Android applications are executed on smartphones equipped with a variety of resources that must be properly accessed and controlled, otherwise the correctness of the executions and the stability of the entire environment might be negatively affected. For example, apps must properly acquire, use, and release microphones, cameras, and other multimedia devices otherwise the behavior of the apps that use the same resources might be compromised. Unfortunately, several apps do not use resources correctly, for instance due to faults and inaccurate design decisions. By interacting with these apps users may experience unexpected behaviors, which in turn may cause instability and sporadic failures, especially when resources are accessed. In this paper, we present an approach that lets users protect their environment from the apps that use resources improperly by enforcing the correct usage protocol. This is achieved by using software enforcers that can observe executions and change them when necessary. For instance, enforcers can detect that a resource has been acquired but not released, and automatically perform the release operation, thus giving the possibility to use that same resource to the other apps. The main idea is that software libraries, in particular the ones controlling access to resources, can be augmented with enforcers that can be activated and deactivated on demand by users to protect their environment from unwanted app behaviors. We call the software libraries augmented with one or more enforcers proactive libraries because the activation of the enforcer decorates the library with proactive behaviors that can guarantee the correctness of the execution despite the invocation of the operations implemented by the library.