怎么在Android中实现断点续传？相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
1.导入依赖
compile 'com.loopj.android:android-async-http:1.4.9'
2.导入权限
<uses-permission android:name="android.permission.INTERNET"></uses-permission> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
3.布局文件
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" tools:context="com.five.fashion.duandianxuchuan.MainActivity"> <ProgressBar android:layout_width="match_parent" android:layout_height="100dp" android:id="@+id/pb" /> <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/tv_info" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="下载" android:id="@+id/bt_download" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="暂停" android:id="@+id/bt_pause" /> </LinearLayout>
4.主要代码
package com.five.fashion.duandianxuchuan;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.os.SystemClock;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import com.loopj.android.http.HttpGet;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.InputStream;
import java.io.RandomAccessFile;
import cz.msebera.android.httpclient.Header;
import cz.msebera.android.httpclient.HttpResponse;
import cz.msebera.android.httpclient.client.HttpClient;
import cz.msebera.android.httpclient.client.methods.HttpHead;
import cz.msebera.android.httpclient.impl.client.DefaultHttpClient;
public class MainActivity extends AppCompatActivity {
protected static final String TAG = "OtherActivity";
//下载线程的数量
private final static int threadsize = 3;
protected static final int SET_MAX = 0;
public static final int UPDATE_VIEW = 1;
private ProgressBar pb;
private Button bt_download;
private Button bt_pause;
private TextView tv_info;
//显示进度和更新进度
private Handler mHandler = new Handler(){
public void handleMessage(android.os.Message msg) {
switch (msg.what) {
case SET_MAX://设置进度条的最大值
int filelength = msg.arg1;
pb.setMax(filelength);
break;
case UPDATE_VIEW://更新进度条 和 下载的比率
int len = msg.arg1;//新下载的长度
pb.setProgress(pb.getProgress()+len);//设置进度条的刻度
int max = pb.getMax();//获取进度的最大值
int progress = pb.getProgress();//获取已经下载的数据量
// 下载：30 总：100
int result = (progress*100)/max;
tv_info.setText("下载:"+result+"%");
break;
default:
break;
}
};
};
String uri = "https://cache.yisu.com/upload/information/20200623/125/122219.gif";
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//找到控件
pb = (ProgressBar) findViewById(R.id.pb);
tv_info = (TextView) findViewById(R.id.tv_info);
bt_download = (Button) findViewById(R.id.bt_download);
bt_pause = (Button) findViewById(R.id.bt_pause);
//数据的回显
//确定下载的文件
String name = getFileName(uri);
File file = new File(Environment.getExternalStorageDirectory(), name);
if (file.exists()){//文件存在回显
//获取文件的大小
int filelength = (int) file.length();
pb.setMax(filelength);
try {
//统计原来所有的下载量
int count = 0;
//读取下载记录文件
for (int threadid = 0; threadid < threadsize; threadid++) {
//获取原来指定线程的下载记录
int existDownloadLength = readDownloadInfo(threadid);
count = count + existDownloadLength;
}
//设置进度条的刻度
pb.setProgress(count);
//计算比率
int result = (count * 100) / filelength;
tv_info.setText("下载:" + result + "%");
} catch (Exception e) {
e.printStackTrace();
}
}
bt_download.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
download(v);
}
});
bt_pause.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
pause(v);
}
});
}
//暂停
private boolean flag = false;//是否在下载
public void pause(View v){
flag = false;
bt_download.setEnabled(true);
bt_pause.setEnabled(false);
}
//下载
public void download(View v){
flag = true; //是在下载
bt_download.setEnabled(false);//一点击变成不可点击
bt_pause.setEnabled(true);//一点击变成可点击
new Thread(){//子线程
public void run() {
try {
//获取服务器上文件的大小
HttpClient client = new DefaultHttpClient();
HttpHead request = new HttpHead(uri);
HttpResponse response = client.execute(request);
//response 只有响应头 没有响应体
if(response.getStatusLine().getStatusCode() == 200){
