在学习四大组件之一的service时，正好可以利用asyncTask 和OKhttp来进行断点续传，并在手机的前台显示下载进度。

    尝试下载的是Oracle官网上的jdk1.7

    在AS中使用OKhttp，只需要简单的在app/build.gradle里加入一句就可以了，如下代码，就最后一行加入即可

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-v7:25.3.1'    compile 'com.android.support.constraint:constraint-layout:1.0.2'    testCompile 'junit:junit:4.12'    compile 'com.squareup.okhttp3:okhttp:3.8.1'}

1、DownloadTask.java

    在该类里主要进行了文件是否存在，存在的话是否已经下载完成等判断，还有利用OKhttp进行文件下载，最经典是是在文件写入RandomAccessFile里时，判断的当前状态，如果是isPaused是true,表示点了暂停键，那么就要暂停下载等等判断；还有使用asyncTask的方法，传递进度给前置通知显示下载进度。

package com.yuanlp.servicebestproject;import android.os.AsyncTask;import android.os.Environment;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 原立鹏 on 2017/7/1. */public class DownloadTask extends AsyncTask
     
       {    private static final String TAG = "DownloadTask";    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_CANCELD=3;    private DownLoadListener listener;    private boolean isCanceld=false;    private boolean isPaused=false;    private int lastProgress;    public DownloadTask(DownLoadListener downloadListener){        this.listener=downloadListener;    }    @Override    protected Integer doInBackground(String... params) {        InputStream is=null;        RandomAccessFile savedFile=null;  //RandomAccessFile 用来访问指定的文件的        File file=null;        try{            long downloadLength=0;  //记录已经下载的文件中长度            String downloadUrl=params[0];            String fileName=downloadUrl.substring(downloadUrl.lastIndexOf("/")); //获取文件名            String directory= Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath(); //获取文件保存路径            file=new File(directory+fileName);  //创建文件            //如果文件存在            if (file.exists()){                downloadLength=file.length();  //获取已经存在的文件大小            }            long contentLength=getContentLength(downloadUrl);  //获取文件总大小            if (contentLength==0){                return TYPE_FAILED;  //已下载的文件异常，返回失败            }else if (downloadLength==contentLength){                return TYPE_SUCCESS;  //说明下载的文件和总长度一样，返回成功            }            OkHttpClient client=new OkHttpClient();            Request request=new Request.Builder()                    .header("RANGE","bytes="+downloadLength+"-")  //从下载之后的地方开始                    .url(downloadUrl)                    .build();            Response response=client.newCall(request).execute();            if (response!=null){                is=response.body().byteStream();  //获取response中的输入流                savedFile=new RandomAccessFile(file,"rw");  //开始访问指定的文件                savedFile.seek(downloadLength);  //跳过已经下载的文件长度                byte[] b=new byte[1024];                long total=0;                int len;                while ((len=is.read(b))!=-1){   //这个时候说明还没有读取到输入流的最后                    if (isCanceld){  //说明取消了下载                        return TYPE_FAILED;                    }else if (isPaused){                        return TYPE_PAUSED;                    }else{                        total+=len;                        savedFile.write(b,0,len);                        int progress= (int) ((total+downloadLength)*100/contentLength);  //计算下载的百分比                        publishProgress(progress);  //调用onProgressUpdate()更新下载进度                    }                }                response.body().close();  //关闭reponse                return TYPE_SUCCESS;  //返回下载成功            }        }catch (Exception e){            e.printStackTrace();        }finally {            try{                if (is!=null){                    is.close();  //关闭输入流                }                if (savedFile!=null){                    savedFile.close();  //关闭查看文件                }                if (isCanceld&&file!=null){                    file.delete();  //如果点击取消下载并且已经下载的文件存在，就删除文件                }            }catch(Exception e){                e.printStackTrace();            }        }        return TYPE_FAILED;    }    /**     * 在doInBackground 里调用ublishProgress时调用此方法，更新UI进度     * @param values     */    public void onProgressUpdate(Integer... values){        int progress=values[0];  //获取传过来的百分比值        if (progress>lastProgress){            listener.onProgress(progress);        }    }    /**     * 当doInBackground 执行完成时，调用此方法     * @param status     */    public void onPostExecute(Integer status){        switch (status){            case TYPE_SUCCESS:                listener.onSuccess();                break;            case TYPE_FAILED:                listener.onFailed();                break;            case TYPE_PAUSED:                listener.onPause();                break;            case TYPE_CANCELD:                listener.onCancled();                break;            default:                break;        }    }    /**     * 按下暂停键时调用,暂停下载     */    public void pausedDownload(){        isPaused=true;    }    public void canceledDownload(){        isCanceld=true;    }    /**     * 根据传入的rul地址，获取文件总长度     * @param url     * @return     */    public long getContentLength(String url) throws IOException {        OkHttpClient client=new OkHttpClient();        Request request=new Request.Builder()                .url(url)                .build();        Response reponse=client.newCall(request).execute();        if (reponse!=null&&reponse.isSuccessful()){  //成功返回reponse            long contentLength=reponse.body().contentLength();  //获取文件中长度            return contentLength;        }            return 0;    }}
     

