Android（java）学习笔记159：多线程断点下载的原理（Android实现）

之前在Android（java）学习笔记215中，我们从JavaSE的角度去实现了多线程断点下载，下面从Android角度实现这个断点下载：

1. 新建一个Android工程：

（1）其中我们先实现布局文件activity_main.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"
     tools:context=".MainActivity" >

     <EditText
         android:id="@+id/et_path"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:text="http://192.168.1.100:8080/" />

     <EditText
         android:inputType="number"
         android:id="@+id/et_count"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:hint="请设置下载线程的数量，默认3" />

     <Button
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:onClick="download"
         android:text="下载" />

     <LinearLayout
         android:paddingLeft="5dip"
         android:paddingRight="5dip"
         android:id="@+id/ll_container"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:orientation="vertical" >
     </LinearLayout>

 </LinearLayout>

布局效果如下：

（2）其中的进度条样式pb.xml如下：

<?xml version="1.0" encoding="utf-8"?>
<ProgressBar xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/progressBar1"
    style="?android:attr/progressBarStyleHorizontal"
    android:layout_width="fill_parent"
    android:layout_marginLeft="5dip"
    android:layout_marginRight="5dip"
    android:layout_height="wrap_content" />

2. MainActivity.java:

 package com.itheima.mutiledownloader;

 import java.io.BufferedReader;
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.io.RandomAccessFile;
 import java.net.HttpURLConnection;
 import java.net.MalformedURLException;
 import java.net.ProtocolException;
 import java.net.URL;

 import android.app.Activity;
 import android.os.Bundle;
 import android.os.Environment;
 import android.text.TextUtils;
 import android.view.View;
 import android.widget.EditText;
 import android.widget.LinearLayout;
 import android.widget.ProgressBar;
 import android.widget.Toast;

 public class MainActivity extends Activity {
     private EditText et_path;
     private EditText et_count;
     private LinearLayout ll_container;
     private int threadCount = 3;
     private String path;
     protected int runningThreadCount;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         et_count = (EditText) findViewById(R.id.et_count);
         et_path = (EditText) findViewById(R.id.et_path);
         ll_container = (LinearLayout) findViewById(R.id.ll_container);
     }

     public void download(View view){
         String str_count = et_count.getText().toString().trim();
         path = et_path.getText().toString().trim();
         if(TextUtils.isEmpty(path)&&!path.startsWith("http://")){
             Toast.makeText(this, "下载路径不合法", 0).show();
             return;
         }
         if(!TextUtils.isEmpty(str_count)){
             threadCount = Integer.parseInt(str_count);
         }
         ll_container.removeAllViews();
         for(int i=0;i<threadCount;i++){
             ProgressBar pb = (ProgressBar) View.inflate(this, R.layout.pb,null);
             ll_container.addView(pb);
         }
         new Thread(){
             public void run() {
                 try {
                     URL url = new URL(path);
                     HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                     conn.setRequestMethod("GET");
                     int code = conn.getResponseCode();
                     if(code == 200){
                         int length = conn.getContentLength();
                         System.out.println("服务器文件的大小为："+length);
                         //创建一个空白文件文件的大小和服务器资源一样
                         RandomAccessFile raf = new RandomAccessFile(Environment.getExternalStorageDirectory().getPath()+"/"+getFileName(path), "rw");
                         raf.setLength(length);
                         raf.close();
                         //每个线程下载的平均区块大小
                         int blocksize = length / threadCount;
                         runningThreadCount = threadCount;
                         for(int threadId = 0;threadId<threadCount;threadId++){
                              int startIndex = threadId*blocksize;
                              int endIndex = (threadId+1)*blocksize-1;
                              if(threadId==(threadCount-1)){//最后一个线程的修正
                                  endIndex = length - 1;
                              }
                              new DownloadThread(startIndex, endIndex, threadId).start();
                         }
                     }
                 } catch (Exception e) {
                     e.printStackTrace();
                     showToast("下载失败");
                 }

             };
         }.start();

