The most common problem with resource management is improper release of acquired resources. The Java language offers a number of language constructs for cleanup activities such as resource release, namely finalizers, the finally clause, and reference objects. 

Close examination of these mechanisms reveals that the amount of control provide over resource release is limited and might be dissatisfactory if reliable resource release is of paramount importance. We will see that there is no guarantee that a finalizer will ever be invoked; we will talk about unwanted references and why they are a problem, and we will explore whether and how soft, weak, and phantom references mitigate the problems.