java 多线程下载图片

设计思路:

1. 首先读取文件的长度, 并判断网站是否支持分段下载

2. 如果支持分段下载则创建多个线程同时下载该文件,否则使用单线程下载

3. 在各个线程中,分别使用 RandomAccessFile对象写入对应的文件位置

 

package img.test;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class DownloadManager implements Runnable {
	// 保存路径
	private String savePath;
	// 总的下载线程数
	private int threadNum;
	// 下载的链接地址
	private String urlFile;
	// 是否下载开始
	private boolean isStarted;
	// 用于监视何时合并文件存放Thread的list
	private List<DownloadThread> downloadList = new ArrayList<DownloadThread>();

	public DownloadManager(String savePath, int threadNum, String urlFile) {
		super();
		this.savePath = savePath;
		this.threadNum = threadNum;
		this.urlFile = urlFile;
	}

	// 最终调用线程下载。本线程中调用分线程。
	public void action() {
		new Thread(this).start();
	}

	public void run() {
		long t1 = System.currentTimeMillis();
		System.out.println(t1);
		// 如果没有下载 ， 就开始 ， 并且将已经下载的变量值设为true
		if (!isStarted) {
			startDownload();
			isStarted = true;
		}
		while (true) {
			// 初始化认为所有线程下载完成，逐个检查
			boolean finish = true;
			// 如果有任何一个没完成，说明下载没完成，不能合并文件
			for (DownloadThread thread : downloadList) {
				if (!thread.isFinish()) {
					finish = false;
					break;
				}
			}
			// 全部下载完成才为真
			if (finish) {
				// 合并文件
				mergeFiles();
				// 跳出循环 ， 下载结束
				break;
			}
			// 休息一会 ， 减少cpu消耗
			try {
				Thread.sleep(1000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		long t2 = System.currentTimeMillis();
		System.out.println(t2);
		 System.out.println("下载用时：" + (t2 -t1));
	}

	public void startDownload() {
		// 得到每个线程开始值 ， 下载字节数大小
		int[][] posAndLength = getPosAndLength();
		// 根据下载信息创建每个下载线程，并且启动他们。
		for (int i = 0; i < posAndLength.length; i++) {
			int pos = posAndLength[i][0];
			int length = posAndLength[i][1];
			DownloadThread downloadThread = new DownloadThread(i + 1, length,
					pos, savePath, urlFile);
			new Thread(downloadThread).start();
			downloadList.add(downloadThread);
		}
	}

	/**
	 * 获得文件大小
	 * 
	 * @return 文件大小
	 */
	public long getFileLength() {
		System.out.println("获得文件大小  start......");
		HttpURLConnection conn = null;
		long result = 0;
		try {
			URL url = new URL(urlFile);
			conn = (HttpURLConnection) url.openConnection();
			// 使用Content-Length头信息获得文件大小
			result = Long.parseLong(conn.getHeaderField("Content-Length"));
		} catch (MalformedURLException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			if (conn != null) {
				conn.disconnect();
			}
		}
		System.out.println("获得文件大小  end......" + result);
		return result;
	}

	// 具体细节求出每个线程的开始位置和文件下载大小
	public int[][] getPosAndLength() {
		int[][] result = new int[threadNum][2];
		int fileLength = (int) getFileLength();
		int every = fileLength % threadNum == 0 ? fileLength / threadNum
				: fileLength / threadNum + 1;
		for (int i = 0; i < result.length; i++) {
			int length = 0;
			if (i != result.length - 1) {
				length = every;
			} else {
				length = fileLength - i * every;
			}
			result[i][0] = i * every;
			result[i][1] = length;
		}
		return result;
	}

	// 合并文件
	public void mergeFiles() {
		System.out.println("合并文件  start......");
		OutputStream out = null;
		try {
			out = new FileOutputStream(savePath);
			for (int i = 1; i <= threadNum; i++) {
				InputStream in = new FileInputStream(savePath + i);
				byte[] bytes = new byte[2048];
				int read = 0;
				while ((read = in.read(bytes)) != -1) {
					out.write(bytes, 0, read);
					out.flush();
				}
				if (in != null) {
					in.close();
					new File(savePath + i).delete();
				}
			}
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			if (out != null) {
				try {
					out.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
		}
		System.out.println("合并文件  end......");
	}

	public String getSavePath() {
		return savePath;
	}

	public void setSavePath(String savePath) {
		this.savePath = savePath;
	}

	public int getThreadNum() {
		return threadNum;
	}

	public void setThreadNum(int threadNum) {
		this.threadNum = threadNum;
	}

	public String getUrlFile() {
		return urlFile;
	}

	public void setUrlFile(String urlFile) {
		this.urlFile = urlFile;
	}

	public boolean isStarted() {
		return isStarted;
	}

	public void setStarted(boolean isStarted) {
		this.isStarted = isStarted;
	}

	public List<DownloadThread> getDownloadList() {
		return downloadList;
	}

	public void setDownloadList(List<DownloadThread> downloadList) {
		this.downloadList = downloadList;
	}
}

