權限

<uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES"/>   //允許安裝未知來源的app

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

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
清單文件

<application>
　　................

　　<!--適配7.0以上安裝app-->

　　<provider
    android:name="android.support.v4.content.FileProvider"
    android:authorities="${applicationId}.fileprovider"
    android:exported="false"
    android:grantUriPermissions="true">
    <meta-data
        android:name="android.support.FILE_PROVIDER_PATHS"
        android:resource="@xml/filepaths" />   //說明：這裏的
 
filepaths是res下面新建的xml名稱的文件夾下的文件
　　</provider>
</application>

res資源文件

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <paths>
        <external-path path="." name="download" />  //說明：name的值是.apk文件的上一級目錄的名稱（不確定，但是跟.apk文件路徑有關系，不是隨便寫的）。
    </paths>
</resources>

下載    
private void loadFromServer() {
        DialogUtil.getInstance().showDialogText(this,"0%");
        OkGo.<File>get(appurl)
                .tag(this)
                .execute(new FileCallback() {
                    @Override
                    public void onSuccess(Response<File> response) {
                        dialogDismiss();
                        installApp(response.body());
                    }

                    @Override
                    public void onError(Response<File> response) {
                        super.onError(response);
                        if (newVersionBean!=null&&newVersionBean.getData()!=null&&"1".equals(newVersionBean.getData().getIs_force_update())){
                            android.os.Process.killProcess(android.os.Process.myPid());    //獲取PID
                            System.exit(0);
                        }else{
                            dialogDismiss();
                            displayMessage(getResources().getString(R.string.prompt_downloadfauil));
                        }
                    }

                    @Override
                    public void downloadProgress(Progress progress) {
                        super.downloadProgress(progress);
                        DialogUtil.getInstance().showContent((int)((progress.currentSize/(float)progress.totalSize)*100)+"%");
                    }
                });
    }

安裝app
    private void installApp(File file){
//        setPermission(file.getPath());
        Intent intent = new Intent(Intent.ACTION_VIEW);
        intent.putExtra("name", "");
        intent.addCategory("android.intent.category.DEFAULT");
        String packageName = getPackageName();
        Uri    data;
        if (Build.VERSION.SDK_INT>=Build.VERSION_CODES.O){            //8.0以上需要在清單文件中寫上權限：REQUEST_INSTALL_PACKAGES。小米的時候並不需要getPackageManager().canRequestPackageInstalls()的結果為true才做安裝操作，會自動彈出允許安裝未知來源的App的系統彈窗

//            boolean b = getPackageManager().canRequestPackageInstalls();
            // 臨時允許
            intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
            data = FileProvider.getUriForFile(this, packageName + ".fileprovider", file);
        }else if (Build.VERSION.SDK_INT >= 24){
            // 臨時允許
            intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
            data = FileProvider.getUriForFile(this, packageName + ".fileprovider", file);
        }else {
            data = Uri.fromFile(file);
        }
        intent.setDataAndType(data, "application/vnd.android.package-archive");
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        startActivity(intent);
    }
    /**
     * 提升讀寫權限
     *
     * @param filePath 文件路徑
     */
    private static void setPermission(String filePath) {
        String  command = "chmod " + "777" + " " + filePath;
        Runtime runtime = Runtime.getRuntime();
        try {
            runtime.exec(command);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    // 獲取當前App版本
    private String getLocalVersion() {
        // 獲取包管理者對象
        PackageManager pm = getPackageManager();
        try {
            // 獲取包的詳細信息
            PackageInfo info = pm.getPackageInfo(getPackageName(), 0);
            // 獲取版本號和版本名稱
            return info.versionName;
        } catch (Exception e) {
            return "";
        }
    }

應用中對APK進行安裝