Header[] headers = response.getHeaders("Content-Length");
String value = headers[0].getValue();
//文件大小
int filelength = Integer.parseInt(value);
Log.i(TAG, "filelength:"+filelength);
//设置进度条的最大值
Message msg_setmax = Message.obtain(mHandler, SET_MAX, filelength, 0);
msg_setmax.sendToTarget();
//处理下载记录文件
for(int threadid=0;threadid<threadsize;threadid++){
//对应的下载记录文件
File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");
//判断文件是否存在
if(!file.exists()){
//创建文件
file.createNewFile();
}
}
//在sdcard创建和服务器大小一样的文件
String name = getFileName(uri);
File file = new File(Environment.getExternalStorageDirectory(),name);
//随机访问文件
RandomAccessFile raf = new RandomAccessFile(file, "rwd");
//设置文件的大小
raf.setLength(filelength);
//关闭
raf.close();
//计算每条线程的下载量
int block = (filelength%threadsize == 0)?(filelength/threadsize):(filelength/threadsize+1);
//开启三条线程执行下载
for(int threadid=0;threadid<threadsize;threadid++){
new DownloadThread(threadid, uri, file, block).start();
}
}
} catch (Exception e) {
e.printStackTrace();
}
};
}.start();
}
//线程下载类
private class DownloadThread extends Thread{
private int threadid;//线程的id
private String uri;//下载的地址
private File file;//下载文件
private int block;//下载的块
private int start;
private int end;
public DownloadThread(int threadid, String uri, File file, int block) {
super();
this.threadid = threadid;
this.uri = uri;
this.file = file;
this.block = block;
//计算下载的开始位置和结束位置
start = threadid * block;
end = (threadid + 1)*block -1;
try {
//读取该条线程原来的下载记录
int existDownloadLength = readDownloadInfo(threadid);
//修改下载的开始位置 从新下载
start = start + existDownloadLength;
} catch (Exception e) {
e.printStackTrace();
}
}
//下载 状态码：200是普通的下载 206是分段下载 Range:范围
@Override
public void run() {
super.run();
try {
RandomAccessFile raf = new RandomAccessFile(file, "rwd");
//跳转到起始位置
raf.seek(start);
//分段下载
HttpClient client = new DefaultHttpClient();
HttpGet request = new HttpGet(uri);
request.addHeader("Range", "bytes:"+start+"-"+end);//添加请求头
HttpResponse response = client.execute(request);
if(response.getStatusLine().getStatusCode() == 200){
InputStream inputStream = response.getEntity().getContent();
//把流写入到文件
byte[] buffer = new byte[1024];
int len = 0;
while((len = inputStream.read(buffer)) != -1){
//如果暂停下载 点击暂停 false 就直接return 点击下载true接着下载
if(!flag){
return;//标准线程结束
}
//写数据
raf.write(buffer, 0, len);
//读取原来下载的数据量 这里读取是为了更新下载记录
int existDownloadLength = readDownloadInfo(threadid);//原来下载的数据量
//计算最新的下载
int newDownloadLength = existDownloadLength + len;
//更新下载记录 从新记录最新下载位置
updateDownloadInfo(threadid, newDownloadLength);
//更新进度条的显示 下载的百分比
Message update_msg = Message.obtain(mHandler, UPDATE_VIEW, len, 0);
update_msg.sendToTarget();
//模拟 看到进度条动的效果
SystemClock.sleep(50);
}
inputStream.close();
raf.close();
Log.i(TAG, "第"+threadid+"条线程下载完成");
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
/**
* 读取指定线程的下载数据量
* @param threadid 线程的id
* @return
* @throws Exception
*/
public int readDownloadInfo(int threadid) throws Exception{
//下载记录文件
File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");
BufferedReader br = new BufferedReader(new FileReader(file));
//读取一行数据
String content = br.readLine();
int downlength = 0;
//如果该文件第一次创建去执行读取操作 文件里面的内容是 null
if(!TextUtils.isEmpty(content)){
downlength = Integer.parseInt(content);
}
//关闭流
br.close();
return downlength;
}
/**
* 更新下载记录
* @param threadid
* @param newDownloadLength
*/
public void updateDownloadInfo(int threadid,int newDownloadLength) throws Exception{
//下载记录文件
File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");
FileWriter fw = new FileWriter(file);
fw.write(newDownloadLength+"");
fw.close();
}
/**
* 获取文件的名称
* @param uri
* @return
*/
private String getFileName(String uri){
return uri.substring(uri.lastIndexOf("/")+1);
}
}Android是一种基于Linux内核的自由及开放源代码的操作系统，主要使用于移动设备，如智能手机和平板电脑，由美国Google公司和开放手机联盟领导及开发。
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