最近做的项目中需要实现断点下载，即用户一次下载可以分多次进行，下载过程可以中断，在目前大多数的带离线缓存的软件都是需要实现这一功能。本文阐述了通过sqlite3简单实现了一个具有断点下载功能的demo。言归正传，开始正文。
设计
数据库表存储元数据
DBHelper.java
用于业务存储的Dao
Dao.java
抽象下载信息的Bean
LoadInfo.java
呈现下载信息View
MainActivity.java
存储下载信息Bean
DownloadInfo.java
封装好的下载类
Downloader.java
代码结构
具体实现
下载信息类：DownloadInfo.java
这里的代码很简单，就是一个用来保存下载信息的类，很简单，没啥好说的，具体看注释
package entity;
public class DownloadInfo {
private int threadId;//线程ID
private int startPos;//下载起始位置
private int endPos;//下载结束位置
private int completeSize;//下载完成量
private String url;//资源URL
public DownloadInfo(int tId,int sp, int ep,int cSize,String address){
threadId=tId;
startPos=sp;
endPos=ep;
completeSize = cSize;
url=address;
}
/**
* @return the threadId
*/
public int getThreadId() {
return threadId;
}
/**
* @param threadId the threadId to set
*/
public void setThreadId(int threadId) {
this.threadId = threadId;
}
/**
* @return the startPos
*/
public int getStartPos() {
return startPos;
}
/**
* @param startPos the startPos to set
*/
public void setStartPos(int startPos) {
this.startPos = startPos;
}
/**
* @return the endPos
*/
public int getEndPos() {
return endPos;
}
/**
* @param endPos the endPos to set
*/
public void setEndPos(int endPos) {
this.endPos = endPos;
}
/**
* @return the completeSize
*/
public int getCompleteSize() {
return completeSize;
}
/**
* @param completeSize the completeSize to set
*/
public void setCompleteSize(int completeSize) {
this.completeSize = completeSize;
}
/**
* @return the url
*/
public String getUrl() {
return url;
}
/**
* @param url the url to set
*/
public void setUrl(String url) {
this.url = url;
}
@Override
public String toString() {
// TODO Auto-generated method stub
return "threadId:"+threadId+",startPos:"+startPos+",endPos："+endPos+",completeSize:"+completeSize+",url:"+url;
}
}
数据库 DBHelper.java
package db;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DBHelper extends SQLiteOpenHelper {
String sql ="create table download_info (id integer PRIMARY KEY AUTOINCREMENT,"
+ "thread_id integer,"
+ "start_pos integer,"
+ "end_pos integer,"
+ "complete_size integer,"
+ "url char)";
public DBHelper(Context context) {
// TODO Auto-generated constructor stub
super(context, "download.db", null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub
db.execSQL(sql);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
}
}
数据库业务管理类 Dao.java
package db;
import java.util.ArrayList;
import java.util.List;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import entity.DownloadInfo;
public class Dao {
private DBHelper dbHelper;
public Dao(Context context){
dbHelper = new DBHelper(context);
}
public boolean isNewTask(String url){
SQLiteDatabase db = dbHelper.getReadableDatabase();
String sql = "select count(*) from download_info where url=?";
Cursor cursor = db.rawQuery(sql, new String[]{url});
cursor.moveToFirst();
int count = cursor.getInt(0);
cursor.close();
return count == 0;
}
public void saveInfo(List infos){
SQLiteDatabase db = dbHelper.getWritableDatabase();
String sql = "insert into download_info(thread_id,start_pos,end_pos,complete_size,url) values(?,?,?,?,?)";
Object[] bindArgs= null;
for(DownloadInfo info:infos){
bindArgs=new Object[]{info.getThreadId(),info.getStartPos(),info.getEndPos(),info.getCompleteSize(),info.getUrl()};
db.execSQL(sql, bindArgs);
}
}
public List getInfo(String url){
SQLiteDatabase db = dbHelper.getReadableDatabase();
List infos = new ArrayList();
String sql ="select thread_id,start_pos,end_pos,complete_size,url from download_info where url=?";
