Android学习笔记（八）——文件下载

文件下载的关键步骤：

1、建立一个URL类对象；

2、用URL类对象打开链接：openConnection();

3、获取其输入流，然后对输入流进行操作即可；

下面的代码实现了下载文本的内容和下载一个文件：

用强调的是一定要在配置文件中允许访问互联网和允许将内容写入sd卡中：

<?xml version="1.0" encoding="utf-8"?><manifest xmlns:android="http://schemas.android.com/apk/res/android"      package="gap.download"      android:versionCode="1"      android:versionName="1.0">    <uses-sdk android:minSdkVersion="4" />    <application android:icon="@drawable/icon" android:label="@string/app_name">        <activity android:name=".DownloadActivity"                  android:label="@string/app_name">            <intent-filter>                <action android:name="android.intent.action.MAIN" />                <category android:name="android.intent.category.LAUNCHER" />            </intent-filter>        </activity>    </application>        <!-- 访问互联网的权限 -->    <uses-permission android:name="android.permission.INTERNET"/>    <!-- 向SD卡写数据的权限 -->    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/></manifest>

一个Activity文件：public class DownloadActivity extends Activity {private Button textfile;private Button mp3file;    /** Called when the activity is first created. */    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        textfile = (Button)findViewById(R.id.download_textfile);        mp3file = (Button)findViewById(R.id.download_mp3file);                textfile.setOnClickListener(new OnClickListener() {@Overridepublic void onClick(View v) {// TODO Auto-generated method stubHttpDownLoader downloader = new HttpDownLoader();String result = downloader.download("http://192.168.19.10:8080/student/1.txt");System.out.println(result);}});                mp3file.setOnClickListener(new OnClickListener() {@Overridepublic void onClick(View v) {// TODO Auto-generated method stubHttpDownLoader downloader = new HttpDownLoader();downloader.downFile("http://192.168.19.10:8080/student/1.txt", "download/", "1.txt");}});    }}

一个下载文件类：package gap.common;import java.io.BufferedReader;import java.io.File;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.net.HttpURLConnection;import java.net.MalformedURLException;import java.net.URL;public class HttpDownLoader {private URL url = null;public String download(String urlStr) {StringBuffer sb = new StringBuffer();String line = null;BufferedReader buffer = null;try {url = new URL(urlStr);HttpURLConnection urlConn = (HttpURLConnection)url.openConnection();buffer = new BufferedReader(new InputStreamReader(urlConn.getInputStream()));while ((line = buffer.readLine()) != null) {sb.append(line);}} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();} finally {try {buffer.close();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();}}return sb.toString();}/** * 该函数返回整形 -1：代表下载文件出错 0：代表下载文件成功 1：代表文件已经存在 */public int downFile(String urlStr, String path, String fileName) {InputStream inputStream = null;try {FileUtils fileUtils = new FileUtils();if (fileUtils.isFileExist(path + fileName)) {return 1;} else {inputStream = getInputStreamFromUrl(urlStr);File resultFile = fileUtils.write2SDFromInput(path,fileName, inputStream);if (resultFile == null) {return -1;}}} catch (Exception e) {e.printStackTrace();return -1;} finally {try {inputStream.close();} catch (Exception e) {e.printStackTrace();}}return 0;}/** * 根据URL得到输入流 *  * @param urlStr * @return * @throws MalformedURLException * @throws IOException */public InputStream getInputStreamFromUrl(String urlStr)throws MalformedURLException, IOException {url = new URL(urlStr);HttpURLConnection urlConn = (HttpURLConnection) url.openConnection();InputStream inputStream = urlConn.getInputStream();return inputStream;}}

一个公用文件操作类：/** * 在SDCARD上面操作文件和目录的方法 * @author geng * */public class FileUtils {private String SDPATH;public FileUtils() {//得到当前外部存储设备的目录SDPATH = Environment.getExternalStorageDirectory() + "/";}public String getSDPATH() {return SDPATH;}/** * 在SD卡上创建文件 * @param fileName * @return * @throws IOException */public File createSDFile(String fileName) throws IOException{File file = new File(SDPATH + fileName);file.createNewFile();return file;}/** * 在SD卡上创建目录 * @param dirName * @return */public File creatSDDir(String dirName) {File dir = new File(SDPATH + dirName);dir.mkdirs();return dir;}/** * 判断文件是否存在,存在返回true * @param fileName * @return */public boolean isFileExist(String fileName){File file = new File(SDPATH + fileName);return file.exists();}/** * 将一个InputStream里面的数据写入到SD卡上 * @param path：要写入的路径 * @param fileName：文件名 * @param input：输入流 * @return */public File write2SDFromInput(String path, String fileName, InputStream input) {File file = null;OutputStream output = null;try{creatSDDir(path);file = createSDFile(path + fileName);output = new FileOutputStream(file);byte buffer [] = new byte[4 * 1024];while((input.read(buffer)) != -1){output.write(buffer);}output.flush();}catch(Exception e){e.printStackTrace();}finally{try{output.close();}catch(Exception e){e.printStackTrace();}}return file;}}