Http协议的下载功能

      采用HttpClient组件实现Http下载，具有一些特有的优势。尽管java.net包提供了基本通过HTTP访问资源的功能，但它没有提供全面的灵活性和其它很多应用程序需要的功能。HttpClient就是寻求弥补这项空白的组件，通过提供一个有效的，保持更新的，功能丰富的软件包来实现客户端最新的HTTP标准和建议。

      很多时候，单一的下载并不能满足我们的需要，而必须采用多线程的机制。但是众所周知，线程的创建和销毁都是需要开销的，例如，若同时打开几十个下载线程，这对系统的资源消耗是十分大的。我们可以采取线程池的方式，让池中下载的线程数目一定，这样就能做到高效的下载。

下载实现代码如下：

package com.yzy.downloader;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import android.content.Context;
import android.content.Intent;
import android.os.Environment;
import android.util.Log;

public class DownloadManager {
    // 下载地址
    private String url;
    // 下载任务线程池
    private ExecutorService executorService;
    // 应用程序上下文
    private Context context;

    public DownloadManager(Context context) {
        super();
        this.context = context;
    }

    public void addTask(String url, DownloadItem item) {
        this.url = url;
        executorService.submit(new DownloadThread(item));
    }

    public void stop() {
        executorService.shutdown();
    }

    public void start() {
        // 创建线程池
        executorService = Executors.newFixedThreadPool(5);
    }

    public boolean isStop() {
        return executorService.isShutdown();
    }

    class DownloadThread implements Runnable {
        // 已经下载的字节数
        private long finished;
        // 文件总的字节数
        private long fileLength;
        // 文件ID号
        private String fileId;
        // 文件存储位置
        private File downloadFile;

        public DownloadThread(DownloadItem item) {
            fileId = item.getFileId();
            finished = item.getFileLength();
            File sdFile = Environment.getExternalStorageDirectory();
            downloadFile = new File(sdFile + File.separator + "image" + fileId
                    + ".png");
            if (!downloadFile.exists()) {
                try {
                    downloadFile.createNewFile();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        @Override
        public void run() {
            Log.v("tag", "fileId=" + fileId + ",fileLength=" + fileLength);
            try {
                HttpClient httpClient = new DefaultHttpClient();
                HttpGet httpGet = new HttpGet(DownloadManager.this.url);
                HttpResponse httpResponse = httpClient.execute(httpGet);
                HttpEntity httpEntity = httpResponse.getEntity();
                fileLength = httpEntity.getContentLength();
                Log.v("tag", "查询得到文件大小为： " + fileLength);
                httpClient.getConnectionManager().shutdown();
            } catch (Exception e) {
                e.printStackTrace();
            }
            try {
                HttpClient httpClient = new DefaultHttpClient();
                HttpGet httpGet = new HttpGet(DownloadManager.this.url);
                httpGet.addHeader("Range", "bytes=" + finished + "-"
                        + (fileLength - 1));
                HttpResponse httpResponse = httpClient.execute(httpGet);
                HttpEntity httpEntity = httpResponse.getEntity();
                FileOutputStream outputStream = new FileOutputStream(
                        downloadFile, true);
                int count = 0;
                int deltaCount = 0; // 增量
                byte[] tmp = new byte[8 * 1024];
                InputStream inputStream = httpEntity.getContent();
                while (finished < fileLength) {
                    if (DownloadManager.this.executorService.isShutdown()) {
                        return;
                    }
                    count = inputStream.read(tmp);
                    finished += count;
                    deltaCount += count;
                    outputStream.write(tmp, 0, count);
                    if (Float.parseFloat(Integer.toString(deltaCount))
                            / Float.parseFloat(Long.toString(fileLength)) > 0.05) {
                        deltaCount = 0;
                        Intent intent = new Intent("setProgressBar");
                        intent.putExtra("fileId", fileId);
                        int progressBarState = (int) (Float.parseFloat(Long
                                .toString(finished))
                                / Float.parseFloat(Long.toString(fileLength)) * 100);
                        intent.putExtra("progressBarState", progressBarState);
                        DownloadManager.this.context.sendBroadcast(intent);
                    }

                }
                outputStream.close();
                httpClient.getConnectionManager().shutdown();
                Intent intent = new Intent("setProgressBar");
                intent.putExtra("fileId", fileId);
                intent.putExtra("progressBarState", 100);
                DownloadManager.this.context.sendBroadcast(intent);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

实现效果如下：