Cursor cursor=db.rawQuery(sql, new String[]{url});
while(cursor.moveToNext()){
DownloadInfo info = new DownloadInfo(cursor.getInt(0), cursor.getInt(1), cursor.getInt(2), cursor.getInt(3),cursor.getString(4));
infos.add(info);
}
cursor.close();
return infos;
}
public void deleteInfo(String url){
SQLiteDatabase db = dbHelper.getWritableDatabase();
db.delete("download_info", "url=?", new String[]{url});
db.close();
}
public void updateInfo(int completeSize,int threadId,String url){
SQLiteDatabase db = dbHelper.getWritableDatabase();
String sql ="update download_info set complete_size=? where thread_id=? and url=?";
db.execSQL(sql, new Object[]{completeSize,threadId,url});
}
public void closeDB(){
dbHelper.close();
}
}
当前状态保存类 LoadInfo.java
package entity;
public class LoadInfo {
private int fileSize;
private int completeSize;
private String url;
public LoadInfo(int fs,int cSize,String address){
fileSize=fs;
completeSize = cSize;
url=address;
}
/**
* @return the fileSize
*/
public int getFileSize() {
return fileSize;
}
/**
* @param fileSize the fileSize to set
*/
public void setFileSize(int fileSize) {
this.fileSize = fileSize;
}
/**
* @return the completeSize
*/
public int getCompleteSize() {
return completeSize;
}
/**
* @param completeSize the completeSize to set
*/
public void setCompleteSize(int completeSize) {
this.completeSize = completeSize;
}
/**
* @return the url
*/
public String getUrl() {
return url;
}
/**
* @param url the url to set
*/
public void setUrl(String url) {
this.url = url;
}
}
下载助手类：Downloader.java
package com.winton.downloadmanager;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import android.content.Context;
import android.os.Handler;
import android.os.Message;
import db.Dao;
import entity.DownloadInfo;
import entity.LoadInfo;
public class Downloader {
private String url;
private String localPath;
private int threadCount;
private Handler mHanler;
private Dao dao;
private int fileSize;
private List infos;
private final static int INIT = 1;
private final static int DOWNLOADING =2;
private final static int PAUSE =3;
private int state = INIT;
public Downloader(String address,String lPath,int thCount,Context context,Handler handler){
url =address;
localPath = lPath;
threadCount = thCount;
mHanler = handler;
dao = new Dao(context);
}
public boolean isDownloading(){
return state == DOWNLOADING;
}
public LoadInfo getDownLoadInfos(){
if(isFirstDownload(url)){
init();
int range = fileSize/threadCount;
infos = new ArrayList();
for(int i=0;i<=threadCount-1;i++){
DownloadInfo info = new DownloadInfo(i, i*range, (i+1)*range-1, 0, url);
infos.add(info);
}
dao.saveInfo(infos);
return new LoadInfo(fileSize, 0, url);
}else{
infos = dao.getInfo(url);
int size = 0;
int completeSize =0;
for(DownloadInfo info:infos){
completeSize += info.getCompleteSize();
size += info.getEndPos()-info.getStartPos()+1;
}
return new LoadInfo(size, completeSize, url);
}
}
public boolean isFirstDownload(String url){
return dao.isNewTask(url);
}
public void init(){
try {
URL mUrl = new URL(this.url);
HttpURLConnection connection = (HttpURLConnection)mUrl.openConnection();
connection.setConnectTimeout(5000);
connection.setRequestMethod("GET");
fileSize = connection.getContentLength();
File file = new File(localPath);
if(!file.exists()){
file.createNewFile();
}
RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");
accessFile.setLength(fileSize);
accessFile.close();
connection.disconnect();
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public void download(){
