首先给大家分享多线程下载核心类：

  1.  package com.example.urltest;
  2.  
  3.  import java.io.IOException;
  4.  import java.io.InputStream;
  5.  import java.io.RandomAccessFile;
  6.  import java.net.HttpURLConnection;
  7.  import java.net.MalformedURLException;
  8.  import java.net.URL;
  9.  import java.net.URLDecoder;
  10.  
  11.  public class DownUtil {
  12.      private String urlPath;
  13.      private String defaultTargetPath;
  14.      private int threadNum;
  15.      private DownThread[] threads;
  16.      private int fileSize = -100;
  17.      private String fileName = "未知文件";
  18.      private boolean isGetFileInformation = false;
  19.  
  20.      public DownUtil(String urlPath, int threadNum) {
  21.          this.urlPath = urlPath;
  22.          this.defaultTargetPath = "/mnt/sdcard/";
  23.          this.threadNum = threadNum;
  24.          this.threads = new DownThread[threadNum];
  25.      }
  26.  
  27.      private HttpURLConnection connection() throws IOException {
  28.  
  29.          URL url = new URL(urlPath);
  30.          HttpURLConnection connection = (HttpURLConnection) url.openConnection();
  31.          connection.setConnectTimeout(5 * 1000);
  32.          connection.setRequestMethod("GET");
  33.          connection.setRequestProperty(
  34.                  "Accept",
  35.                  "image/gif, image/jpeg, image/pjpeg, image/pjpeg, "
  36.                          + "application/x-shockwave-flash, application/xaml+xml, "
  37.                          + "application/vnd.ms-xpsdocument, application/x-ms-xbap, "
  38.                          + "application/x-ms-application, application/vnd.ms-excel, "
  39.                          + "application/vnd.ms-powerpoint, application/msword, */*");
  40.          connection.setRequestProperty("Accept-Language", "zh-CN");
  41.          connection.setRequestProperty("Charset", "UTF-8");
  42.          connection.setRequestProperty("Connection", "Keep-Alive");
  43.  
  44.          return connection;
  45.      }
  46.  
  47.      public void getFileInformation() throws IOException {
  48.  
  49.          HttpURLConnection connection = connection();
  50.  
  51.          connection.setInstanceFollowRedirects(false);
  52.  
  53.          int status = connection.getResponseCode();
  54.          if (status != -1) {
  55.  
  56.              if (status / 100 == 3) {// 当响应码是302时，说明可以获得重定向的资源地址
  57.                  // 得到文件名(此方法不能正确的获取所有url的资源文件名)
  58.                  String name = connection.getHeaderField("Location");
  59.                  name = URLDecoder.decode(name.substring(name.lastIndexOf('/')), "UTF-8");
  60.                  this.fileName = name;
  61.              }
  62.  
  63.              // 得到文件大小
  64.              this.fileSize = connection.getContentLength();
  65.              if (fileSize <= 0) {
  66.                  isGetFileInformation = false;
  67.              } else {
  68.                  isGetFileInformation = true;
  69.              }
  70.              connection.disconnect();
  71.  
  72.          } else {
  73.  
  74.              connection.disconnect();
  75.              isGetFileInformation = false;
  76.  
  77.          }
  78.  
  79.      }
  80.  
  81.      public boolean download(String targetPath, String fileName) throws IOException {
  82.  
  83.          if (isGetFileInformation == false) {
  84.              getFileInformation();
  85.          }
  86.  
  87.          if (isGetFileInformation) {
  88.  
  89.              String absFilePath = targetPath + fileName;
  90.  
  91.              int currentPartSize = (fileSize / threadNum) + 1;// 每一部分需要下载的大小，注意此处加1是为了避免不能整除带来的误差
  92.              RandomAccessFile file = new RandomAccessFile(absFilePath, "rw");
