这篇文章给大家介绍怎么在Android中利用AsyncTask实现一个多线程断点续传功能，内容非常详细，感兴趣的小伙伴们可以参考借鉴，希望对大家能有所帮助。
一、实现原理
多线程下载首先要通过每个文件总的下载线程数（我这里设定5个）来确定每个线程所负责下载的起止位置。
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long blockLength = mFileLength / DEFAULT_POOL_SIZE;
for (int i = 0; i < DEFAULT_POOL_SIZE; i++) {
long beginPosition = i * blockLength;//每条线程下载的开始位置
long endPosition = (i + 1) * blockLength;//每条线程下载的结束位置
if (i == (DEFAULT_POOL_SIZE - 1)) {
endPosition = mFileLength;//如果整个文件的大小不为线程个数的整数倍，则最后一个线程的结束位置即为文件的总长度
}
......
}这里需要注意的是，文件大小往往不是线程个数的整数倍，所以最后一个线程的结束位置需要设置为文件长度。
确定好每个线程的下载起止位置之后，需要设置http请求头来下载文件的指定位置：
//设置下载的数据位置beginPosition字节到endPosition字节
Header header_size = new BasicHeader("Range", "bytes=" + beginPosition + "-" + endPosition);
request.addHeader(header_size);以上是多线程下载的原理，但是还要实现断点续传需要在每次暂停之后记录每个线程已下载的大小，下次继续下载时从上次下载后的位置开始下载。一般项目中都会存数据库中，我这里为了简单起见直接存在了SharedPreferences中，已下载url和线程编号作为key值。
@Override
protected void onPostExecute(Long aLong) {
Log.i(TAG, "download success ");
//下载完成移除记录
mSharedPreferences.edit().remove(currentThreadIndex).commit();
}
@Override
protected void onCancelled() {
Log.i(TAG, "download cancelled ");
//记录已下载大小current
mSharedPreferences.edit().putLong(currentThreadIndex, current).commit();
}下载的时候，首先获取已下载位置，如果已经下载过，就从上次下载后的位置开始下载：
//获取之前下载保存的信息，从之前结束的位置继续下载
//这里加了判断file.exists()，判断是否被用户删除了，如果文件没有下载完，但是已经被用户删除了，则重新下载
long downedPosition = mSharedPreferences.getLong(currentThreadIndex, 0);
if(file.exists() && downedPosition != 0) {
beginPosition = beginPosition + downedPosition;
current = downedPosition;
synchronized (mCurrentLength) {
mCurrentLength += downedPosition;
}
}二、完整代码
package com.bbk.lling.multithreaddownload;
import android.app.Activity;
import android.content.Context;
import android.content.SharedPreferences;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
import org.apache.http.Header;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicHeader;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.RandomAccessFile;
import java.net.MalformedURLException;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
public class MainActivity extends Activity {
private static final String TAG = "MainActivity";
private static final int DEFAULT_POOL_SIZE = 5;
private static final int GET_LENGTH_SUCCESS = 1;
//下载路径
private String downloadPath = Environment.getExternalStorageDirectory() +
File.separator + "download";
// private String mUrl = "http://ftp.neu.edu.cn/mirrors/eclipse/technology/epp/downloads/release/juno/SR2/eclipse-java-juno-SR2-linux-gtk-x86_64.tar.gz";
private String mUrl = "http://p.gdown.baidu.com/c4cb746699b92c9b6565cc65aa2e086552651f73c5d0e634a51f028e32af6abf3d68079eeb75401c76c9bb301e5fb71c144a704cb1a2f527a2e8ca3d6fe561dc5eaf6538e5b3ab0699308d13fe0b711a817c88b0f85a01a248df82824ace3cd7f2832c7c19173236";
private ProgressBar mProgressBar;
private TextView mPercentTV;
SharedPreferences mSharedPreferences = null;
long mFileLength = 0;
Long mCurrentLength = 0L;
private InnerHandler mHandler = new InnerHandler();
//创建线程池
private Executor mExecutor = Executors.newCachedThreadPool();
private List<DownloadAsyncTask> mTaskList = new ArrayList<DownloadAsyncTask>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mProgressBar = (ProgressBar) findViewById(R.id.progressbar);
mPercentTV = (TextView) findViewById(R.id.percent_tv);
mSharedPreferences = getSharedPreferences("download", Context.MODE_PRIVATE);
//开始下载
findViewById(R.id.begin).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread() {
@Override
public void run() {
//创建存储文件夹
File dir = new File(downloadPath);
if (!dir.exists()) {
dir.mkdir();
}
//获取文件大小
HttpClient client = new DefaultHttpClient();
HttpGet request = new HttpGet(mUrl);
HttpResponse response = null;
try {
response = client.execute(request);
mFileLength = response.getEntity().getContentLength();
} catch (Exception e) {
Log.e(TAG, e.getMessage());
} finally {
if (request != null) {
request.abort();
}
}
Message.obtain(mHandler, GET_LENGTH_SUCCESS).sendToTarget();
}
}.start();
}
});
//暂停下载
findViewById(R.id.end).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
for (DownloadAsyncTask task : mTaskList) {
if (task != null && (task.getStatus() == AsyncTask.Status.RUNNING || !task.isCancelled())) {
task.cancel(true);
}
}
mTaskList.clear();
}
});
}
/**
* 开始下载
* 根据待下载文件大小计算每个线程下载位置，并创建AsyncTask
*/
private void beginDownload() {
mCurrentLength = 0L;
mPercentTV.setVisibility(View.VISIBLE);
mProgressBar.setProgress(0);
