我们平时都用"迅雷"下载软件，当下载到一半的时候突然断网，下次开启的时候能够从上次下载的地方继续下载，而且下载速度很快，那么这是怎么做到的呢！
其实它的“快”其实就是多线程的下载实现的，断点下载的原理是将每次下载的字节数存取下来，保证存取的子节点跟下载的同步，并在用户下次下载的时候自动读取
存储点，并以存储点为开始值继续下载。那么android里面如何实现这么断点的下载呢？
在res的布局文件里面先画一个带有进度条的下载界面
activity_main.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.example.thread.MainActivity" >
<EditText
android:id="@+id/et_path"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="请输入下载路径"
android:text="下载文件" />
<Button
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:onClick="downLoad"
android:text="下载" />
<ProgressBar
android:id="@+id/pd"
style="?android:attr/progressBarStyleHorizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content" />
<TextView
android:id="@+id/tv"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="下载进度" />
</LinearLayout>
package com.example.thread;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.text.TextUtils;
import android.view.View;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends Activity {
private static EditText et_path;
private static ProgressBar pb;
private static TextView tv;
public static int runningThread = 3;
protected static final int DOWN_LOAD_ERROR = 1;
protected static final int SERVER_ERROR = 2;
protected static final int DOWN_LOAD_FINSIH = 3;
protected static final int SHOW_TEXT = 4;
public static int currentProcess = 0;// 当前进度
private Handler handler = new Handler() {
public void handleMessage(android.os.Message msg) {
switch (msg.what) {
case DOWN_LOAD_ERROR:
Toast.makeText(getApplicationContext(), "下载失败", 0).show();
break;
case SERVER_ERROR:
Toast.makeText(getApplicationContext(), "服务器异常", 0).show();
break;
case DOWN_LOAD_FINSIH:
Toast.makeText(getApplicationContext(), "文件下载完毕", 0).show();
break;
case SHOW_TEXT:
Toast.makeText(getApplicationContext(),
"下载进度" + pb.getProgress() * 100 / pb.getMax() + "%", 0)
.show();
break;
default:
break;
}
};
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
et_path = (EditText) findViewById(R.id.et_path);
pb = (ProgressBar) findViewById(R.id.pd);
tv = (TextView) findViewById(R.id.tv);
}
public void downLoad(View view) {
String path = et_path.getText().toString().trim();
final int thread = 3;
if (TextUtils.isEmpty(path)) {
Toast.makeText(this, "路径读取失败", 0).show();
return;
}
new Thread() {
public void run() {
try {
// 1:连接服务器，获取一个文件，获取文件的长度，在本地创建一个大小跟服务器文件大小
String path = "";
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url
.openConnection();
conn.setConnectTimeout(5000);
conn.setRequestMethod("GET");
int code = conn.getResponseCode();
if (code == 200) {
// 服务器返回的数据的长度
int length = conn.getContentLength();
pb.setMax(length);// 设置进度条的最大值
System.out.println("文件总长度" + length);
// 在客户端本地创建出来一个大小跟服务器端文件大小的临时文件
RandomAccessFile raFile = new RandomAccessFile(
"/sdcard/spider_demo.rar", "rwd");
// 指定创建的这个文件的长度
raFile.setLength(length);
raFile.close();
// 平均每一个线程下载的文件的大小
int blockSize = length / thread;
for (int i = 1; i <= thread - 1; i++) {
// 第一个线程下载的开始位置
int startIndex = (i - 1) * blockSize;
int endIndex = i * blockSize - 1;
if (i == thread) {// 最后一个线程的长度要稍微长一点
endIndex = length;
}
System.out.println("线程:" + i + "下载:----"
+ startIndex + "----->" + endIndex);
new DownLoadthread(i, startIndex, endIndex, path)
.start();
}
} else {
Message msg = new Message();
msg.what = SERVER_ERROR;
handler.sendMessage(msg);
}
} catch (Exception e) {