  93.              file.setLength(fileSize);
  94.              file.close();
  95.              for (int i = 0; i < threadNum; i++) {
  96.                  int startPos = i * currentPartSize;
  97.                  RandomAccessFile currentPart = new RandomAccessFile(absFilePath, "rw");// 打开目标文件
  98.                  currentPart.seek(startPos);
  99.                  threads[i] = new DownThread(startPos, currentPartSize, currentPart);
  100.                  threads[i].start();
  101.  
  102.              }
  103.  
  104.              return true;
  105.          } else {
  106.              return false;
  107.  
  108.          }
  109.      }
  110.  
  111.      public boolean download() throws IOException {
  112.          if (isGetFileInformation) {
  113.  
  114.              return download(this.defaultTargetPath, this.getFileName());
  115.  
  116.          } else {
  117.              getFileInformation();
  118.              return download(this.defaultTargetPath, this.getFileName());
  119.  
  120.          }
  121.  
  122.      }
  123.  
  124.      public double getCompleteRate() {
  125.  
  126.          int sumSize = 0;
  127.          for (int i = 0; i < threadNum; i++) {
  128.  
  129.              sumSize += threads[i].length;
  130.          }
  131.  
  132.          return sumSize * 1.0 / fileSize;
  133.  
  134.      }
  135.  
  136.      public String getDefaultTargetPath() {
  137.          return defaultTargetPath;
  138.      }
  139.  
  140.      public int getFileSize() {
  141.          return fileSize;
  142.      }
  143.  
  144.      public String getFileName() {
  145.          return fileName;
  146.      }
  147.  
  148.      public void setFileName(String fileName) {
  149.          this.fileName = fileName;
  150.      }
  151.  
  152.      public boolean isGetFileInformation() {
  153.          return isGetFileInformation;
  154.      }
  155.  
  156.      private class DownThread extends Thread {
  157.          private int startPos;
  158.          private int currentPartSize;
  159.          private RandomAccessFile currentPart;
  160.          int length; // 该线程已经下载的字节数
  161.  
  162.          public DownThread(int startPos, int currentPartSize, RandomAccessFile currentPart) {
  163.  
  164.              this.startPos = startPos;
  165.              this.currentPartSize = currentPartSize;
  166.              this.currentPart = currentPart;
  167.          }
  168.  
  169.          @Override
  170.          public void run() {
  171.  
  172.              try {
  173.  
  174.                  HttpURLConnection connection = connection();
  175.                  int endPos = startPos + currentPartSize;
  176.                  connection.setRequestProperty("Range", "bytes=" + startPos + "-" + endPos);// 使用http来设置一个文件的下载范围（startPos-endPos）
  177.                  InputStream inStream = connection.getInputStream();
  178.                  // inStream.skip(startPos); // skip函数有时候不起作用
  179.                  byte[] buffer = new byte[1024];
  180.                  int hasRead = 0;
  181.                  while (length < currentPartSize && (hasRead = inStream.read(buffer)) > 0) {
  182.  
  183.                      currentPart.write(buffer, 0, hasRead);
  184.                      length = length + hasRead;
  185.  
  186.                  }
  187.  
  188.                  inStream.close();
  189.                  currentPart.close();
  190.                  connection.disconnect();
  191.  
  192.              } catch (MalformedURLException e2) {
  193.                  e2.printStackTrace();
  194.              } catch (IOException e) {
  195.                  e.printStackTrace();
  196.              }
  197.  
  198.          }
  199.  
  200.      }
  201.  
  202.  }

