利用近期學(xué)習(xí)的知識(shí)做一個(gè)下載器，輸入網(wǎng)址即可下載的下載器。
首先復(fù)習(xí)上節(jié)課的知識(shí)。
多線程的那些事：
->何為線程，多線程？
->為什么要使用多線程？
幫助提高效率，耗時(shí)操作并行。多線程可防止一條線卡死。
->ANR（Application Not Responding）
->什么時(shí)候使用多線程？
->Android當(dāng)中的多線程。
總結(jié)來說：線程就相當(dāng)于做飯的過程，OnCreate一步一步按順序執(zhí)行：摘菜->洗菜->切菜->做菜->吃菜。 電飯鍋相當(dāng)于另外一個(gè)線程，幫做米飯，同時(shí)你吃菜，電飯鍋會(huì)在蒸熟米飯后語音或其他方式提醒。
如何使用多線程？
->Main/UI Thread和Worker Thread。
->Main/UI Thread和Worker Thread之間的通信。
->Thread/Runnable.
->線程安全
->Activity.runOnUiThread(Runnable)
->View.post(Runnable)
->View.post Delayed(Runnable,long)
->Handler
->Async Task
->同步代碼塊（Synchronized block）.
下載器的制作方法：
在activity_main.xml文件中做好下載器的下載界面：
下載按鈕：

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/download"
    android:id="@+id/download_button"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true" />

輸入網(wǎng)址的文本框：

<EditText
    android:layout_width="400dp"
    android:layout_height="50dp"
    android:id="@+id/EditText"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_marginTop="84dp" />

進(jìn)度條：

<ProgressBar
    style="?android:attr/progressBarStyleHorizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/progressBar"
    android:max="100"
    android:layout_below="@+id/download_button"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true"
    android:layout_marginTop="72dp" />

進(jìn)度更新顯示：

<TextView
    android:id="@+id/text_view"
    android:layout_width="200dp"
    android:layout_height="wrap_content"
    android:text=" "
    android:textColor="@color/blue"
    android:layout_below="@+id/progressBar"
    android:layout_alignRight="@+id/download_button"
    android:layout_alignEnd="@+id/download_button" />

輸入網(wǎng)址提示：

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceMedium"
    android:text="請(qǐng)輸入網(wǎng)址："
    android:id="@+id/textView"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

在MainActivity.java文件中：

public class MainActivity extends Activity implements View.OnClickListener{
    public static final String GEEK_BAND = "RENHONG";
    private Handler mHandler = new DownloadHandler(this);
    private TextView mTextView;
    private Button mDownloadButton;
    private ProgressBar mProgressBar;
    private EditText mEditText;
    public TextView getmTextView() {
        return mTextView;
    }
    public ProgressBar getmProgressBar() {
        return mProgressBar;
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView) findViewById(R.id.text_view);
        mEditText=(EditText) findViewById(R.id.EditText);
        mDownloadButton = (Button) findViewById(R.id.download_button);
        mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
        mDownloadButton.setOnClickListener(this);
    }
    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.download_button:
                String apkurl=mEditText.getText().toString();
                new TestTask().execute(apkurl);
                break;
        }
    }
    class TestTask extends AsyncTask<String,Integer,String>{
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }
        @Override
        protected String doInBackground(String...prams) {
            try {
                URL url = new URL(prams[0]);
                URLConnection urlConnection = url.openConnection();
                InputStream inputStream = urlConnection.getInputStream();
                int contentLength = urlConnection.getContentLength();
 // 要下載的文件的大小
                String downloadFoldersName = Environment.getExternalStorageDirectory() + File.separator + GEEK_BAND + File.separator;
                File file = new File(downloadFoldersName);
                if(!file.exists()){
                    file.mkdir();
                }
                String fileName = downloadFoldersName + "test.apk";
                File apkFile = new File(fileName);
                if(apkFile.exists()){
                    apkFile.delete();
                }
                int downloadSize = 0;
                byte[] bytes = new byte[1024];
                int length = 0;
                OutputStream outputStream = new FileOutputStream(fileName);
                while ((length = inputStream.read(bytes)) != -1){
                    outputStream.write(bytes, 0, length);
                    downloadSize += length;
                    int progress = downloadSize * 100/ contentLength;
                    Message message = mHandler.obtainMessage();
                    message.obj = progress;
                    message.what = 0;
                    mHandler.sendMessage(message);
                }
                inputStream.close();
                outputStream.close();
            }
       catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }
        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
        }
    }
    public static class DownloadHandler extends Handler{
    public final WeakReference<MainActivity>mActivity;
    public DownloadHandler(MainActivity activity){
        mActivity = new WeakReference<>(activity);
    }
        public void handleMessage(Message msg){
            super.handleMessage(msg);
            MainActivity activity = mActivity.get();
            switch (msg.what){
                case 0:
                    int progress = (int)msg.obj;
                    activity.getmProgressBar().setProgress(progress);
                    activity.getmTextView().setText("已下載："+progress);
                    if (progress == 100){
                        Toast.makeText(activity,"下載完成",Toast.LENGTH_SHORT).show();
                    }
                    break;
            }
        }
    }
}

在Mainifest.xml文件中注冊(cè)網(wǎng)絡(luò)：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" />
<uses-permission android:name="android.permission.INTERNET" />