e.printStackTrace();
Message msg = new Message();
msg.what = DOWN_LOAD_ERROR;
handler.sendMessage(msg);
}
};
}.start();
}
/**
* 下载文件的子线程，每一个线程下载对应位置的文件
*
* */
public class DownLoadthread extends Thread {
private int threadId;
private int startIndex;
private int endIndex;
private String path;
/**
* @param threadId线程id
* @param startIndex
* 线程下载的开始位置
* @param endIndex
* 线程下载的结束位置
* @param path下载文件在服务器上的路径
*
* */
public DownLoadthread(int threadId, int startIndex, int endIndex,
String path) {
this.threadId = threadId;
this.startIndex = startIndex;
this.endIndex = endIndex;
this.path = path;
}
@Override
public void run() {
try {
// 检查是否存在记录下载的文件,如果存在读取这个文件的数据
// ----------------------替换成数据库----------------------------
File tempFile = new File("/sdcard/" + threadId + ".txt");
if (tempFile.exists() && tempFile.length() > 0) {
FileInputStream fis = new FileInputStream(tempFile);
byte[] temp = new byte[1024];
int leng = fis.read(temp);
String downloadLen = new String(temp, 0, leng);
int downloadInt = Integer.parseInt(downloadLen);
startIndex += downloadInt;// 修改下载的真正的开始位置
fis.close();
}
// ----------------------替换成数据库----------------------------
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url
.openConnection();
conn.setConnectTimeout(5000);
conn.setRequestMethod("GET");
// 重要:请求服务器下载部分的文件，需要置顶文件的位置
conn.setRequestProperty("Range", "bytes=" + startIndex + "-"
+ endIndex);
// 从服务器请求全部资源,如果从服务器请求部分资源为206
int code = conn.getResponseCode();
System.out.println("code=" + code);
/*
* if (code == 200) { InputStream is =
* conn.getInputStream();//返回全部的资源 }
*/
InputStream is = conn.getInputStream();// 返回部分的资源
RandomAccessFile raFile = new RandomAccessFile(
"/sdcard/spider_demo.rar", "rwd");
// 随机写文件的时候，从哪个文件开始写
raFile.seek(startIndex);
int len = 0;
byte[] buffer = new byte[1024];
int total = 0;// 已经下载的数据的长度
File file = new File("/sdcard/" + threadId + ".txt");// 作用:记录当前线程下载的数据长度
while ((len = is.read(buffer)) != -1) {
// 如果你是固态硬盘,那么必须这样写数据
// RandomAccessFile info = new RandomAccessFile(
// threadId + ".txt", "rwd");
FileOutputStream fos = new FileOutputStream(file);
raFile.write(buffer, 0, len);
total += len;
fos.write(String.valueOf(total).getBytes());
fos.close();
// 更新进度条
synchronized (MainActivity.this) {
currentProcess += len;// 获取所以进程下载的总进度总进度
pb.setProgress(currentProcess);// 更该界面上progressbar进度条的进度
Message message = Message.obtain();// 返回一个消息实例，防止创建太多对象
message.what = SHOW_TEXT;
// message.obj
handler.sendMessage(message);
// 特殊情况，progressbar
// progressdialog进度条对话框可以直接在子线程里面更新界面，它内部处理过了
}
}
is.close();
raFile.close();
System.out.println("线程:" + threadId + "下载完毕了...");
File deleteFile = new File("/sdcard/" + threadId + ".txt");
deleteFile.delete();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
threadFinish();
}
}
private synchronized void threadFinish() {
runningThread--;
if (runningThread == 0) {
for (int i = 1; i <= 3; i++) {
File file = new File("/sdcard/" + i + ".txt");
file.delete();
}
System.out.println("文件下载完毕,删除所以的下载记录");
Message msg = new Message();
msg.what = DOWN_LOAD_FINSIH;
handler.sendMessage(msg);
}
}
}
}在清单文件里面加入权限
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.thread"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="21" />
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
最终显示断点下载
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