使用HttpURLConnection实现多线程下载

HttpURLConnection继承了URLConnection，因此也可用于向指定网站发送GET请求、POST请求，而且它在URLConnection基础上提供了如下便捷方法：

实现多线程下载的步骤：

下面用一个示例来示范使用HttpURLConnection实现多线程下载。此代码来源疯狂讲义一书，该代码主要思路：在Activity中点击按钮,调用DownUtil的download()方法，在download()中启动四个线程去下载资源，每个线程负责下载自己的那部分资源，代码如下：

Activity：

package com.home.activity;

import java.util.Timer;
import java.util.TimerTask;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;

import com.home.multithreaddown.R;
import com.home.util.DownUtil;

public class MultiThreadDownActivity extends Activity {
	private EditText urlText;
	private EditText targetText;
	private Button downBtn;
	private ProgressBar bar;
	private DownUtil downUtil;
	private int mDownStatus;
	private Handler handler;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		// 获取界面中控件
		targetText = (EditText) findViewById(R.id.main_et_name);
		urlText = (EditText) findViewById(R.id.main_et_url);
		downBtn = (Button) findViewById(R.id.main_btn_download);
		bar = (ProgressBar) findViewById(R.id.main_progressBar);
		// 创建一个Handler对象
		handler = new Handler() {
			public void handleMessage(Message msg) {
				if (msg.what == 0x123) {
					bar.setProgress(mDownStatus);
				}
			}
		};
		downBtn.setOnClickListener(new OnClickListener() {

			@Override
			public void onClick(View v) {
				// 初始化DownUtil对象
				downUtil = new DownUtil(urlText.getText().toString(),
						targetText.getText().toString(), 4);
				try {
					// 开始下载
					downUtil.download();
				} catch (Exception e) {
					e.printStackTrace();
				}
				// 定义每秒调度获取一次系统的完成进度
				final Timer timer = new Timer();
				timer.schedule(new TimerTask() {
					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);
						// 下载完成后取消任务调度
						if (mDownStatus >= 100) {
							timer.cancel();
						}
					}
				}, 0, 100);

			}
		});
	}

}

下载的工具类（DownUtil）：

package com.home.util;

import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

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;
	// 定义下载的文件的总大小
	private int fileSize;
	// 定义下载的线程对象
	private DownloadThread[] threads;

	public DownUtil(String path, String targetFile, int threadNum) {
		this.path = path;
		this.threadNum = threadNum;
		// 初始化threads数组
		threads = new DownloadThread[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,application/x-shockwaveflash,application/x-ms-xbap,application/xaml+xml,application/vnd.ms-xpsdocument,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(
				"User-Agent",
				"Mozilla/4.0(compatible;MSIE7.0;Windows NT 5.2;Trident/4.0;.NET CLR 1.1.4322;.NET CLR 2.0.50727;.NET CLR 3.0.04506.30;.NET CLR 3.0.4506.2152;.NET CLR 3.5.30729)");

		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 DownloadThread(startPos, currentPartSize,
					currentPart);
			// 启动下载线程
			threads[i].start();
		}
	}

	/**
	 * 获取下载完成的百分比
	 *
	 * @return
	 */
	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 DownloadThread 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;
		// 定义该线程已下载的字节数
		private int length = 0;

		public DownloadThread(int startPos, int currentPartSize,
				RandomAccessFile currentPart) {
			this.startPos = startPos;
			this.currentPartSize = currentPartSize;
			this.currentPart = currentPart;
		}

		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,application/x-shockwaveflash,application/x-ms-xbap,application/xaml+xml,application/vnd.ms-xpsdocument,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 is = conn.getInputStream();
				// 跳过startPos个字符，表明该线程只下载自己负责那部分文件
				is.skip(startPos);
				byte[] by = new byte[1024];
				int hasRead = 0;
				// 读取网络数据，并写入本地文件
				while (length < currentPartSize
						&& (hasRead = is.read(by)) != -1) {
					currentPart.write(by, 0, hasRead);
					// 累计该线程下载的总大小
					length += hasRead;
				}
				currentPart.close();
				is.close();
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}
}

Activity布局XML：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="输入url:" />

        <EditText
            android:id="@+id/main_et_url"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="输入保存的文件名:" />

        <EditText
            android:id="@+id/main_et_name"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />
    </LinearLayout>

    <Button
        android:id="@+id/main_btn_download"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="下载" />

    <ProgressBar
        android:id="@+id/main_progressBar"
        style="@android:style/Widget.ProgressBar.Horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</LinearLayout>

权限：

<!-- 在SD卡中创建与删除文件权限 -->
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>
<!-- 向SD卡写入数据权限 -->
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<!-- 授权访问网络 -->
<uses-permission android:name="android.permission.INTERNET"/>