通过Handler进行文件下载（出自菜鸟窝）

public class FileDownloadActivity extends AppCompatActivity {

    public String url="http://112.124.22.238:8081/course_api/css/net_music.apk";
    public String fileName="net_music.apk";

    @BindView(R.id.btn_file)
    Button btnFile;
    @BindView(R.id.progressbar)
    ProgressBar progressbar;

    private OkHttpClient client;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_file_download);
        ButterKnife.bind(this);
        initClient();
    }

    private void initClient() {
        client=new OkHttpClient();
    }

    @OnClick(R.id.btn_file)
    public void onViewClicked() {
        downloadFile();
    }

    private void downloadFile() {


        Request request=new Request
                .Builder()
                .url(url)
                .build();

        client.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                Log.e("aaaaaaaa:1 ","文件出差" );
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {

                writeFile(response);
            }
        });
    }

    Handler mHandler=new Handler(){
        @Override
        public void handleMessage(Message msg) {
           if (msg.what==1){
               int progress=msg.arg1;
               progressbar.setProgress(progress);
           }

        }
    };


    private void writeFile(Response response) {
        InputStream is=null;
        FileOutputStream fos=null;
        is=response.body().byteStream();

//        获取存储路径
        String path= Environment.getExternalStorageDirectory().getAbsolutePath();



        File file=new File(path,fileName);

        try {
            fos=new FileOutputStream(file);
            byte[] bytes=new byte[1024];
            int len=0;

            long contentSize=response.body().contentLength();

            long sum=0;
            while ((len=is.read(bytes))!=-1){

                fos.write(bytes);

                sum+=len;
                int progress= (int) ((sum*1.f/contentSize)*100);
                Message msg=mHandler.obtainMessage(1);
                msg.arg1=progress;
                mHandler.sendMessage(msg);


            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            try {
                if (is!=null){
                    is.close();
                }
                if (fos!=null){
                    fos.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}