Two App with Shared Userid

Two app with shared UserID

You can use android:sharedUserId in AndroidManifest.xml to let your application share the same user id with another application.

android:sharedUserId

The name of a Linux user ID that will be shared with other
applications. By default, Android assigns each application its own
unique user ID. However, if this attribute is set to the same value
for two or more applications, they will all share the same ID —
provided that they are also signed by the same certificate.
Application with the same user ID can access each other's data and, if
desired, run in the same process.

Notice that they need to be signed by the same certificate.

Two applications share the same user id may access each other's resource.

For example:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.shareusertesta"
    android:versionCode="1"
    android:versionName="1.0" 
    android:sharedUserId="com.example">

Then we can init a new context of com.example by:

Context friendContext = this.createPackageContext( "com.example",Context.CONTEXT_IGNORE_SECURITY);

And access some resources of that application:

friendContext.getResources().getString(id);
friendContext.getResources().getDrawable(id);
friendContext.registerReceiver(...);

Two Android applications with the same user ID

You can do this by setting the sharedUserId and sharedUserLabel in the AndroidManifest.xml file to the same value. As an example, if I have the following 2 manifest files (I only included the beginning):

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.example.package1" 
      android:sharedUserId="userId"
      android:sharedUserLabel="@string/label_shared_user" 
      android:versionCode="1" 
      android:versionName="1.0.0">

and

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.example.package2" 
      android:sharedUserId="userId"
      android:sharedUserLabel="@string/label_shared_user" 
      android:versionCode="1" 
      android:versionName="1.0.0">

then they will both share the same user.

How do you make two Android apps share the same user id?

Looks like you can with the sharedUserId manifest attribute. The two apps have to also be signed with the same signature.

How to use one account with multiple apps (AccountAuthenticator, sharedUserId, Google Play App Signing)?

Google Play App signing allows you to re-use the same signature across multiple apps, for precisely this sort of problem.

Just choose "Reuse signing key" when you see this screen:

Two Application's using sharedUserID - createPackageContext issue

First, I really do not recommend this technique. sharedUserId is mostly for pre-installed apps. In particular, once you publish an app, you can never change the sharedUserId (including defining one where you did not have one before). If you change sharedUserId, and the user upgrades from the old sharedUserId to the new one, your app no longer has access to your app's own files, as they are owned by the old sharedUserId.

That being said, android:sharedUserId is an attribute on <manifest>, not <application>.

Android Share User ID between two apps

You probably want to learn a bit about android Intents. They one way you could share data between the two apps.
http://developer.android.com/training/sharing/send.html

If all you are passing is a simple bit of text it should be easy.

Another approach is using SharedPreferences, discussed in this Stack thread:
The easiest way to pass data between application in Android
Although the poster seems confident, I am not sure that another app can listen to the SharedPreferences of another app. And if you could it could constitute a serious security risk unless there is some way to assure that ONLY your apps can share that data.

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