if(infos != null){
if(state ==DOWNLOADING){
return;
}
state = DOWNLOADING;
for(DownloadInfo info:infos){
new DownloadThread(info.getThreadId(), info.getStartPos(), info.getEndPos(), info.getCompleteSize(), info.getUrl()).start();
}
}
}
class DownloadThread extends Thread{
private int threadId;
private int startPos;
private int endPos;
private int completeSize;
private String url;
public DownloadThread(int tId,int sp,int ep,int cSize,String address) {
// TODO Auto-generated constructor stub
threadId=tId;
startPos=sp;
endPos = ep;
completeSize = cSize;
url = address;
}
@Override
public void run() {
// TODO Auto-generated method stub
HttpURLConnection connection = null;
RandomAccessFile randomAccessFile = null;
InputStream is = null;
try {
URL mUrl = new URL(url);
connection = (HttpURLConnection)mUrl.openConnection();
connection.setConnectTimeout(5000);
connection.setRequestMethod("GET");
connection.setRequestProperty("Range", "bytes="+(startPos+completeSize)+"-"+endPos);
randomAccessFile = new RandomAccessFile(localPath, "rwd");
randomAccessFile.seek(startPos+completeSize);
is=connection.getInputStream();
byte[] buffer = new byte[4096];
int length =-1;
while((length=is.read(buffer)) != -1){
randomAccessFile.write(buffer, 0, length);
completeSize +=length;
dao.updateInfo(threadId, completeSize, url);
Message msg = Message.obtain();
msg.what=1;
msg.obj=url;
msg.arg1=length;
mHanler.sendMessage(msg);
if(state==PAUSE){
return;
}
}
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}finally{
try {
is.close();
randomAccessFile.close();
connection.disconnect();
dao.closeDB();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
public void delete(String url){
dao.deleteInfo(url);
}
public void reset(){
state=INIT;
}
public void pause(){
state=PAUSE;
}
}
View呈现类：MainActivity.java
package com.winton.downloadmanager;
import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
import entity.LoadInfo;
public class MainActivity extends Activity implements OnClickListener{
private TextView name;
private ProgressBar process;
private Button start,stop;
private Downloader downloader;
//处理下载进度UI的
Handler handler = new Handler(){
public void handleMessage(android.os.Message msg) {
if(msg.what==1){
name.setText((String)msg.obj);
int lenght = msg.arg1;
process.incrementProgressBy(lenght);
}
if(msg.what==2){
int max =msg.arg1;
process.setMax(max);
Toast.makeText(getApplicationContext(), max+"", 1).show();
}
};
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
name=(TextView)findViewById(R.id.tv_name);
process=(ProgressBar)findViewById(R.id.pb_download);
start = (Button)findViewById(R.id.bt_start);
stop = (Button)findViewById(R.id.bt_stop);
start.setOnClickListener(this);
stop.setOnClickListener(this);
downloader=new Downloader("http://img4.duitang.com/uploads/item/201206/11/20120611174542_5KRMj.jpeg", Environment.getExternalStorageDirectory().getPath()+"/test1.jpg", 1, getApplicationContext(), handler);
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
if(v==start){
new Thread(new Runnable() {
@Override
public void run() {
// TODO Auto-generated method stub
LoadInfo loadInfo = downloader.getDownLoadInfos();
Message msg =handler.obtainMessage();
msg.what=2;
msg.arg1=loadInfo.getFileSize();
handler.sendMessage(msg);
downloader.download();
}
}).start();
return;
}
if(v==stop){
downloader.pause();
return;
}
}
}
运行效果
总体比较简单，基本实现了 断点下载的功能，而且开启了多个线程去实现分块下载，对初学的同学有一定的帮助。
这些代码我也是从网上各种搜集而来，自己亲自动手敲了一遍，并做了一些小的改动，对整个断点下载的过程有了一个深刻的认识。因此平时要多敲代码，善于总结，必能有所收获。