   

package img.test;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class DownloadThread implements Runnable {
	// 当前第几个线程 ， 用于给下载文件起名 file1 file2 file3 ...
	private int whichThread;
	// 监听单一线程下载是否完成
	private boolean isFinish;
	// 本线程要下载的文件字节数
	private int length;
	// 本线程向服务器发送请求时输入流的首位置
	private int startPosition;
	// 保存的路径
	private String savePath;
	// 要下载的文件 ， 用于创建连接
	private String url;

	public void run() {
		HttpURLConnection conn = null;
		InputStream in = null;
		OutputStream out = null;
		try {
			System.out.println("正在执行的线程：" + whichThread);
			URL fileUrl = new URL(url);
			// 与服务器创建连接
			conn = (HttpURLConnection) fileUrl.openConnection();
			// 下载使用get请求
			conn.setRequestMethod("GET");
			// 告诉服务器 ， 我是火狐 ， 不要不让我下载。
			conn.setRequestProperty(
							"User-Agent",
							"Firefox Mozilla/5.0 (Windows; U; Windows NT 5.1; zh-CN; rv:1.9.1.3) Gecko/20090824 Firefox/3.5.3");
			// 这里是设置文件输入流的首位置
			conn.setRequestProperty("Range", "bytes=" + startPosition + "-");
			// 与服务器创建连接
			conn.connect();
			// 获得输入流
			in = conn.getInputStream();
			// 在硬盘上创建file1 , file2 , ...这样的文件 ， 准备往里面写东西
			out = new FileOutputStream(savePath + whichThread);
			// 用于写入的字节数组
			byte[] bytes = new byte[4096];
			// 一共下载了多少字节
			int count = 0;
			// 单次读取的字节数
			int read = 0;
			while ((read = in.read(bytes)) != -1) {
				// 检查一下是不是下载到了本线程需要的长度
				if (length - count < bytes.length) {
					// 比如说本线程还需要900字节，但是已经读取1000
					// 字节，则用要本线程总下载长度减去
					// 已经下载的长度
					read = length - count;
				}
				// 将准确的字节写入输出流
				out.write(bytes, 0, read);
				// 已经下载的字节数加上本次循环字节数
				count = count + read;
				// 如果下载字节达到本线程所需要字节数，消除循环，
				// 停止下载
				if (count == length) {
					break;
				}
			}
			// 将监视变量设置为true
			isFinish = true;
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			// 最后进行输入、输出、连接的关闭
			if (in != null) {
				try {
					in.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			if (out != null) {
				try {
					out.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			if (conn != null) {
				conn.disconnect();
			}
		}
	}

	public int getStartPosition() {
		return startPosition;
	}

	public void setStartPosition(int startPosition) {
		this.startPosition = startPosition;
	}

	public String getUrl() {
		return url;
	}

	public void setUrl(String url) {
		this.url = url;
	}

	public int getWhichThread() {
		return whichThread;
	}

	public void setWhichThread(int whichThread) {
		this.whichThread = whichThread;
	}

	public int getLength() {
		return length;
	}

	public void setLength(int length) {
		this.length = length;
	}

	public String getSavePath() {
		return savePath;
	}

	public void setSavePath(String savePath) {
		this.savePath = savePath;
	}

	public DownloadThread(int whichThread, int length, int startPosition,
			String savePath, String url) {
		super();
		this.whichThread = whichThread;
		this.length = length;
		this.startPosition = startPosition;
		this.savePath = savePath;
		this.url = url;
	}

	public DownloadThread() {
		super();
	}

	public boolean isFinish() {
		return isFinish;
	}

	public void setFinish(boolean isFinish) {
		this.isFinish = isFinish;
	}
}

   

package img.test;
public class TestDownload {

	public static void main(String[] args) {
		DownloadManager downloadManager = new DownloadManager("f:/test.jpg" , 8 , "");
		downloadManager.action();
	}
}