使用HttpURLConnection
HttpURLConnection继承自URLConnection，也可用于向指定的网站发送GET请求
POST请求。
方法：
int getResponseCode ():
获取服务器的响应代码
String getResponseMessage ():
获取服务器的响应消息
String getResponseMethod ():
获取发送请求的方法
void setRequestMethod ():
设置发送请求的方法
多线程下载
步骤：
1.创建URL对象
2.获取指定URL对象所指向资源的大小 由getContentLength()实现
3.在本地磁盘上创建一个与网络资源相同大小的空文件
4.计算每条线程应该下载网络资源的那个部分（从哪个字节开始，到哪个字节结束）
5.依次创建、启动多条线程来下载网络资源的指定部分。
下载工具类：
public class DownUtil{ // 定义下载资源的路径 private String path; // 指定所下载的文件的保存位置 private String targetFile; // 定义需要使用多少线程下载资源 private int threadNum; // 定义下载的线程对象 private DownThread[] threads; // 定义下载的文件的总大小 private int fileSize; public DownUtil(String path, String targetFile, int threadNum) { this.path = path; this.threadNum = threadNum; // 初始化threads数组 threads = new DownThread[threadNum]; this.targetFile = targetFile; } public void download() throws Exception { URL url = new URL(path); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setConnectTimeout(5 * 1000); conn.setRequestMethod("GET"); conn.setRequestProperty( "Accept", "image/gif, image/jpeg, image/pjpeg, image/pjpeg, " + "application/x-shockwave-flash, application/xaml+xml, " + "application/vnd.ms-xpsdocument, application/x-ms-xbap, " + "application/x-ms-application, application/vnd.ms-excel, " + "application/vnd.ms-powerpoint, application/msword, */*"); conn.setRequestProperty("Accept-Language", "zh-CN"); conn.setRequestProperty("Charset", "UTF-8"); conn.setRequestProperty("Connection", "Keep-Alive"); // 得到文件大小 fileSize = conn.getContentLength(); conn.disconnect(); int currentPartSize = fileSize / threadNum + 1; RandomAccessFile file = new RandomAccessFile(targetFile, "rw"); // 设置本地文件的大小 file.setLength(fileSize); file.close(); for (int i = 0; i < threadNum; i++) { // 计算每条线程的下载的开始位置 int startPos = i * currentPartSize; // 每个线程使用一个RandomAccessFile进行下载 RandomAccessFile currentPart = new RandomAccessFile(targetFile, "rw"); // 定位该线程的下载位置 currentPart.seek(startPos); // 创建下载线程 threads[i] = new DownThread(startPos, currentPartSize, currentPart); // 启动下载线程 threads[i].start(); } } // 获取下载的完成百分比 public double getCompleteRate() { // 统计多条线程已经下载的总大小 int sumSize = 0; for (int i = 0; i < threadNum; i++) { sumSize += threads[i].length; } // 返回已经完成的百分比 return sumSize * 1.0 / fileSize; } private class DownThread extends Thread { // 当前线程的下载位置 private int startPos; // 定义当前线程负责下载的文件大小 private int currentPartSize; // 当前线程需要下载的文件块 private RandomAccessFile currentPart; // 定义已经该线程已下载的字节数 public int length; public DownThread(int startPos, int currentPartSize, RandomAccessFile currentPart) { this.startPos = startPos; this.currentPartSize = currentPartSize; this.currentPart = currentPart; } @Override public void run() { try { URL url = new URL(path); HttpURLConnection conn = (HttpURLConnection)url .openConnection(); conn.setConnectTimeout(5 * 1000); conn.setRequestMethod("GET"); conn.setRequestProperty( "Accept", "image/gif, image/jpeg, image/pjpeg, image/pjpeg, " + "application/x-shockwave-flash, application/xaml+xml, " + "application/vnd.ms-xpsdocument, application/x-ms-xbap, " + "application/x-ms-application, application/vnd.ms-excel, " + "application/vnd.ms-powerpoint, application/msword, */*"); conn.setRequestProperty("Accept-Language", "zh-CN"); conn.setRequestProperty("Charset", "UTF-8"); InputStream inStream = conn.getInputStream(); // 跳过startPos个字节，表明该线程只下载自己负责哪部分文件。 inStream.skip(this.startPos); byte[] buffer = new byte[1024]; int hasRead = 0; // 读取网络数据，并写入本地文件 while (length < currentPartSize && (hasRead = inStream.read(buffer)) > 0) { currentPart.write(buffer, 0, hasRead); // 累计该线程下载的总大小 length += hasRead; } currentPart.close(); inStream.close(); } catch (Exception e) { e.printStackTrace(); } } }}
Activity：
public class MainActivity extends Activity { EditText url; EditText target; Button downButton; ProgressBar bar; DownUtil downUtil; private int mDownStatus; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); url = (EditText) findViewById(R.id.url); target = (EditText) findViewById(R.id.target); downButton = (Button) findViewById(R.id.down); bar = (ProgressBar) findViewById(R.id.bar); final Handler handler = new Handler() { @Override public void handleMessage(Message msg) { if (msg.what == 0x123) { bar.setProgress(mDownStatus); } } }; downButton.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { downUtil = new DownUtil(url.getText().toString(), target .getText().toString(), 6); new Thread() { public void run() { try { downUtil.download(); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } final Timer timer = new Timer(); timer.schedule(new TimerTask() { @Override public void run() { // 获取下载任务的完成比率 double completeRate = downUtil .getCompleteRate(); mDownStatus = (int) (completeRate * 100); // 发送消息通知界面更新进度条 handler.sendEmptyMessage(0x123); // 下载完全后取消任务调度 if (mDownStatus >= 100) { timer.cancel(); } } }, 0, 100); }; }.start(); } }); }}