1.配置：

1.1 AndroidManifest.xml中添加权限和FileProvider：

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

        <provider
            android:name="androidx.core.content.FileProvider"
            android:authorities="com.fengzhi.wuyemanagement.fileprovider"
            android:grantUriPermissions="true"
            android:exported="false">
            <meta-data
                android:name="android.support.FILE_PROVIDER_PATHS"
                android:resource="@xml/file_paths" />
        </provider>

1.2 新建文件（路径：res\xml\file_paths.xml）:

<paths>
    <external-path path="." name="external_storage_root" />
</paths>

1.3 (app的)build.gradle:

    implementation "com.lzy.net:okgo:3.0.4"//okgo 网络请求
    implementation 'com.google.code.gson:gson:2.8.2'//gson
    implementation "org.permissionsdispatcher:permissionsdispatcher:4.3.1"//权限
    annotationProcessor "org.permissionsdispatcher:permissionsdispatcher-processor:4.3.1"//权限

2.这里以点击按钮进行更新为例：

2.1 核心代码：

    private int version;
    /* 更新进度条 */
    private ProgressBar mProgress;
    private AlertDialog mDownloadDialog;
    
--------------------------------------------------------------------------------------------------------------------
    
    //点击按钮，检查权限，，，检查更新的方法
    @NeedsPermission({Manifest.permission.READ_EXTERNAL_STORAGE,
            Manifest.permission.WRITE_EXTERNAL_STORAGE,
            Manifest.permission.REQUEST_INSTALL_PACKAGES})
    protected void checkUpdate() {
        showLoadingDialog("检测更新中...");
        version = AppUpdateUtil.getAppVersionCode(this);//检查当前版本号
//      调用方法，，，接口的具体实现，接收传过来的参数，再调自己的方法，
        requestAppUpdate(version, new DataRequestListener<UpdateAppBean>() {
            @Override
            public void success(UpdateAppBean data) {
//                返回的json，getStatus为0时，去下载apk文件,这里是下载apk文件的方法
                updateApp(data.getData().getApk_url());
            }

            @Override
            public void fail(String msg) {
//                返回的json，getStatus为1时，提示："已是最新版本！"
                SToast(msg);
                dismissLoadingDialog();
            }
        });
    }

    //检查版本号，第一次请求(post)，，，UpdateAppBean根据服务器返回生成
    private void requestAppUpdate(int version, final DataRequestListener<UpdateAppBean> listener) {
        OkGo.<String>post(Const.HOST_URL + Const.UPDATEAPP).params("version", version).execute(new StringCallback() {
            @Override
            public void onSuccess(Response<String> response) {
                Gson gson = new Gson();
                UpdateAppBean updateAppBean = gson.fromJson(response.body(), UpdateAppBean.class);
                if (updateAppBean.getStatus() == 0) {
                    listener.success(updateAppBean);
                } else {
                    listener.fail(updateAppBean.getMsg());
                }
            }

            @Override
            public void onError(Response<String> response) {
                listener.fail("服务器连接失败");
                dismissLoadingDialog();
            }
        });
    }

    //如果有新版本，提示有新的版本，然后下载apk文件
    private void updateApp(String apk_url) {
        dismissLoadingDialog();
        DialogUtils.getInstance().showDialog(this, "发现新的版本，是否下载更新？",
                new DialogUtils.DialogListener() {
            @Override
            public void positiveButton() {
                downloadApp(apk_url);
            }
        });
    }

    //下载apk文件并跳转(第二次请求，get)
    private void downloadApp(String apk_url) {
        OkGo.<File>get(apk_url).tag(this).execute(new FileCallback() {
            @Override
            public void onSuccess(Response<File> response) {
                String filePath = response.body().getAbsolutePath();
                Intent intent = IntentUtil.getInstallAppIntent(mContext, filePath);
//                测试过这里必须用startactivity，不能用stratactivityforresult
                mContext.startActivity(intent);
                dismissLoadingDialog();
                mDownloadDialog.dismiss();
                mDownloadDialog=null;
            }

            @Override
            public void downloadProgress(Progress progress) {
//                        showDownloadDialog();
//                        mProgress.setProgress((int) (progress.fraction * 100));
                if (mDownloadDialog == null) {
                    // 构造软件下载对话框
                    AlertDialog.Builder builder = new AlertDialog.Builder(mContext);
                    builder.setTitle("正在更新");
                    // 给下载对话框增加进度条
                    final LayoutInflater inflater = LayoutInflater.from(mContext);
                    View v = inflater.inflate(R.layout.item_progress, null);
                    mProgress = (ProgressBar) v.findViewById(R.id.update_progress);
                    builder.setView(v);
                    mDownloadDialog = builder.create();
                    mDownloadDialog.setCancelable(false);
                    mDownloadDialog.show();
                }
                mProgress.setProgress((int) (progress.fraction * 100));
            }
        });
    }

2.2 DataRequestListener：

public interface DataRequestListener<T> {
    //请求成功
    void success(T data);
    //请求失败
    void fail(String msg);
}

接下来是工具类，来自github，参考，https://github.com/vondear/RxTool

2.3 AppUpdateUtil：

    /**
     * 获取App版本码
     *
     * @param context 上下文
     * @return App版本码
     */
    public static int getAppVersionCode(Context context) {
        return getAppVersionCode(context, context.getPackageName());
    }

2.4 IntentUtil：

public class IntentUtil {

    /**
     * 获取安装App(支持7.0)的意图
     *
     * @param context
     * @param filePath
     * @return
     */
    public static Intent getInstallAppIntent(Context context, String filePath) {
        //apk文件的本地路径
        File apkfile = new File(filePath);
        if (!apkfile.exists()) {
            return null;
        }
        Intent intent = new Intent(Intent.ACTION_VIEW);
        Uri contentUri = FileUtil.getUriForFile(context, apkfile);
        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
            intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION | Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
        }
        intent.setDataAndType(contentUri, "application/vnd.android.package-archive");
        return intent;
    }

2.5 FileUtil：

    /**
     * 将文件转换成uri(支持7.0)
     *
     * @param mContext
     * @param file
     * @return
     */
    public static Uri getUriForFile(Context mContext, File file) {
        Uri fileUri = null;
        if (Build.VERSION.SDK_INT >= 24) {
            fileUri = FileProvider.getUriForFile(mContext, mContext.getPackageName() + ".fileprovider", file);
        } else {
            fileUri = Uri.fromFile(file);
        }
        return fileUri;
    }

3.遇到的问题

9.0手机authorities配置出错，导致无法安装，解决办法：

1.项目中使用了Androidx，AndroidManifest.xml的配置中就必须使用androidx的fileprovider

2.这里的authorities与FileUtil.java中的要一样，我就是字母P大写了导致错误