     }
     class DownloadThread extends Thread{
         /**
          * 理论上第一次开始的位置
          */
         int firstStartIndex;
         int startIndex;
         int endIndex;
         /**
          * 当前线程下载到文件的位置
          */
         int filePosition;
         int threadId;
         /**
          * 当前线程需要下载的总文件大小
          */
         int threadTotal;
         ProgressBar pb;//当前线程对应的进度条
         /**
          *
          * @param startIndex 开始位置
          * @param endIndex 结束位置
          * @param threadId 线程id
          */
         public DownloadThread(int startIndex, int endIndex, int threadId) {
             this.startIndex = startIndex;
             this.firstStartIndex = startIndex;
             this.endIndex = endIndex;
             threadTotal = endIndex - startIndex;
             this.threadId = threadId;
             pb = (ProgressBar) ll_container.getChildAt(threadId);
             pb.setMax(threadTotal);
             filePosition = startIndex;
         }

         @Override
         public void run() {
             //System.out.println("线程："+threadId+"理论下载的位置："+startIndex+"~~~"+endIndex);
             //读取，看看有没有下载的历史数据，读下载的进度
             try {
             File file = new File(Environment.getExternalStorageDirectory().getPath()+"/"+threadId+getFileName(path)+".txt");//用一个文本记录当前线程下载的进程
             if(file.exists()&&file.length()>0){
                 FileInputStream fis = new FileInputStream(file);
                 BufferedReader br = new BufferedReader(new InputStreamReader(fis));
                 filePosition = Integer.parseInt(br.readLine());//上一次下载到文件的哪个位子。
                 startIndex = filePosition;
                 fis.close();
             }
             System.out.println("线程："+threadId+"实际上下载的位置："+startIndex+"~~~"+endIndex);
                 URL url = new URL(path);
                 HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                 conn.setRequestMethod("GET");
                 conn.setRequestProperty("Range", "bytes="+startIndex+"-"+endIndex);
                 int code = conn.getResponseCode();//2XX 成功 3XX重定向 4XX资源找不到 5XX服务器异常
                 if(code == 206){
                     InputStream is = conn.getInputStream();
                     RandomAccessFile raf = new RandomAccessFile(Environment.getExternalStorageDirectory().getPath()+"/"+getFileName(path), "rwd");
                     //一定要记得定位文件写的位置
                     raf.seek(startIndex);
                     byte[] buffer = new byte[1024*4];//缓冲区的大小
                     int len = -1;
                     while((len = is.read(buffer))!=-1){
                         raf.write(buffer, 0, len);
                         filePosition+=len;//记录写入数据的进度
                         int process = filePosition - firstStartIndex;
                         pb.setProgress(process);
                         RandomAccessFile rafinfo = new RandomAccessFile(file, "rwd");
                         rafinfo.write(String.valueOf(filePosition).getBytes());
                         rafinfo.close();
                     }
                     raf.close();
                     is.close();
                     System.out.println("线程:"+threadId+"下载完毕了。");
                     synchronized (MainActivity.this) {//加锁，同步运行，保证下面一段代码在同一个时间片运行，原子性执行
                         runningThreadCount--;
                         if(runningThreadCount==0){
                             System.out.println("所有的线程都下载完毕了");
                             showToast("下载完毕了");
                             for(int i =0;i<threadCount;i++){
                                 File f = new File(Environment.getExternalStorageDirectory().getPath()+"/"+i+getFileName(path)+".txt");
                                 System.out.println(f.delete());
                             }
                         }
                     }

                 }
             } catch (Exception e) {
                 e.printStackTrace();
             }
         }
     }
     /**
      * 获取路径对应的文件名
      * @param path
      * @return
      */
     private static String getFileName(String path){
         int beginIndex = path.lastIndexOf("/")+1;
         return path.substring(beginIndex);
     }

     private void showToast(final String text){
         runOnUiThread(new Runnable() {
             @Override
             public void run() {
                 Toast.makeText(MainActivity.this, text, 0).show();
             }
         });
     }
 }

3. 其中AndroidMainfest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.itheima.mutiledownloader"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="17" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.INTERNET"/>

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.itheima.mutiledownloader.MainActivity"
            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>

</manifest>