media/lib/signer/README.txt

There are two libraries defined in this directory:
First, com.android.mediadrm.signer.jar is a shared java library
containing classes required by unbundled apps running on devices that use
the certficate provisioning and private key signing capabilities provided
by the MediaDrm API.
Second, com.android.mediadrm.signer.stubs.jar is a stub for the shared library
which provides build-time APIs to the unbundled clients.

At runtime, the shared library is added to the classloader of the app via the
<uses-library> tag. And since Java always tries to load a class from the
parent classloader, regardless of whether the stub library is linked to the
app statically or dynamically, the real classes are loaded from the shared
library.

--- Rules of this library ---
o The stub library is effectively a PUBLIC API for unbundled CAST receivers
  that may be distributed outside the system image. So it MUST BE API STABLE.
  You can add but not remove. The rules are the same as for the
  public platform SDK API.
o This library can see and instantiate internal platform classes, but it must not
  expose them in any public method (or by extending them via inheritance). This would
  break clients of the library because they cannot see the internal platform classes.

This library is distributed in the system image, and loaded as
a shared library. So you can change the implementation, but not
the interface. In this way it is like framework.jar.

--- Why does this library exist? ---

Unbundled apps cannot use internal platform classes.

This library will eventually be replaced when the provisioned certificate-
based signing infrastructure that is currently defined in the support library
is reintegrated with the framework in a new API.  That API isn't ready yet so
this library is a compromise to make new capabilities available to the system
without exposing the full surface area of the support library.