下面是界面的逻辑代码：

  1.  package com.example.urltest;
  2.  
  3.  import android.app.Activity;
  4.  import android.app.AlertDialog;
  5.  import android.app.ProgressDialog;
  6.  import android.content.DialogInterface;
  7.  import android.os.Bundle;
  8.  import android.os.Message;
  9.  import android.view.View;
  10.  import android.view.View.OnClickListener;
  11.  import android.widget.Button;
  12.  import android.widget.EditText;
  13.  import android.widget.ProgressBar;
  14.  import android.widget.TextView;
  15.  import android.widget.Toast;
  16.  
  17.  import java.io.IOException;
  18.  import java.util.Timer;
  19.  import java.util.TimerTask;
  20.  
  21.  public class MultiThreadDown extends Activity {
  22.      EditText url, target;
  23.      Button downButton;
  24.      ProgressBar bar;
  25.      ProgressDialog progressDialog;
  26.      View downView;
  27.      DownUtil downUtil;
  28.      private int mDownStatus;
  29.      private int threadNum = 6; // 默认的线程数
  30.      android.os.Handler handler = new android.os.Handler() {
  31.  
  32.          @Override
  33.          public void handleMessage(Message msg) {
  34.              if (msg.what == 0x123) {
  35.                  bar.setProgress(mDownStatus);
  36.                  if (mDownStatus >= 100) {
  37.                      Toast.makeText(MultiThreadDown.this, "下载完成", Toast.LENGTH_SHORT).show();
  38.                  }
  39.                  // Log.i("csx", "" + mDownStatus);
  40.  
  41.              }
  42.          }
  43.  
  44.      };
  45.  
  46.      @Override
  47.      protected void onCreate(Bundle savedInstanceState) {
  48.  
  49.          super.onCreate(savedInstanceState);
  50.          setContentView(R.layout.layout_down);
  51.          url = (EditText) findViewById(R.id.url);
  52.  
  53.          downButton = (Button) findViewById(R.id.down);
  54.          bar = (ProgressBar) findViewById(R.id.bar);
  55.          progressDialog = new ProgressDialog(this);
  56.          progressDialog.setTitle("尝试连接");
  57.          progressDialog.setMessage("正在连接...");
  58.          downButton.setOnClickListener(new DownButtonOnClickListener());
  59.  
  60.      }
  61.  
  62.      private class DownButtonOnClickListener implements OnClickListener {
  63.  
  64.          EditText targetFilePath, fileName;
  65.          TextView fileSize;
  66.          Thread connectionThread;
  67.  
  68.          public Thread instanceOfConnectionThread() {
  69.              return new Thread() {
  70.  
  71.                  @Override
  72.                  public void run() {
  73.                      try {
  74.                          downUtil.getFileInformation();
  75.  
  76.                      } catch (IOException e1) {
  77.                          e1.printStackTrace();
  78.                      }
  79.  
  80.                  }
  81.  
  82.              };
  83.          }
  84.  
  85.          @Override
  86.          public void onClick(View v) {
  87.  
  88.              String urlPath = url.getText().toString();
  89.              if (urlPath == null || urlPath.equals("")) {
  90.                  return;
  91.              }
  92.              progressDialog.show();
  93.              downUtil = new DownUtil(urlPath, threadNum);
  94.              connectionThread = instanceOfConnectionThread();
  95.              connectionThread.start();
  96.  
  97.              int connectionNum = 3;
  98.              while (!downUtil.isGetFileInformation() && connectionNum > 0) {// 循环请求连接,如果3次之后还没有连接成功，就退出
  99.                  if (!connectionThread.isAlive()) {
  100.                      connectionThread = null;
  101.                      connectionThread = instanceOfConnectionThread();
  102.                      connectionThread.start();
  103.                      connectionNum--;
  104.                  }
  105.  
  106.              }
  107.  
  108.              progressDialog.cancel();
  109.              if (!downUtil.isGetFileInformation()) {
  110.                  Toast.makeText(MultiThreadDown.this, "请求失败！", Toast.LENGTH_SHORT).show();
  111.                  return;
  112.              }
  113.              downView = getLayoutInflater().inflate(R.layout.layout_download_view, null);
  114.              targetFilePath = (EditText) downView.findViewById(R.id.editText_target_path);
  115.              fileName = (EditText) downView.findViewById(R.id.editText_file_name);
  116.              fileSize = (TextView) downView.findViewById(R.id.textView_file_size);
  117.              targetFilePath.setText(downUtil.getDefaultTargetPath());
  118.              fileName.setText(downUtil.getFileName());
  119.              fileSize.append("" + ((double) downUtil.getFileSize()) / 1024 + "k");
  120.  
  121.              new AlertDialog.Builder(MultiThreadDown.this).setView(downView)
  122.                      .setPositiveButton("确定", new DialogInterface.OnClickListener() {
  123.  
  124.                          @Override
  125.                          public void onClick(DialogInterface dialog, int which) {
  126.                              if (!downUtil.isGetFileInformation()) {
  127.                                  dialog.dismiss();
  128.                                  return;
  129.                              }
  130.                              final String path = targetFilePath.getText().toString();
  131.                              final String name = fileName.getText().toString();
  132.  
  133.                              new Thread() {
  134.  
  135.                                  @Override
  136.                                  public void run() {
  137.                                      try {
  138.                                          downUtil.download(path, name);
  139.                                      } catch (IOException e) {
  140.                                          // TODO Auto-generated catch block
  141.                                          e.printStackTrace();
  142.                                      }
  143.  
  144.                                      final Timer timer = new Timer();
  145.                                      TimerTask task = new TimerTask() {
  146.  
  147.                                          @Override
  148.                                          public void run() {
  149.  
  150.                                              mDownStatus = (int) (downUtil.getCompleteRate() * 100);
  151.                                              handler.sendEmptyMessage(0x123);
  152.                                              if (mDownStatus >= 100) {
  153.                                                  timer.cancel();
  154.                                              }
  155.  
  156.                                          }
  157.                                      };
  158.                                      timer.schedule(task, 0, 100);
  159.  
  160.                                  }
  161.  
  162.                              }.start();
  163.  
  164.                          }
  165.                      }).setNegativeButton("取消", null)
  166.                      .setTitle(downUtil.isGetFileInformation() ? "链接可用" : "链接不可用").show();
  167.  
  168.          }
  169.      }
  170.  
  171.  }

