java中怎么利用多线程实现文件下载功能

java中怎么利用多线程实现文件下载功能，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。

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1、DownloadManager类

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 downloadList = new ArrayList(); 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 getDownloadList() { return downloadList; } public void setDownloadList(List downloadList) { this.downloadList = downloadList; }}

2、DownloadThread类

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; }}

3、TestDownload测试类

public class TestDownload { public static void main(String[] args) { DownloadManager downloadManager = new DownloadManager("d:/upload/09018417.zip" , 5 , "http://10.1.2.65:8080/cetvossFront/09018417.zip"); downloadManager.action(); }}

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