本文主要向大家介绍了如何用Java语言实现多线程断点下载，通过具体的内容向大家展示，希望对大家学习JAVA语言有所帮助。

JAVA多线程断点下载原理如图：

代码如下：

import java.io.BufferedReader;

import java.io.File;

import java.io.FileInputStream;

import java.io.InputStream;

import java.io.InputStreamReader;

import java.io.RandomAccessFile;

import java.net.HttpURLConnection;

import java.net.URL;

public class MutileThreadDownload {

/**

* 线程的数量

*/

private static int threadCount = 3;

/**

* 每个下载区块的大小

*/

private static long blocksize;

/**

* 正在运行的线程的数量

*/

private static int runningThreadCount;

/**

* @param args

* @throws Exception

*/

public static void main(String[] args) throws Exception {

// 服务器文件的路径

String path = "//192.168.1.100:8080/ff.exe";

URL url = new URL(path);

HttpURLConnection conn = (HttpURLConnection) url.openConnection();

conn.setRequestMethod("GET");

conn.setConnectTimeout(5000);

int code = conn.getResponseCode();

if (code == 200) {

long size = conn.getContentLength();// 得到服务端返回的文件的大小

System.out.println("服务器文件的大小：" + size);

blocksize = size / threadCount;

// 1.首先在本地创建一个大小跟服务器一模一样的空白文件。

File file = new File("temp.exe");

RandomAccessFile raf = new RandomAccessFile(file, "rw");

raf.setLength(size);

// 2.开启若干个子线程分别去下载对应的资源。

runningThreadCount = threadCount;

for (int i = 1; i <= threadCount; i++) {

long startIndex = (i - 1) * blocksize;

long endIndex = i * blocksize - 1;

if (i == threadCount) {

// 最后一个线程

endIndex = size - 1;

}

System.out.println("开启线程：" + i + "下载的位置：" + startIndex + "~"

+ endIndex);

new DownloadThread(path, i, startIndex, endIndex).start();

}

}

conn.disconnect();

}

private static class DownloadThread extends Thread {

private int threadId;

private long startIndex;

private long endIndex;

private String path;

public DownloadThread(String path, int threadId, long startIndex,

long endIndex) {

this.path = path;

this.threadId = threadId;

this.startIndex = startIndex;

this.endIndex = endIndex;

}

@Override

public void run() {

try {

// 当前线程下载的总大小

int total = 0;

File positionFile = new File(threadId + ".txt");

URL url = new URL(path);

HttpURLConnection conn = (HttpURLConnection) url

.openConnection();

conn.setRequestMethod("GET");

// 接着从上一次的位置继续下载数据

if (positionFile.exists() && positionFile.length() > 0) {// 判断是否有记录

FileInputStream fis = new FileInputStream(positionFile);

BufferedReader br = new BufferedReader(

new InputStreamReader(fis));

// 获取当前线程上次下载的总大小是多少

String lasttotalstr = br.readLine();

int lastTotal = Integer.valueOf(lasttotalstr);

System.out.println("上次线程" + threadId + "下载的总大小："

+ lastTotal);

startIndex += lastTotal;

total += lastTotal;// 加上上次下载的总大小。

fis.close();

}

conn.setRequestProperty("Range", "bytes=" + startIndex + "-"

+ endIndex);

conn.setConnectTimeout(5000);

int code = conn.getResponseCode();

System.out.println("code=" + code);

InputStream is = conn.getInputStream();

File file = new File("temp.exe");

RandomAccessFile raf = new RandomAccessFile(file, "rw");

// 指定文件开始写的位置。

raf.seek(startIndex);

System.out.println("第" + threadId + "个线程：写文件的开始位置："

+ String.valueOf(startIndex));

int len = 0;

byte[] buffer = new byte[512];

while ((len = is.read(buffer)) != -1) {

RandomAccessFile rf = new RandomAccessFile(positionFile,

"rwd");

raf.write(buffer, 0, len);

total += len;

rf.write(String.valueOf(total).getBytes());

rf.close();

}

is.close();

raf.close();

} catch (Exception e) {

e.printStackTrace();

} finally {

// 只有所有的线程都下载完毕后 才可以删除记录文件。

synchronized (MutileThreadDownload.class) {

System.out.println("线程" + threadId + "下载完毕了");

runningThreadCount--;

if (runningThreadCount < 1) {

System.out.println("所有的线程都工作完毕了。删除临时记录的文件");

for (int i = 1; i <= threadCount; i++) {

File f = new File(i + ".txt");

System.out.println(f.delete());

}

}

}

}

}

}

}

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