来源:http://1679554191.iteye.com/admin/blogs/1751065
Android开发中经常会遇到文件的下载,而下载的时间与网络状态和被下载文件的大小等因素有关。本文会对单线程下载和多线程下载做简要说明。无论哪种操作,最终都是基于HTTP(HTTPS)的网络访问。
先看看基本的一个操作流程,然后对于每一个操作步骤逐一实现就可以了。
基本流程:
- 设置连接属性;
- 建立连接;
- 获得需要的资源;
- 释放资源;
说明:这个流程是相对的,每个人可以根据具体情况而定。:)
1、在普通Java项目中下载文件: 为了简化问题,我们先不考虑在Android中实现过程,仅仅实现一个下载并保存网络资源的操作。
代码如下:
import java.io.ByteArrayOutputStream; import java.io.File; import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStream; import java.net.HttpURLConnection; import java.net.URL; public class FileDownload { /** * @param args */ public static void main(String[] args) { downloadFile("http://image.tianjimedia.com/uploadImages/2012/153/2H4Y79KZF2X3.jpg", "image.jpg"); downloadFile("http://www.google.com", "message.txt"); } private static void downloadFile(String urlPath, String filePath) { InputStream inputStream = null; ByteArrayOutputStream byteArrayOutputStream = null; byte[] data = null; try { URL url = new URL(urlPath); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setRequestMethod("GET"); conn.setConnectTimeout(6 * 1000); conn.connect(); if (conn.getResponseCode() == 200) { inputStream = conn.getInputStream(); byteArrayOutputStream = new ByteArrayOutputStream(); byte[] buffer = new byte[1024]; int length = -1; while ((length = inputStream.read(buffer)) != -1) { byteArrayOutputStream.write(buffer, 0, length); } data = byteArrayOutputStream.toByteArray(); } } catch (Exception e) { e.printStackTrace(); } finally { if (inputStream != null) { try { inputStream.close(); } catch (IOException e1) { e1.printStackTrace(); } } if (byteArrayOutputStream != null) { try { byteArrayOutputStream.close(); } catch (IOException e1) { e1.printStackTrace(); } } } if (data != null) { File file = new File(filePath); FileOutputStream outputStream = null; try { outputStream = new FileOutputStream(file); outputStream.write(data); } catch (Exception e) { e.printStackTrace(); } finally { if (outputStream != null) { try { outputStream.close(); } catch (IOException e) { e.printStackTrace(); } } } } } }
说明:
1、InputStream inputStream = null;和ByteArrayOutputStream byteArrayOutputStream = null;之所以在try-catch外定义,是为了可以在finally代码块中可以访问,并释放该资源。在回收资源时,最好把不同资源放在单独的 try-catch块中做资源释放操作,否则可能造成资源泄漏。
InputStream inputStream = null; ByteArrayOutputStream byteArrayOutputStream = null; try { ...... } catch (Exception e) { e.printStackTrace(); } finally { try { if (inputStream != null) { inputStream.close(); } if (byteArrayOutputStream != null) { byteArrayOutputStream.close(); } } catch (Exception e) { e.printStackTrace(); } }
如果采用上述资源释放代码,当inputStream和byteArrayOutputStream都不为null时,如果 inputStream.close();发生异常,将导致byteArrayOutputStream.close();不被执行,从而没有释放该资 源。
2、conn.setRequestMethod("GET");和conn.setConnectTimeout(6 * 1000);是设置连接属性。还有其它属性,如果需要的话,可以进行设定。
3、conn.connect();建立连接。
4、conn.getInputStream();获得连接后的输入流,从中可以获得数据。
5、在finally代码块中对申请的资源进行释放。
运行该项目,会在项目的根目录下看到两个文件,image.jpg和message.txt,如图所示:
2、在Android项目中下载文件:
a、对UI的设计:
主Activity中有两个Button,分别启动另外的两个Activity,来实现图片的下载显示和网页内容的获得并显示。布局文件就补贴出来了,会在附件中提供源码。
b、主Activity没有什么好说的,下面对另外两个Activity做详细说明:
DownloadImageActivity:
package com.anhuioss.download; import java.io.ByteArrayOutputStream; import java.io.File; import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStream; import java.net.HttpURLConnection; import java.net.URL; import android.app.Activity; import android.graphics.drawable.Drawable; import android.os.Bundle; import android.os.Environment; import android.os.Handler; import android.text.TextUtils; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; import android.widget.EditText; import android.widget.ImageView; public class DownloadImageActivity extends Activity implements OnClickListener { private EditText filePathEditText; private Button downloadButton; private ImageView imageDisplay; private Handler handler = new Handler(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_download_image); filePathEditText = (EditText) findViewById(R.id.adi_image_path); downloadButton = (Button) findViewById(R.id.adi_button_download); imageDisplay = (ImageView) findViewById(R.id.adi_image); downloadButton.setOnClickListener(this); } @Override public void onClick(View v) { if (v == downloadButton) { if (!Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED)) { return; } String path = filePathEditText.getText().toString(); if (!TextUtils.isEmpty(path)) { downloadButton.setEnabled(false); imageDisplay.setImageDrawable(null); downloadFile(path, Environment.getExternalStorageDirectory() + File.separator + "image.jpg"); } } } private void downloadFile(final String urlPath, final String filePath) { new Thread(new Runnable() { @Override public void run() { InputStream inputStream = null; ByteArrayOutputStream byteArrayOutputStream = null; byte[] data = null; try { URL url = new URL(urlPath); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setRequestMethod("GET"); conn.setConnectTimeout(6 * 1000); conn.connect(); if (conn.getResponseCode() == 200) { inputStream = conn.getInputStream(); byteArrayOutputStream = new ByteArrayOutputStream(); byte[] buffer = new byte[1024]; int length = -1; while ((length = inputStream.read(buffer)) != -1) { byteArrayOutputStream.write(buffer, 0, length); } data = byteArrayOutputStream.toByteArray(); } } catch (Exception e) { e.printStackTrace(); } finally { if (inputStream != null) { try { inputStream.close(); } catch (IOException e1) { e1.printStackTrace(); } } if (byteArrayOutputStream != null) { try { byteArrayOutputStream.close(); } catch (IOException e1) { e1.printStackTrace(); } } } if (data != null) { File file = new File(filePath); FileOutputStream outputStream = null; try { outputStream = new FileOutputStream(file); outputStream.write(data); handler.post(new Runnable() { @Override public void run() { downloadButton.setEnabled(true); Drawable drawable = null; try { drawable = Drawable.createFromPath(filePath); } catch (Error e) { e.printStackTrace(); } imageDisplay.setImageDrawable(drawable); } }); } catch (Exception e) { e.printStackTrace(); } finally { if (outputStream != null) { try { outputStream.close(); } catch (IOException e) { e.printStackTrace(); } } } } } }).start(); } }
说明:
1、private Handler handler = new Handler();handler的作用是在下载线程中完成下载后在UI线程里面更新UI。
2、由于示例中需要用到SDCARD,所以在做下载操作时,如果没有挂载SDCARD时,不进行下载处理:
if (!Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED)) { return; }
3、
handler.post(new Runnable() { @Override public void run() { downloadButton.setEnabled(true); Drawable drawable = null; try { drawable = Drawable.createFromPath(filePath); } catch (Error e) { e.printStackTrace(); } imageDisplay.setImageDrawable(drawable); } });
利用handler的post方法,在UI线程里面更新UI。首先使能按钮,其次创建drawable,最后显示图片。
3、运行项目,查看效果: 在运行前,不要忘记在Manifest文件中添加网络访问和访问SDCARD的权限哦!
<uses-permission android:name="android.permission.INTERNET" /> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
下载图片的效果如下:
获取网页内容的页面效果:
4、多线程文件下载,直接看普通Java项目中相关操作的代码:
import java.io.IOException; import java.io.InputStream; import java.io.RandomAccessFile; import java.net.HttpURLConnection; import java.net.URL; public class FileDownloadMultiThread { public static void main(String[] args) { downloadFile(); } private static void downloadFile() { String fileName = "mysql.chm"; String urlPath = "http://192.168.1.106/mysql.chm"; // 下载开始前,创建RandomAccessFile并设置文件大小 RandomAccessFile file = null; try { // 创建URL对象 URL url = new URL(urlPath); // 创建HttpURL连接 HttpURLConnection conn = (HttpURLConnection) url.openConnection(); // 设置请求属性 conn.setConnectTimeout(6 * 1000); conn.setRequestMethod("GET"); // 连接 conn.connect(); // 需要下载文件的长度 int fileLength = conn.getContentLength(); // 创建RandomAccessFile并设置文件大小 file = new RandomAccessFile(fileName, "rw"); file.setLength(fileLength); // 启动的线程数量 int threadCount = 6; // 计算每个线程需要下载数据的长度 int threadDownloadLength = fileLength % 3 == 0 ? fileLength / 3 : fileLength / 3 + 1; // 创建threadCount个下载线程 for (int i = 0; i < threadCount; i++) { // 计算开始位置 int startPosition = i * threadDownloadLength; // 创建匿名下载线程对象 new MultiDownloadThread(fileName, urlPath, startPosition).start(); } } catch (Exception e) { e.printStackTrace(); } finally { // 资源释放 if (file != null) { try { file.close(); } catch (IOException e) { e.printStackTrace(); } } } } static class MultiDownloadThread extends Thread { private String urlPath; private int startPosition; private RandomAccessFile accessFile; public MultiDownloadThread(String fileName, String urlPath, int startPosition) { this.urlPath = urlPath; this.startPosition = startPosition; try { accessFile = new RandomAccessFile(fileName, "rw"); accessFile.seek(startPosition); } catch (Exception e) { e.printStackTrace(); } } public void run () { InputStream inputStream = null; try { URL url = new URL(urlPath); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setConnectTimeout(6 * 1000); conn.setRequestMethod("GET"); // 设置HTTP的Range字段,指定该线程从文件的什么位置开始下载 conn.setRequestProperty("Range", "bytes=" + startPosition + "-"); conn.connect(); inputStream = conn.getInputStream(); byte[] buffer = new byte[1024]; int readLength = -1; int totalLength = 0; while ((readLength = inputStream.read(buffer)) != -1) { accessFile.write(buffer, 0, readLength); totalLength = totalLength + readLength; } } catch (Exception e) { e.printStackTrace(); } finally { if (accessFile != null) { try { accessFile.close(); } catch (IOException e) { e.printStackTrace(); } } if (inputStream != null) { try { inputStream.close(); } catch (IOException e) { e.printStackTrace(); } } } } } }
说明:
1、RandomAccessFile类创建目标文件并设置文件大小;
2、HttpURLConnection.setRequestProperty("Range", "bytes=" + startPosition + "-");方法指定下载位置;
3、RandomAccessFile.seek();设置文件指针偏移量,保存下载内容到指定位置;
4、由于线程的创建和销毁过程,线程之间的上下文切换等都需要系统资源,所以threadCount不是越大越好,而是根据具体情况进行设定;
4、在Android上实现多线程下载的方法类似,注意Android环境的特性就可以了,再次不再赘述!:)
5、多说一句: HTTP(HTTPS)的连接到网络上的数据资源的方式和设置多种多样,本例仅仅是从一个角度进行说明,希望对你有所帮助!如果需要Android项目源码,请查看附件!:)