这可以帮助别人很多!

第一:

private static final String APP_DIR = Environment.getExternalStorageDirectory().getAbsolutePath() + "/MyAppFolderInStorage/";

private void install() {
    File file = new File(APP_DIR + fileName);

    if (file.exists()) {
        Intent intent = new Intent(Intent.ACTION_VIEW);
        String type = "application/vnd.android.package-archive";

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
            Uri downloadedApk = FileProvider.getUriForFile(getContext(), "ir.greencode", file);
            intent.setDataAndType(downloadedApk, type);
            intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
        } else {
            intent.setDataAndType(Uri.fromFile(file), type);
            intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        }

        getContext().startActivity(intent);
    } else {
        Toast.makeText(getContext(), "ّFile not found!", Toast.LENGTH_SHORT).show();
    }
}

第二:对于android 7及以上，你应该在manifest中定义一个提供商，如下所示!

    <provider
        android:name="android.support.v4.content.FileProvider"
        android:authorities="ir.greencode"
        android:exported="false"
        android:grantUriPermissions="true">
        <meta-data
            android:name="android.support.FILE_PROVIDER_PATHS"
            android:resource="@xml/paths" />
    </provider>

第三:在res/xml文件夹中定义path.xml，如下所示! 我使用这个路径内部存储，如果你想改变它的东西有一些方法!你可以进入这个链接: FileProvider

<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
    <external-path name="your_folder_name" path="MyAppFolderInStorage/"/>
</paths>

第四:您应该在manifest中添加此权限:

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

允许应用程序请求安装包。针对api大于25的应用程序必须拥有此权限才能使用Intent.ACTION_INSTALL_PACKAGE。 请确保提供者权限相同!

It's worth noting that if you use the DownloadManager to kick off your download, be sure to save it to an external location e.g. setDestinationInExternalFilesDir(c, null, "<your name here>).apk";. The intent with a package-archive type doesn't appear to like the content: scheme used with downloads to an internal location, but does like file:. (Trying to wrap the internal path into a File object and then getting the path doesn't work either, even though it results in a file: url, as the app won't parse the apk; looks like it must be external.)

例子:

int uriIndex = cursor.getColumnIndex(DownloadManager.COLUMN_LOCAL_URI);
String downloadedPackageUriString = cursor.getString(uriIndex);
File mFile = new File(Uri.parse(downloadedPackageUriString).getPath());
Intent promptInstall = new Intent(Intent.ACTION_VIEW)
        .setDataAndType(Uri.fromFile(mFile), "application/vnd.android.package-archive")
        .addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
appContext.startActivity(promptInstall);

UpdateNode provides an API for Android to install APK packages from inside another App. You can just define your Update online and integrate the API into your App - that's it. Currently the API is in Beta state, but you can already do some tests yourself. Beside that, UpdateNode offers also displaying messages though the system - pretty useful if you want to tell something important to your users. I am part of the client dev team and am using at least the message functionality for my own Android App.

这里介绍了如何集成API

只是一个扩展，如果有人需要一个库，那么这可能会有所帮助。感谢Raghav

File file = new File(dir, "App.apk");
Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setDataAndType(Uri.fromFile(file), "application/vnd.android.package-archive");
startActivity(intent);

我也遇到过同样的问题，在尝试了几次之后，结果是这样的。我不知道为什么，但是分别设置数据和类型破坏了我的意图。

首先在AndroidManifest.xml中添加以下行:

<uses-permission android:name="android.permission.INSTALL_PACKAGES"
    tools:ignore="ProtectedPermissions" />

然后使用以下代码安装apk:

File sdCard = Environment.getExternalStorageDirectory();
            String fileStr = sdCard.getAbsolutePath() + "/MyApp";// + "app-release.apk";
            File file = new File(fileStr, "TaghvimShamsi.apk");
            Intent promptInstall = new Intent(Intent.ACTION_VIEW).setDataAndType(Uri.fromFile(file),
                    "application/vnd.android.package-archive");
            startActivity(promptInstall);