OkHttp、Retrofit、HttpURLConnectio

1、添加依赖

// okhttp
implementation 'com.squareup.okhttp3:okhttp:3.12.0'

// retrofit
implementation 'com.squareup.retrofit2:retrofit:2.5.0'
implementation 'com.squareup.retrofit2:converter-gson:2.3.0' // 转换器，请求结果转换成 Model
implementation 'com.squareup.retrofit2:adapter-rxjava2:2.3.0' // 配合 Rxjava 使用

// Rxjava
implementation 'io.reactivex.rxjava2:rxjava:2.2.4'
implementation 'io.reactivex.rxjava2:rxandroid:2.1.0'

2、添加权限

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

3、获取 SD 卡权限和 SD 卡根目录

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
        initSD();
    }
    private void initSD() {
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) ==
                PackageManager.PERMISSION_GRANTED) {
            openSD();
        }else {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 100);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions,
                                           @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == 100 && grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            openSD();
        }
    }

    private void openSD() {
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            sd = Environment.getExternalStorageDirectory();
        }
    }

4、OkHttp

    private void loadFileByOk() {
        OkHttpClient okHttpClient = new OkHttpClient.Builder()
                .build();

        Request request = new Request.Builder()
                .url("http://cdn.banmi.com/banmiapp/apk/banmi_330.apk")
                .build();

        Call call = okHttpClient.newCall(request);

        // 异步调用，不用再新建线程了
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                Log.e(TAG, "onFailure: " + e.getMessage());
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                InputStream inputStream = response.body().byteStream();

                // 获取要下载文件的大小
                long max = response.body().contentLength();

                //保存文件
                saveFiles(inputStream, sd+"/"+"abc123.apk", max);
            }
        });
    }

5、Retrofit

    private void loadFileByRetrofit() {
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(HttpService.url)
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();

        HttpService httpService = retrofit.create(HttpService.class);

        httpService.getData()
                .subscribeOn(Schedulers.io()) // io线程
                .subscribe(new Observer<ResponseBody>() {
                    // 因为要写文件，所以 Observer 不切换到主线程
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onNext(ResponseBody responseBody) {
                        InputStream inputStream = responseBody.byteStream();
                        long max = responseBody.contentLength();
                        saveFiles(inputStream, sd+"/"+"abc456.apk", max);
                    }

                    @Override
                    public void onError(Throwable e) {
                        Log.e(TAG, "onError: " + e.getMessage());
                    }

                    @Override
                    public void onComplete() {

                    }
                });
    }

6、HttpURLConnection

    private void loadFileByHttp() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    URL url = new URL("http://cdn.banmi.com/banmiapp/apk/banmi_330.apk");
                    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                    int i = connection.getResponseCode();
                    if (i == 200){
                        InputStream inputStream = connection.getInputStream();
                        int max = connection.getContentLength();
                        saveFiles(inputStream, sd+"/"+"abc789.jpk", max);
                    }
                } catch (MalformedURLException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }

7、下载文件（ saveFiles() 方法 ）

    /**
     * 保存文件
     * @param inputStream
     * @param string  文件保存的路径
     * @param max
     */
    private void saveFiles(InputStream inputStream, final String string, long max) {
        // ProgressBar 设置最大值
        pb.setMax((int) max);

        // 读写的进度
        int count = 0;

        try {
            // 输出流
            FileOutputStream fileOutputStream = new FileOutputStream(new File(string));

            byte[] bytes = new byte[1024];
            int len = 0;

            while ((len=inputStream.read(bytes)) != -1) {
                fileOutputStream.write(bytes,0,len);

                // 传递当前读写的进度
                count += len;

                Log.e(TAG, "saveFiles: " + count + "       max: " + max);

                pb.setProgress(count);
            }

            fileOutputStream.close();
            inputStream.close();

            runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    Toast.makeText(MainActivity.this, "下载完成", Toast.LENGTH_SHORT).show();

                    InstallUtil.installApk(MainActivity.this, string);
                }
            });
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

8、InstallUtil 类（ 可以实现下载完应用后跳出是否安装的界面 ）

import android.app.Activity;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.net.Uri;
import android.os.Build;
import android.provider.Settings;
import android.support.annotation.RequiresApi;
import android.support.v4.content.FileProvider;

import java.io.File;

public class InstallUtil {

    public static final int UNKNOWN_CODE = 2019;
    public static void installApk(Context context, String path) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O){
            startInstallO(context, path);
        }else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N){
            startInstallN(context, path);
        }else {
            startInstall(context, path);
        }
    }

    /**
     *android 1.x —— 6.x
     *@param path 文件的路径
     */
    private static void startInstall(Context context, String path) {
        Intent install = new Intent(Intent.ACTION_VIEW);
        install.setDataAndType(Uri.parse("file://" + path), "application/vnd.android.package-archive");
        install.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        context.startActivity(install);
    }

    /**
     * android 7.x
     * @param path 文件路径
     */
    @RequiresApi(api = Build.VERSION_CODES.N)
    private static void startInstallN(Context context, String path) {
        // 参数 1：上下文，参数 2：在 AndroidManifest 中的 android:authorities 值，参数 3：共享的文件
        Uri apkUri = FileProvider.getUriForFile(context, "com.baidu.download.provider", new File(path));
        Intent install = new Intent(Intent.ACTION_VIEW);
        // 由于没有在 Activity 环境下启动 Activity，设置下面的标签
        install.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        // 添加这一句表示对目标应用临时授权该 Uri 所代表的文件
        install.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
        install.setDataAndType(apkUri, "application/vnd.android.package-archive");
        context.startActivity(install);
    }

    /**
     * android 8.x
     */
    @RequiresApi(api = Build.VERSION_CODES.O)
    private static void startInstallO(final Context context, String path) {
        boolean isGranted = context.getPackageManager().canRequestPackageInstalls();
        if (isGranted) startInstallN(context, path); // 安装应用的逻辑（写自己的就可以）
        else new AlertDialog.Builder(context)
                .setCancelable(false)
                .setTitle("安装应用需要打开未知来源权限，请去设置中开启权限")
                .setPositiveButton("确定", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface d, int w) {
                        Intent intent = new Intent(Settings.ACTION_MANAGE_UNKNOWN_APP_SOURCES);
                        Activity act = (Activity) context;
                        act.startActivityForResult(intent, UNKNOWN_CODE);
                    }
                })
                .show();
    }
}

9、HttpService

public interface HttpService {
    String url="http://cdn.banmi.com/banmiapp/";
    @GET("apk/banmi_330.apk")
    Observable<ResponseBody> getData();
}

10、ProgressBar

<ProgressBar
        android:id="@+id/pb"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        style="?android:progressBarStyleHorizontal"/>