* @return
*/
public boolean isFinish() {
return finish;
}
/**
* 已經下載的內容大小
* @return 如果返回值爲-1,代表下載失敗
*/
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;
/**
* 文件下載器
*
try {
FileDownloader loader = new FileDownloader(context, "http://192.168.1.110:8080/jyt2013.rar",
new File("D:\\文件\\test"), 2);
loader.getFileSize();//得到文件總大小
loader.download(new DownloadProgressListener(){
public void onDownloadSize(int size) {
print("已經下載："+ size);
}
});
} catch (Exception e) {
e.printStackTrace();
}
*/
public class FileDownloader {
private static final String TAG = "FileDownloader";
private Context context;
private FileService fileService;//文件下載進度保存至數據庫
/* 停止下載 */
private boolean exit;
/* 已下載文件長度 */
private int downloadSize = 0;
/* 原始文件長度 */
private int fileSize = 0;
/* 線程數 */
private DownloadThread[] threads;
/* 本地保存文件 */
private File saveFile;
/* 緩存各線程下載的長度*/
private Map<Integer, Integer> data = new ConcurrentHashMap<Integer, Integer>();
/* 每條線程下載的長度 */
private int block;
/* 下載路徑 */
private String downloadUrl;
/**
* 獲取線程數
*/
public int getThreadSize() {
return threads.length;
}
/**
* 退出下載
*/
public void exit(){
this.exit = true;
}
public boolean getExit(){
return this.exit;
}
/**
* 獲取文件大小
* @return
*/
public int getFileSize() {
return fileSize;
}
/**
* 累計已下載大小
* @param size
*/
protected synchronized void append(int size) {
downloadSize += size;
}
/**
* 更新指定線程最後下載的位置
* @param threadId 線程id
* @param pos 最後下載的位置
*/
protected synchronized void update(int threadId, int pos) {
this.data.put(threadId, pos);
this.fileService.update(this.downloadUrl, threadId, pos);
}
/**
* 構建文件下載器
* @param downloadUrl 下載路徑
* @param fileSaveDir 文件保存目錄
* @param threadNum 下載線程數
*/
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();//根據響應獲取文件大小
if (this.fileSize <= 0) throw new RuntimeException("Unkown file size ");
String filename = getFileName(conn);//獲取文件名稱
this.saveFile = new File(fileSaveDir, filename);//構建保存文件
Map<Integer, Integer> logdata = fileService.getData(downloadUrl);//獲取下載記錄
if(logdata.size()>0){//如果存在下載記錄
for(Map.Entry<Integer, Integer> entry : logdata.entrySet())
data.put(entry.getKey(), entry.getValue());//把各條線程已經下載的數據長度放入data中
}
if(this.data.size()==this.threads.length){//下面計算所有線程已經下載的數據總長度
for (int i = 0; i < this.threads.length; i++) {
this.downloadSize += this.data.get(i+1);
}
print("已經下載的長度"+ this.downloadSize);
}
//計算每條線程下載的數據長度
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");
}
}
/**
* 獲取文件名
*/
private String getFileName(HttpURLConnection conn) {
String filename = this.downloadUrl.substring(this.downloadUrl.lastIndexOf('/') + 1);
if(filename==null || "".equals(filename.trim())){//如果獲取不到文件名稱
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";//默認取一個文件名
}
return filename;
}
/**
* 開始下載文件
* @param listener 監聽下載數量的變化,如果不需要了解實時下載的數量,可以設置爲null
* @return 已下載文件大小
* @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){//如果原先未曾下載或者原先的下載線程數與現在的線程數不一致
this.data.clear();
for (int i = 0; i < this.threads.length; i++) {
this.data.put(i+1, 0);//初始化每條線程已經下載的數據長度爲0
}
this.downloadSize = 0;
}
for (int i = 0; i < this.threads.length; i++) {//開啓線程進行下載
int downLength = this.data.get(i+1);
if(downLength < this.block && this.downloadSize<this.fileSize){//判斷線程是否已經完成下載,否則繼續下載
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);//如果存在下載記錄，刪除它們，然後重新添加
fileService.save(this.downloadUrl, this.data);
boolean notFinish = true;//下載未完成
while (notFinish) {// 循環判斷所有線程是否完成下載
Thread.sleep(900);
notFinish = false;//假定全部線程下載完成
for (int i = 0; i < this.threads.length; i++){
if (this.threads[i] != null && !this.threads[i].isFinish()) {//如果發現線程未完成下載
notFinish = true;//設置標誌爲下載沒有完成
if(this.threads[i].getDownLength() == -1){//如果下載失敗,再重新下載
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);//通知目前已經下載完成的數據長度