long blockLength = mFileLength / DEFAULT_POOL_SIZE;
for (int i = 0; i < DEFAULT_POOL_SIZE; i++) {
long beginPosition = i * blockLength;//每条线程下载的开始位置
long endPosition = (i + 1) * blockLength;//每条线程下载的结束位置
if (i == (DEFAULT_POOL_SIZE - 1)) {
endPosition = mFileLength;//如果整个文件的大小不为线程个数的整数倍，则最后一个线程的结束位置即为文件的总长度
}
DownloadAsyncTask task = new DownloadAsyncTask(beginPosition, endPosition);
mTaskList.add(task);
task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, mUrl, String.valueOf(i));
}
}
/**
* 更新进度条
*/
synchronized public void updateProgress() {
int percent = (int) Math.ceil((float)mCurrentLength / (float)mFileLength * 100);
// Log.i(TAG, "downloading " + mCurrentLength + "," + mFileLength + "," + percent);
if(percent > mProgressBar.getProgress()) {
mProgressBar.setProgress(percent);
mPercentTV.setText("下载进度：" + percent + "%");
if (mProgressBar.getProgress() == mProgressBar.getMax()) {
Toast.makeText(MainActivity.this, "下载结束", Toast.LENGTH_SHORT).show();
}
}
}
@Override
protected void onDestroy() {
for(DownloadAsyncTask task: mTaskList) {
if(task != null && task.getStatus() == AsyncTask.Status.RUNNING) {
task.cancel(true);
}
mTaskList.clear();
}
super.onDestroy();
}
/**
* 下载的AsyncTask
*/
private class DownloadAsyncTask extends AsyncTask<String, Integer , Long> {
private static final String TAG = "DownloadAsyncTask";
private long beginPosition = 0;
private long endPosition = 0;
private long current = 0;
private String currentThreadIndex;
public DownloadAsyncTask(long beginPosition, long endPosition) {
this.beginPosition = beginPosition;
this.endPosition = endPosition;
}
@Override
protected Long doInBackground(String... params) {
Log.i(TAG, "downloading");
String url = params[0];
currentThreadIndex = url + params[1];
if(url == null) {
return null;
}
HttpClient client = new DefaultHttpClient();
HttpGet request = new HttpGet(url);
HttpResponse response = null;
InputStream is = null;
RandomAccessFile fos = null;
OutputStream output = null;
try {
//本地文件
File file = new File(downloadPath + File.separator + url.substring(url.lastIndexOf("/") + 1));
//获取之前下载保存的信息，从之前结束的位置继续下载
//这里加了判断file.exists()，判断是否被用户删除了，如果文件没有下载完，但是已经被用户删除了，则重新下载
long downedPosition = mSharedPreferences.getLong(currentThreadIndex, 0);
if(file.exists() && downedPosition != 0) {
beginPosition = beginPosition + downedPosition;
current = downedPosition;
synchronized (mCurrentLength) {
mCurrentLength += downedPosition;
}
}
//设置下载的数据位置beginPosition字节到endPosition字节
Header header_size = new BasicHeader("Range", "bytes=" + beginPosition + "-" + endPosition);
request.addHeader(header_size);
//执行请求获取下载输入流
response = client.execute(request);
is = response.getEntity().getContent();
//创建文件输出流
fos = new RandomAccessFile(file, "rw");
//从文件的size以后的位置开始写入，其实也不用，直接往后写就可以。有时候多线程下载需要用
fos.seek(beginPosition);
byte buffer [] = new byte[1024];
int inputSize = -1;
while((inputSize = is.read(buffer)) != -1) {
fos.write(buffer, 0, inputSize);
current += inputSize;
synchronized (mCurrentLength) {
mCurrentLength += inputSize;
}
this.publishProgress();
if (isCancelled()) {
return null;
}
}
} catch (MalformedURLException e) {
Log.e(TAG, e.getMessage());
} catch (IOException e) {
Log.e(TAG, e.getMessage());
} finally{
try{
/*if(is != null) {
is.close();
}*/
if (request != null) {
request.abort();
}
if(output != null) {
output.close();
}
if(fos != null) {
fos.close();
}
} catch(Exception e) {
e.printStackTrace();
}
}
return null;
}
@Override
protected void onPreExecute() {
Log.i(TAG, "download begin ");
super.onPreExecute();
}
@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
//更新界面进度条
updateProgress();
}
@Override
protected void onPostExecute(Long aLong) {
Log.i(TAG, "download success ");
//下载完成移除记录
mSharedPreferences.edit().remove(currentThreadIndex).commit();
}
@Override
protected void onCancelled() {
Log.i(TAG, "download cancelled ");
//记录已下载大小current
mSharedPreferences.edit().putLong(currentThreadIndex, current).commit();
}
@Override
protected void onCancelled(Long aLong) {
Log.i(TAG, "download cancelled(Long aLong)");
super.onCancelled(aLong);
mSharedPreferences.edit().putLong(currentThreadIndex, current).commit();
}
}
private class InnerHandler extends Handler {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case GET_LENGTH_SUCCESS :
beginDownload();
break;
}
super.handleMessage(msg);
}
}
}关于怎么在Android中利用AsyncTask实现一个多线程断点续传功能就分享到这里了，希望以上内容可以对大家有一定的帮助，可以学到更多知识。如果觉得文章不错，可以把它分享出去让更多的人看到。
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