Android完整的下载实例

1）新建一个dowmloadfile的项目

2）在app/build中添加Okhttp的依赖

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:24.2.1'
    testCompile 'junit:junit:4.12'
    compile 'com.squareup.okhttp3:okhttp:3.4.1'//添加依赖
}

3）定义一个回掉接口DownloadListener 用于事件的监听

public interface DownloadListener {
    void onProgress(int progress);//通知当前下载进度
    void onSuccess();//通知下载成功
    void onFiled();//通知下载失败
    void onPaused();//下载暂停
    void inCanceled();//下载取消
}

4）使用来实现下载

package com.example.dowmloadfile.Myclass;

import android.os.AsyncTask;
import android.os.Environment;


import com.example.dowmloadfile.imp.DownloadListener;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

/**
 * Created by 海绵宝宝 on 2019/5/13.
 */
/**三个参数第一个为需要传入给后台的数据类型，第二个是进度显示类型，第三个为使用Integer作为反馈结果类型**/
public class DownLoadTask extends AsyncTask<String,Integer,Integer> {

    public static final int TYPE_SUCCESS=0;
    public static final int TYPE_FAILED=1;
    public static final int TYPE_PAUSED=2;
    public static final int TYPE_CANCELED=3;

    private DownloadListener downloadListener;

    private boolean isCanceled=false;

    private boolean isPaused=false;

    private int lastProgress;
    //通过DownLoadTask回调下载状态
    public DownLoadTask(DownloadListener listener){
        downloadListener=listener;
    }
    @Override//后台执行具体的下载逻辑
    protected Integer doInBackground(String... params) {
        InputStream is=null;
        RandomAccessFile savedFile=null;
        File file=null;
        try{
            long downloadLength=0;
            String downloadUrl=params[0];
            String fileName=downloadUrl.substring(downloadUrl.lastIndexOf("/"));//解析出下载的文件名
            String derectory= Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();//获取本地的下载文件夹的路径
            file=new File(derectory+fileName);
            if (file.exists()){
                downloadLength=file.length();
            }
            long contentLength=getContentLength(downloadUrl);//读取下载文件的字节数
            if (contentLength==0){
                return TYPE_FAILED;
            }else if (contentLength==downloadLength){
                return TYPE_SUCCESS;
            }
            OkHttpClient client=new OkHttpClient();
            Request request=new Request.Builder().addHeader("RANGE","bytes="+downloadLength+"-").url(downloadUrl).build();//head用于告诉服务器我们从那个字节开始下载
            Response response=client.newCall(request).execute();
            if (response!=null){
                is=response.body().byteStream();
                savedFile=new RandomAccessFile(file,"rw");
                savedFile.seek(downloadLength);//跳过已经下载的字节
                byte[] b=new byte[1024];
                int total=0;
                int len;
                while((len=is.read(b))!=-1){
                    if (isCanceled){
                        return TYPE_CANCELED;
                    }else  if(isPaused){
                        return TYPE_PAUSED;
                    }else {
                        total+=len;
                        savedFile.write(b,0,len);
                        int progress=(int)((total+downloadLength)*100/contentLength);
                        publishProgress(progress);
                    }
                }
                response.body().close();
                return TYPE_SUCCESS;
            };
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            try {
                if (is!=null){
                    is.close();
                }else if (savedFile!=null){
                    savedFile.close();
                }else if (isCanceled&&file!=null){
                    file.delete();
                }
            }catch (Exception e){
                e.printStackTrace();
            }
        }
        return TYPE_FAILED;
    }

    @Override//用于比较下载进度然后使用onProgress来更改下载进度的通知
    protected void onProgressUpdate(Integer... values) {
        int progress=values[0];
        if (progress>lastProgress){
            downloadListener.onProgress(progress);
            lastProgress=progress;
        }
    }

    @Override//根据传入的参数进行回调
    protected void onPostExecute(Integer integer) {
        switch (integer) {
            case TYPE_SUCCESS:
                downloadListener.onSuccess();
                break;
            case TYPE_FAILED:
                downloadListener.onFiled();
                break;
            case TYPE_PAUSED:
                downloadListener.onPaused();
                break;
            case TYPE_CANCELED:
                downloadListener.inCanceled();
            default:
                break;
        };
    }
    public void pauseDownload(){
        isPaused=true;
    }
    public void cancelDownload(){
        isCanceled=true;
    }

    //获取需要下载的文件长度
    private  long getContentLength(String downloadUrl)throws IOException{
        OkHttpClient client=new OkHttpClient();
        Request request=new Request.Builder().url(downloadUrl).build();
        Response response=client.newCall(request).execute();
        if(response!=null&&response.isSuccessful()){
            long contentLength=response.body().contentLength();
            response.close();
            return contentLength;
        }
        return 0;
    }

}

5）为了保证下载可以一直运行我们需要创建一个服务

package com.example.dowmloadfile.Myservice;

import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Intent;
import android.graphics.BitmapFactory;
import android.os.Binder;
import android.os.Environment;
import android.os.IBinder;
import android.support.v4.app.NotificationCompat;
import android.widget.Toast;

import com.example.dowmloadfile.MainActivity;
import com.example.dowmloadfile.Myclass.DownLoadTask;
import com.example.dowmloadfile.R;
import com.example.dowmloadfile.imp.DownloadListener;

import java.io.File;

public class DownloadService extends Service {
    private DownLoadTask downLoadTask;
    private String downloadUrl;
    //创建一个DownloadListener并实现其中的方法
    private DownloadListener downloadListener=new DownloadListener() {
        @Override//以通知的方式显示进度条
        public void onProgress(int progress) {
            //使用getNotificationManager函数构建一个用于显示下载进度的通知
            //使用notify去触发这个通知
            getNotificationManager().notify(1,getNotification("Download...",progress));
        }

