Android多线程实现文件断点下载详解编程语言

download_main_layout.xml:

<LinearLayout xmlns:android=”http://schemas.android.com/apk/res/android”
    xmlns:tools=”http://schemas.android.com/tools”
    android:layout_width=”match_parent”
    android:layout_height=”match_parent”
    android:orientation=”vertical” >

    <EditText
        android:id=”@+id/down_load_edt”
        android:layout_width=”match_parent”
        android:layout_height=”wrap_content”
        android:layout_marginTop=”20dp”
        android:hint=”请输入下载文件的网址!” />

    <Button
        android:layout_marginTop=”20dp”
        android:id=”@+id/down_load_btn”
        android:layout_width=”match_parent”
        android:layout_height=”wrap_content”
        android:text=”点击下载” />

    <ProgressBar
        android:id=”@+id/down_load_pbar”
        style=”?android:attr/progressBarStyleHorizontal”
        android:layout_width=”match_parent”
        android:layout_height=”wrap_content” />

</LinearLayout>

下载入口Activity:

public class MainActivity extends Activity implements OnClickListener {
/**下载文件按钮 */
private Button downLoadBtn;
/**下载文件地址 */
private EditText downLoadEdt;
/**下载进度 */
private ProgressBar downLoadPbar;
/**定义发送消息的字段*/
private static final int MESSAGE_NUMBER = 1;
/**所要下载文件总大小 */
private int fileSize;
/**已下载大小 */
private int downLoadSize;
/**所下载文件的保存路径 */
private String path;
/**更新进度条的值 */
private Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if (msg.what == MESSAGE_NUMBER) {
/**设置进度*/
downLoadPbar.setProgress(Double.valueOf(downLoadSize * 1.0 / fileSize * 100).intValue());
}
}

};

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.download_activity_main);
findViews();
setListeners();
initdata();
}

private void findViews() {
downLoadEdt=(EditText) findViewById(R.id.down_load_edt);
downLoadBtn = (Button) findViewById(R.id.down_load_btn);
downLoadPbar = (ProgressBar) findViewById(R.id.down_load_pbar);
}

private void setListeners() {
downLoadBtn.setOnClickListener(this);
}

private void initdata() {
downLoadPbar.setVisibility(View.GONE);
downLoadPbar.setMax(100);
downLoadPbar.setProgress(0);
}

@Override
public void onClick(View v) {
/**获取SDcard */
path = Environment.getExternalStorageDirectory() + “/downloads/”;
File file = new File(path);
if (!file.exists()) {
file.mkdir();
}
/**下载操作 */
new Thread(new DownLoadTask()).start();
/**显示进度条 */
downLoadPbar.setVisibility(View.VISIBLE);
}

/**子线程,计算下载量,更新UI */
class DownLoadTask implements Runnable {
/**线程块大小,每个线程的下载量 */
private int blockSize;
/** 默认为5个线程 */
private int threadNum = 5;
/** 下载后的文件名 */
private String fileName = “myDownLoad.zip”;

@Override
public void run() {
/**数组保存线程对象，便于后面的每个线程下载量计算总和 */
DownLoadThread[] fileDownLoads = new DownLoadThread[threadNum];
/** 计算总大小 */
URL url;
try {
url = new URL(downLoadEdt.getText().toString().trim());
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
/**计算下载量 */
fileSize = conn.getContentLength();
/** 计算每个线程的下载量 */
blockSize = fileSize / threadNum;
/** 执行下载操作 */
for (int i = 0; i < threadNum; i++) {
/** 每个线程的开始位置 */
int begin = i * blockSize;
/** 每个线程的结束位置 */
int end = (i + 1) * blockSize – 1;
DownLoadThread thread = new DownLoadThread(url, begin, end, path + fileName);
thread.start();
fileDownLoads[i] = thread;
}

/** 更新UI */
boolean flag = false;
while (!flag) {
flag = true;
for (int i = 0; i < threadNum; i++) {
downLoadSize += fileDownLoads[i].getDownloadsize();
if (!fileDownLoads[i].isFinish()) {
flag = false;
}
}
MainActivity.this.handler.sendEmptyMessage(MESSAGE_NUMBER);
Thread.sleep(1000);
}
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}

下载线程工具类：

public class DownLoadThread extends Thread {
/**文件年在Url*/
private URL url;
/**文件下载开始位置*/
private int startPostion;
/**文件下载结束位置*/
private int endPostion;
/**文件下载当前位置*/
private int currentPostion;
/**文件名*/
private String filename;
/**缓冲输入流*/
private BufferedInputStream buffin;
/**写入流，支持断点*/
private RandomAccessFile raf;
/**缓冲大小*/
byte[] buffsize = new byte[1024];
/**已经下载大小*/
private int downloadsize;
/**下载是否完成*/
private boolean isFinish;

public int getDownloadsize() {
return downloadsize;
}
public boolean isFinish() {
return isFinish;
}

public DownLoadThread(URL url, int begin, int end, String filename) {
super();
this.url = url;
this.startPostion = begin;
this.endPostion = end;
this.filename = filename;
}
@Override
public void run() {
try {
/**
* 抽象类 URLConnection 是所有类的超类，它代表应用程序和 URL 之间的通信链接。此类的实例可用于读取和写入此 URL 引用的资源。通常，创建一个到 URL 的连接需要几个步骤： 
* openConnection() connect()对影响到远程资源连接的参数进行操作。 与资源交互；查询头字段和内容。 
* 通过在 URL 上调用 openConnection 方法创建连接对象。 处理设置参数和一般请求属性。 使用 connect 方法建立到远程对象的实际连接。 远程对象变为可用。远程对象的头字段和内容变为可访问。 
*/
URLConnection conn = url.openConnection();
/**指定下载的范围*/
conn.setRequestProperty(“Range”, “bytes=”+startPostion+”-“+endPostion);
/**此URL将在一定的上下文环境中被检验，是否允许用户交互，比如弹出一个认证对话框。如果为 false，那么不允许用户交互。*/
conn.setAllowUserInteraction(true);
/**做缓冲优化处理*/
buffin = new BufferedInputStream(conn.getInputStream());
/**写入本地文件*/
raf = new RandomAccessFile(new File(filename), “rw”);
/**移动到新的位置*/
raf.seek(startPostion);

while(currentPostion<endPostion){//如果当前下载没有结束
int size = buffin.read(buffsize, 0, 1024);
/**已经读完*/
if(size==-1){
break;
}
raf.write(buffsize, 0, size);
currentPostion+=size;
/**下载量*/
downloadsize+=size;
}
isFinish = true;
/**下载完成就要关闭输入流与写入流*/
buffin.close();
raf.close();

} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
super.run();
}

}

不要忘记在清单文件AndroidManifest.xml中设置相应权限：

 <!– 网络访问权限 –>

    <uses-permission android:name=”android.permission.INTERNET” />

    <!– sdcard权限 –>

    <uses-permission android:name=”android.permission.MOUNT_UNMOUNT_FILESYSTEMS” />

    <uses-permission android:name=”android.permission.WRITE_EXTERNAL_STORAGE” />