Android实现多线程断点下载的方法

package cn.itcast.download;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.ProtocolException;
import java.net.URL;
import cn.itcast.mutiledownload.StreamTool;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
public class MutiledownloadActivity extends Activity implements OnClickListener {
private ProgressBar pb;
private Button bt;
private TextView tv;
private EditText et;
boolean flag = true;
boolean stopflag = false;
private Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
pb.setProgress(total);
int max = pb.getMax();
if (total >= (max - 1)) {
total = max;
flag = false;
}
int result = total * 100 / max;
tv.setText("当前进度 :" + result + "%");
super.handleMessage(msg);
}
};
int total = 0;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
pb = (ProgressBar) this.findViewById(R.id.pb);
bt = (Button) this.findViewById(R.id.bt);
tv = (TextView) this.findViewById(R.id.tv_process);
et = (EditText) this.findViewById(R.id.et);
bt.setOnClickListener(this);
}
@Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.bt:
// 创建一个子线程 定期的更新ui
if("开始下载".equals(bt.getText().toString())){
bt.setText("暂停");
stopflag = false; //开始下载
}
else {
bt.setText("开始下载");
stopflag = true;
}
new Thread() {
@Override
public void run() {
super.run();
while (flag) {
try {
sleep(1000);
// 如果total > = 文件长度
Message msg = new Message();
handler.sendMessage(msg);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}.start();
 
// 开始执行下载的操作
String path = et.getText().toString().trim();
if ("".equals(path)) {
Toast.makeText(this, "路径不能为空", 1).show();
return;
}
try {
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url
.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5000);
conn.setRequestProperty("User-Agent",
"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)");
int code = conn.getResponseCode();
if (code == 200) {
int len = conn.getContentLength();
RandomAccessFile file = new RandomAccessFile(
"/mnt/sdcard/" + getFilenName(path), "rwd");
// 1.设置本地文件大小跟服务器的文件大小一致
file.setLength(len);
// 设置进度条的最大值
pb.setMax(len);
// 2 .假设开启3 个线程
int threadnumber = 3;
int blocksize = len / threadnumber;
/**
* 线程1 0~ blocksize 线程2 1*bolocksize ~ 2*blocksize 线程3
* 2*blocksize ~ 文件末尾
*/
for (int i = 0; i < threadnumber; i++) {
int startposition = i * blocksize;
int endpositon = (i + 1) * blocksize;
if (i == (threadnumber - 1)) {
// 最后一个线程
endpositon = len;
}
DownLoadTask task = new DownLoadTask(i, path,
startposition, endpositon);
task.start();
}
}
} catch (Exception e) {
Toast.makeText(this, "下载出现异常", 0).show();
e.printStackTrace();
}
break;
}
}
class DownLoadTask extends Thread {
int threadid;
String filepath;
int startposition;
int endpositon;
public DownLoadTask(int threadid, String filepath, int startposition,
int endpositon) {
this.threadid = threadid;
this.filepath = filepath;
this.startposition = startposition;
this.endpositon = endpositon;
}
@Override
public void run() {
try {
File postionfile = new File("/mnt/sdcard/" + threadid + ".txt");
URL url = new URL(filepath);
HttpURLConnection conn = (HttpURLConnection) url
.openConnection();
System.out.println("线程" + threadid + "正在下载 " + "开始位置 : "
+ startposition + "结束位置 " + endpositon);
if (postionfile.exists()) {
FileInputStream fis = new FileInputStream(postionfile);
byte[] result = StreamTool.getBytes(fis);
String str = new String(result);
if (!"".equals(str)) {
int newstartposition = Integer.parseInt(str);
if (newstartposition > startposition) {
startposition = newstartposition;
}
}
}
// "Range", "bytes=2097152-4194303")
conn.setRequestProperty("Range", "bytes=" + startposition + "-"
+ endpositon);
conn.setRequestMethod("GET");
conn.setConnectTimeout(5000);
conn.setRequestProperty("User-Agent",
"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)");
InputStream is = conn.getInputStream();
RandomAccessFile file = new RandomAccessFile("/mnt/sdcard/"
+ getFilenName(filepath), "rwd");
// 设置 数据从文件哪个位置开始写
file.seek(startposition);
byte[] buffer = new byte[1024];
int len = 0;
// 代表当前读到的服务器数据的位置 ,同时这个值已经存储的文件的位置
int currentPostion = startposition;
// 创建一个文件对象 ,记录当前某个文件的下载位置
while ((len = is.read(buffer)) != -1) {
if (stopflag) {
return;
}
file.write(buffer, 0, len);
synchronized (MutiledownloadActivity.this) {
total += len;
}
currentPostion += len;
// 需要把currentPostion 信息给持久化到存储设备
String position = currentPostion + "";
FileOutputStream fos = new FileOutputStream(postionfile);
fos.write(position.getBytes());
fos.flush();
fos.close();
}
file.close();
System.out.println("线程" + threadid + "下载完毕");
// 当线程下载完毕后 把文件删除掉
if (postionfile.exists()) {
postionfile.delete();
}
} catch (Exception e) {
e.printStackTrace();
}
super.run();
}
}
public String getFilenName(String path) {
int start = path.lastIndexOf("/") + 1;
return path.substring(start, path.length());
}
}