2、DownloadService.java

    在这个里面，主要是根据Mainactivity里的指令，进行调用downloadTask类里的方法，以及调用前置通知，显示进度。

package com.yuanlp.servicebestproject;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 java.io.File;public class DownloadService extends Service {    private static final String TAG = "DownloadService";    private DownloadTask downloadTask;    private String downloadUrl;    public DownloadService() {    }    @Override    public IBinder onBind(Intent intent) {        return mBinder;    }    private DownLoadListener listener=new DownLoadListener() {        @Override        public void onProgress(int progress) {           getNotifactionManager().notify(1,getNotification("Downloading....",progress));        }        @Override        public void onSuccess() {            downloadTask=null;            //下载成功后，将前台通知关闭，并创建一个下载成功的通告            stopForeground(true);            getNotifactionManager().notify(1,getNotification("Download Success",-1));            Toast.makeText(DownloadService.this,"Download Success",Toast.LENGTH_SHORT).show();        }        @Override        public void onFailed() {            downloadTask=null;            stopForeground(true);            getNotifactionManager().notify(1,getNotification("Down Failed",-1));            Toast.makeText(DownloadService.this,"Down Failed",Toast.LENGTH_SHORT).show();        }        @Override        public void onPause() {            downloadTask=null;            Toast.makeText(DownloadService.this,"Paused",Toast.LENGTH_SHORT).show();        }        @Override        public void onCancled() {            downloadTask=null;            Toast.makeText(DownloadService.this,"Canceled",Toast.LENGTH_SHORT).show();        }    };    private DownloadBinder mBinder=new DownloadBinder();    class DownloadBinder extends Binder{        public void startDownload(String url){            if (downloadTask==null){                downloadUrl=url;                downloadTask=new DownloadTask(listener);                Toast.makeText(DownloadService.this, "Downloading....", Toast.LENGTH_SHORT).show();                downloadTask.execute(downloadUrl);                startForeground(1,getNotification("Downloading...",0));            }        }        public void  pauseDownload(){            if (downloadTask==null){                downloadTask.pausedDownload();            }        }        public void cancelDownload(){            if (downloadTask==null){                downloadTask.canceledDownload();            }else{                if (downloadUrl!=null){                    String filename=downloadUrl.substring(downloadUrl.lastIndexOf("/"));                    String directory= Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS).getPath();                    File file=new File(directory);                    if (file.exists()){                        file.delete();                    }                    getNotifactionManager().cancel(1);                    stopForeground(true);                    Toast.makeText(DownloadService.this,"Canceled",Toast.LENGTH_SHORT).show();                }            }        }    }    private NotificationManager getNotifactionManager(){        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);        }        return builder.build();    }}

3、MainActivity.java

    主要是进行了开启服务和绑定服务，对应按钮的操作，以及运行时权限申请。

package com.yuanlp.servicebestproject;import android.Manifest;import android.content.ComponentName;import android.content.Intent;import android.content.ServiceConnection;import android.content.pm.PackageManager;import android.os.Bundle;import android.os.IBinder;import android.support.v4.app.ActivityCompat;import android.support.v4.content.ContextCompat;import android.support.v7.app.AppCompatActivity;import android.view.View;import android.widget.Toast;public class MainActivity extends AppCompatActivity {    private static final String TAG = "MainActivity";    private DownloadService.DownloadBinder mDownloadBinder;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        Intent intent=new Intent(this,DownloadService.class);        startService(intent);        bindService(intent,conn,BIND_AUTO_CREATE);        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);    }    private ServiceConnection conn=new ServiceConnection() {        @Override        public void onServiceConnected(ComponentName name, IBinder service) {            mDownloadBinder= (DownloadService.DownloadBinder) service;        }        @Override        public void onServiceDisconnected(ComponentName name) {        }    };    /**     * 点击开始下载     * @paam view     */    public void startService(View view){        Toast.makeText(this,"点击下载",Toast.LENGTH_SHORT).show();        if (mDownloadBinder==null){            return;        }        String url="http://download.java.net/java/jdk9/archive/176/binaries/jdk-9+176_windows-x86_bin.exe";        mDownloadBinder.startDownload(url);    }    public void pauseService(View view){        if (mDownloadBinder==null){            return;        }        mDownloadBinder.pauseDownload();    }    public void cancelSerivce(View view){        if (mDownloadBinder==null){            return;        }        mDownloadBinder.cancelDownload();    }    public void onRequestPermissiosResult(int requestCode,String[] permissions,int[] grantResult){        switch (requestCode){            case 1:                if (grantResult.length>0&&grantResult[0]!=PackageManager.PERMISSION_GRANTED){                    Toast.makeText(this,"拒绝授权将无法使用程序",Toast.LENGTH_SHORT).show();                    finish();                }                break;            default:        }    }    public void onDestroy(){        super.onDestroy();        unbindService(conn);    }}