下面是主页面布局：layout_down.xml

  1.  <?xml version="1.0" encoding="utf-8"?>
  2.  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  3.      android:layout_width="match_parent"
  4.      android:layout_height="match_parent"
  5.      android:orientation="vertical" >
  6.  
  7.      <ScrollView
  8.          android:id="@+id/scrollView1"
  9.          android:layout_width="match_parent"
  10.          android:layout_height="wrap_content" >
  11.  
  12.          <LinearLayout
  13.              android:layout_width="match_parent"
  14.              android:layout_height="match_parent"
  15.              android:orientation="vertical" >
  16.  
  17.              <TextView
  18.                  android:layout_width="match_parent"
  19.                  android:layout_height="wrap_content"
  20.                  android:text="要下载的资源的URL：" />
  21.  
  22.              <EditText
  23.                  android:id="@+id/url"
  24.                  android:layout_width="match_parent"
  25.                  android:layout_height="wrap_content"
  26.                  android:text="在在这里输入URL" />
  27.  
  28.              <Button
  29.                  android:id="@+id/down"
  30.                  android:layout_width="match_parent"
  31.                  android:layout_height="wrap_content"
  32.                  android:text="下载" />
  33.              <!-- 定义一个水平进度条,用于显示下载进度 -->
  34.  
  35.              <ProgressBar
  36.                  android:id="@+id/bar"
  37.                  style="?android:attr/progressBarStyleHorizontal"
  38.                  android:layout_width="match_parent"
  39.                  android:layout_height="wrap_content"
  40.                  android:max="100" />
  41.          </LinearLayout>
  42.      </ScrollView>
  43.  
  44.  </LinearLayout>

下面是下载选项dialog布局：layout_download_view.xml

  1.  <?xml version="1.0" encoding="utf-8"?>
  2.  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  3.      android:layout_width="match_parent"
  4.      android:layout_height="match_parent"
  5.      android:orientation="vertical" >
  6.  
  7.      <TextView
  8.          android:id="@+id/textView1"
  9.          android:layout_width="wrap_content"
  10.          android:layout_height="wrap_content"
  11.          android:text="下载路径：" />
  12.  
  13.      <EditText
  14.          android:id="@+id/editText_target_path"
  15.          android:layout_width="match_parent"
  16.          android:layout_height="wrap_content"
  17.          android:ems="10" >
  18.  
  19.          <requestFocus />
  20.      </EditText>
  21.  
  22.      <TextView
  23.          android:id="@+id/textView2"
  24.          android:layout_width="wrap_content"
  25.          android:layout_height="wrap_content"
  26.          android:text="文件名：" />
  27.  
  28.      <EditText
  29.          android:id="@+id/editText_file_name"
  30.          android:layout_width="match_parent"
  31.          android:layout_height="wrap_content"
  32.          android:maxLines="1"
  33.          android:ems="10" />
  34.  
  35.      <TextView
  36.          android:id="@+id/textView_file_size"
  37.          android:layout_width="wrap_content"
  38.          android:layout_height="wrap_content"
  39.          android:text="文件大小：" />
  40.  
  41.  </LinearLayout>

效果图如下：输入URL，点击下载弹出对话框，输入路径和文件名 点击确定开始下载

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