网络编程基础，多线程下载

Bertha 。 2022-08-08 13:44 276阅读 0赞

# 多线程下载图片例子 #

package com.zcwfeng.java.test.netsocket;
 
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.RandomAccessFile;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.URLConnection;
 
 public class MutilDown { 
 public static void main(String[] args) {
 final int DOWN_THREAD_NUM = 4;
 final String OUT_FILE_NAME = "mdown.jpg";
 
 InputStream[] isArr = new InputStream[DOWN_THREAD_NUM];
 RandomAccessFile[] outArr = new RandomAccessFile[DOWN_THREAD_NUM];
 
 try {
 URL url = new URL("http://images.csdn.net/20150519/201.jpg");
 isArr[0] = url.openStream();
 long fileLen = getFileLength(url);
 System.out.println("资源大小" + fileLen);
 outArr[0] = new RandomAccessFile(OUT_FILE_NAME, "rw");
 // 创建一个与下载资源大小相同的空文件
 for (int i = 0; i < fileLen; i++) {
 outArr[0].write(0);
 }
 // 每个线程下载的字节数
 long numPerThread = fileLen / DOWN_THREAD_NUM;
 long left = fileLen % DOWN_THREAD_NUM;
 for (int i = 0; i < DOWN_THREAD_NUM; i++) {
 if (i != 0) {
 isArr[i] = url.openStream();
 outArr[i] = new RandomAccessFile(OUT_FILE_NAME, "rw");
 }
 if (i == DOWN_THREAD_NUM - 1) {
 new DownloadThread(i * numPerThread, (i + 1) * numPerThread
 + left, isArr[i], outArr[i]).start();;
 } else {
 new DownloadThread(i * numPerThread,
 (i + 1) * numPerThread, isArr[i], outArr[i]).start();
 }
 }
 
 } catch (MalformedURLException e) {
 e.printStackTrace();
 } catch (IOException e) {
 e.printStackTrace();
 } 
 }
 
 private static long getFileLength(URL url) throws IOException {
 long length = 0;
 URLConnection conn = url.openConnection();
 long size = conn.getContentLength();
 length = size;
 return length;
 }
 }

package com.zcwfeng.java.test.netsocket;
 
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.RandomAccessFile;
 
 /** * Description: * Copyright (C), 2005-2015,David * Thies program is protected by copyright * David. Program name * Date:2015年5月19日 * * @author David * @version 1.0 * */
 public class DownloadThread extends Thread { 
 private final int BUFF_LEN = 32;
 private InputStream is;
 private long start;
 private long end;
 private RandomAccessFile raf;
 
 public DownloadThread() {
 }
 
 public DownloadThread(long start, long end, InputStream is,
 RandomAccessFile raf) {
 System.out.println(start + "------------>" + end);
 this.is = is;
 this.start = start;
 this.end = end;
 this.raf = raf;
 }
 
 @Override
 public void run() {
 super.run();
 try {
 is.skip(start);
 raf.seek(start);
 byte[] buff = new byte[BUFF_LEN];
 long contentLen = end - start;
 long times = contentLen / BUFF_LEN + 4;// 定义最多几次读取
 int hasRead = 0;
 for (int i = 0; i < times; i++) {
 hasRead = is.read(buff);
 
 if (hasRead <= 0) {
 break;
 }
 raf.write(buff, 0, hasRead);
 }
 } catch (IOException e) {
 e.printStackTrace();
 } finally {
 try {
 if (raf != null) {
 raf.close();
 }
 if (is != null) {
 is.close();
 }
 
 } catch (Exception ex) {
 ex.printStackTrace();
 }
 }
 }
 }