        @Override
        public void onSuccess() {
            downLoadTask=null;
            //关闭前台服务通知
            stopForeground(true);
            getNotificationManager().notify(1,getNotification("download succeed",-1));
            Toast.makeText(DownloadService.this,"Download Succeed",Toast.LENGTH_LONG).show();
        }

        @Override
        public void onFiled() {
            downLoadTask=null;
            //关闭前台服务通知
            stopForeground(true);
            getNotificationManager().notify(1,getNotification("download filed",-1));
            Toast.makeText(DownloadService.this,"Download failed",Toast.LENGTH_LONG).show();
        }

        @Override
        public void onPaused() {
            downLoadTask=null;
            Toast.makeText(DownloadService.this,"Download Paused",Toast.LENGTH_LONG).show();
        }

        @Override
        public void inCanceled() {
            downLoadTask=null;
            //关闭前台服务通知
            stopForeground(true);
            Toast.makeText(DownloadService.this,"Download canceled",Toast.LENGTH_LONG).show();
        }
    };

    private DownloadBinder mBinder=new DownloadBinder();




    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        return mBinder;
    }
    //用于使服务可以和活动通信
    public class DownloadBinder extends Binder{
        public void startDownload(String url){
            if(downLoadTask==null){
                downloadUrl=url;
                downLoadTask=new DownLoadTask(downloadListener);
                //使用execute开启下载
                downLoadTask.execute(downloadUrl);
                //startForeground使服务成为一个前台服务以创建持续运行的通知
                startForeground(1,getNotification("download...",0));
                Toast.makeText(DownloadService.this,"Download",Toast.LENGTH_LONG).show();
            }
        }
        public void pauseDownload(){
            if (downLoadTask!=null){
                downLoadTask.pauseDownload();
            }
        }
        //取消下载后需要将下载中的任务取消
        public void cancelDownload(){
            if(downLoadTask!=null){
                downLoadTask.cancelDownload();
            }else {
                if (downloadUrl!=null)
                {
                    //取消需要将文件删除并将通知关闭
                    String fileName=downloadUrl.substring(downloadUrl.lastIndexOf("/"));
                    String directory= Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();
                    File file=new File(directory+fileName);
                    if(file.exists()){
                        file.delete();
                    }
                    getNotificationManager().cancel(1);
                    stopForeground(true);
                    Toast.makeText(DownloadService.this,"Canceled",Toast.LENGTH_LONG).show();
                }
            }
        }
    }

    private NotificationManager getNotificationManager(){
        return (NotificationManager)getSystemService(NOTIFICATION_SERVICE);
    }
    private Notification getNotification(String title,int progress){
        Intent intent=new Intent(this, MainActivity.class);
        PendingIntent pi=PendingIntent.getActivity(this,0,intent,0);
        NotificationCompat.Builder builder=new NotificationCompat.Builder(this);
        builder.setSmallIcon(R.mipmap.ic_launcher);
        builder.setLargeIcon(BitmapFactory.decodeResource(getResources(),R.mipmap.ic_launcher));
        builder.setContentIntent(pi);
        builder.setContentTitle(title);
        if(progress>0){
            builder.setContentText(progress+"%");
            builder.setProgress(100,progress,false);//最大进度，当前进度，是否使用模糊进度条
        }
        return builder.build();
    }
}

6）前台xml文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    <Button
        android:id="@+id/start"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="start download"/>
    <Button
        android:id="@+id/pause"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="start pause"/>
    <Button
        android:id="@+id/cancel"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="start cancel"/>

</LinearLayout>

7）MainActivity.java中设置具体逻辑和按钮的触发事件

package com.example.dowmloadfile;

import android.Manifest;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.content.pm.PackageManager;
import android.os.IBinder;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import com.example.dowmloadfile.Myservice.DownloadService;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{
    private DownloadService.DownloadBinder downloadBinder;
    private ServiceConnection connection=new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            //获取实例以便于在活动中调用其中的方法
            downloadBinder=(DownloadService.DownloadBinder) service;
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button start=(Button)findViewById(R.id.start);
        Button pause=(Button)findViewById(R.id.pause);
        Button cancel=(Button)findViewById(R.id.cancel);
        start.setOnClickListener(this);
        pause.setOnClickListener(this);
        cancel.setOnClickListener(this);
        Intent intent=new Intent(this, DownloadService.class);
        startService(intent);//启动服务保证服务一直运行
        bindService(intent,connection,BIND_AUTO_CREATE);//绑定服务保证数据在服务和活动中传递
        //申请运行时权限
        if(ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.WRITE_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED){
            ActivityCompat.requestPermissions(MainActivity.this,new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},1);
        }

    }

    @Override
    public void onClick(View view){
        if(downloadBinder==null)
        {
            return;
        }
        switch (view.getId()){
            case R.id.start:
                String url="http://icon.nipic.com/BannerPic/20190513/original/20190513111935_1.jpg";
                downloadBinder.startDownload(url);
                break;
            case R.id.pause:
                downloadBinder.pauseDownload();
                break;
            case R.id.cancel:
                downloadBinder.cancelDownload();
                break;
            default:
                break;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        unbindService(connection);
    }
}

8）申请权限

    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />