多线程下载：

下载速度更快，服务器对每个线程平分资源，故线程越多，得到的资源越多，下载速度越快。

断点续传：

下载中断，再次下载时从上一次下载结束的位置开始下载，防止重复下载

下载结束后

代码：

package com.itheime.phonemultidown;

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;

import android.app.Activity;

import android.os.Bundle;

import android.os.Environment;

import android.view.View;

public class MainActivity extends Activity {

static int threadCount = 3;//下载线程数

static int finishedThread = 0;//完成的下载线程数

String fileName = "baidu.html";//要下载的文件名

//确定下载地址，我们用Tomcat做服务器

String path = "http://192.168.61.254:8080/" + fileName;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

}

//下载按钮点击时，调用此函数

public void click(View v) {

Thread t = new Thread(){//开一个子线程

@Override

public void run() {

try {//发送GET请求，请求这个地址的资源

URL url = new URL(path);

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

conn.setRequestMethod("GET");

conn.setReadTimeout(5000);

conn.setConnectTimeout(5000);

if (conn.getResponseCode() == 200) {

int length = conn.getContentLength();//得到请求资源文件的长度

File file = new File(Environment.getExternalStorageDirectory(), fileName);

//生成临时文件

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

raf.setLength(length);//设置临时文件的大小

raf.close();

int size = length / threadCount;//计算每个线程应该下载多少字节

for (int i = 0; i < threadCount; i++) {

int startIndex = i * size;//计算线程下载的开始和结束位置

int endIndex = (i + 1) * size - 1;

if (i == threadCount - 1) {//最后一个线程，下到文件结束

endIndex = length - 1;

}

new DownLoadThread(startIndex, endIndex, i).start();

}

}

} catch (Exception e) {

e.printStackTrace();

}

}

};

t.start();

}

class DownLoadThread extends Thread {//下载线程内部类

int startIndex;

int endIndex;

int threadId;

public DownLoadThread(int startIndex, int endIndex, int threadId) {

super();

this.startIndex = startIndex;

this.endIndex = endIndex;

this.threadId = threadId;

}

@Override

public void run() {

try {

//生成一个专门记录下载进度的临时文件

File progressFile = new File(Environment.getExternalStorageDirectory(),

threadId + ".txt");

//判断进度临时文件是否存在

if (progressFile.exists()) {

FileInputStream fis = new FileInputStream(progressFile);

BufferedReader br = new BufferedReader(new InputStreamReader(fis));

//从进度临时文件中取出上一次的下载总进度，与原本的开始位置相加，得到新的开始位置

startIndex += Integer.parseInt(br.readLine());

fis.close();

}

URL url = new URL(path);

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

conn.setReadTimeout(5000);

conn.setConnectTimeout(5000);

//设置本次http请求所请求数据的区间

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

//请求部分数据，成功的响应码为206

if (conn.getResponseCode() == 206) {

//流里此时只有1/3原文件的数据

InputStream is = conn.getInputStream();

byte[] b = new byte[1024];

int len = 0;

File file = new File(Environment.getExternalStorageDirectory(), fileName);

//拿到临时文件的输出流

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

//把文件的写入位置移动至startIndex

raf.seek(startIndex);

int total = 0;

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

raf.write(b, 0, len);//每次读取流里的数据后，把数据写入临时文件

total += len;

//System.out.println("线程" + threadId + "下载了" + total);

RandomAccessFile progressRaf = new RandomAccessFile(progressFile, "rwd");

//每次读取流里的数据后，把当前线程下载的总进度写入进度临时文件中

progressRaf.write((total + "").getBytes());

progressRaf.close();

}

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

raf.close();

finishedThread++;

synchronized(path) {

if (finishedThread == threadCount) {//所有线程下载完毕后将进度临时文件删除

for (int i = 0; i < threadCount; i++) {

System.out.println("delete " + i);

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

f.delete();

}

finishedThread = 0;

}

}

}

} catch (Exception e) {

e.printStackTrace();

}

}

}

}

布局文件：

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

tools:context="${relativePackage}.${activityClass}" >

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="开始下载"

android:onClick="click"

/>

配置文件添加权限：