}
if(downloadSize == this.fileSize) fileService.delete(this.downloadUrl);//下載完成刪除記錄
} catch (Exception e) {
print(e.toString());
throw new Exception("file download error");
}
return this.downloadSize;
}
/**
* 獲取Http響應頭字段
* @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頭字段
* @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);
}
}/**
* 文件下載進度接口監聽器
*/
public interface DownloadProgressListener {
public void onDownloadSize(int size);
}/**
* 執行次序：點擊開始按鈕==》dowload方法==》DownloadTask（開啓子線程）==》UIHander（重繪屏幕顯示進度）
*
*/
public class MainActivity extends Activity {
private EditText pathEdit;// 下載路徑
private TextView resultView;// 下載進度顯示
private Button downloadButton;// 下載按鈕
private Button stopButton;//停止按鈕
private ProgressBar progressBar;// 進度條
private Handler handler=new UIHander();//handler的作用是用於往創建Hander對象所在的線程所綁定的消息隊列發送消息
@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);
}
/**
* 開始和停止按鈕監聽器
*
*/
private final class ButtonClickListener implements View.OnClickListener {
public void onClick(View v) {
switch (v.getId()) {
case R.id.downloadbutton://如果是點擊開始按鈕就執行下載操作
String path = pathEdit.getText().toString();
if (Environment.getExternalStorageState().equals(
Environment.MEDIA_MOUNTED)) {
File saveDir = Environment.getExternalStorageDirectory();
dowload(path, saveDir);//開始下載
} else {
Toast.makeText(getApplicationContext(), R.string.sdcarderror, 1)
.show();
}
downloadButton.setEnabled(false);
stopButton.setEnabled(true);
break;
case R.id.stopbutton://如果是點擊停止按鈕，就執行停止下載操作
exit();//停止下載
downloadButton.setEnabled(true);
stopButton.setEnabled(false);
break;
}
}
/*
由於用戶的輸入事件(點擊button, 觸摸屏幕....)是由主線程負責處理的，如果主線程處於工作狀態，
此時用戶產生的輸入事件如果沒能在5秒內得到處理，系統就會報“應用無響應”錯誤。
所以在主線程裏不能執行一件比較耗時的工作，否則會因主線程阻塞而無法處理用戶的輸入事件，
導致“應用無響應”錯誤的出現。耗時的工作應該在子線程裏執行。
*/
private DownloadTask task;
/**
* 退出下載
*/
public void exit(){
if(task!=null)task.exit();
}
private void dowload(String path, File saveDir) {
task=new DownloadTask(path,saveDir);//開始下載任務線程類
new Thread(task).start();//開啓子線程進行下載
}
/**
* 下載任務線程，主要負責任務的下載
* 把耗時的工作交給子線程
* UI控件畫面的重繪（更新）是由主線程負責處理的，
* 如果在子線程中更新UI控件的值，更新後的值不會重繪到屏幕上
* 一定要在主線程裏更新UI控件的值，這樣才能在屏幕上顯示出來
* 不能在子線程中更新UI控件的值
*/
private final class DownloadTask implements Runnable{
private String path;//下載路徑
private File saveDir;//存放位置
private FileDownloader loader;//文件下載器
public DownloadTask(String path, File saveDir) {
this.path=path;
this.saveDir=saveDir;
}
/**
* 退出下載
*/
public void exit(){
if(loader!=null)loader.exit();
}
public void run() {
try {
loader = new FileDownloader(
getApplicationContext(), path, saveDir, 2);//2表示開啓的下載線程數，因爲手機的執行效率有限，不建議開啓過多線程
progressBar.setMax(loader.getFileSize());// 得到文件總大小,設置爲進度條的最大刻度
//監聽下載文件大小變化
loader.download(new DownloadProgressListener() {
public void onDownloadSize(int size) {
Message msg=new Message();//消息對象
msg.what=1;//定義消息標識ID爲1
msg.getData().putInt("size", size);
handler.sendMessage(msg);//將消息發送出去
}
});
} catch (Exception e) {
e.printStackTrace();
handler.sendMessage(handler.obtainMessage(-1));//-1表示定義的消息標識ID
}
}
}
}
/**
* 在下載過程中，實現不斷的重繪UI進度的值
* 如果把顯示進度放在子線程中，重繪不起作用
* 因爲重繪UI只能在主線程中進行
*/
private final class UIHander extends Handler{
public void handleMessage(Message msg){
switch (msg.what) {
case 1://如果是1，則表示重繪UI界面的下載進度
int size=msg.getData().getInt("size");//從消息隊列中取出當前已下載的文件大小，作爲進度條的當前刻度
progressBar.setProgress(size);//進度條的當前刻度
float num=(float)progressBar.getProgress()/(float)progressBar.getMax();//計算當前下載進度
int result=(int)(num*100);
resultView.setText(result+"%");
//如果下載的進度等於進度條的最大刻度。說明下載完成，給出提示
if(progressBar.getProgress()==progressBar.getMax()){
Toast.makeText(getApplicationContext(), R.string.success, 1).show();
}
break;
case -1://如果下載出錯，給出提示
Toast.makeText(getApplicationContext(), R.string.error, 1).show();
break;
}
}
}
}界面如下圖所示：
源碼下載：android多線程斷點下載
