Destructors, finalizers, and synchronization

ABSTRACT

We compare two different facilities for running cleanup actions for objects that are about to reach the end of their life.Destructors, such as we find in C++, are invoked synchronously when an object goes out of scope. They make it easier to implement cleanup actions for objects of well-known lifetime, especially in the presence of exceptions.Languages like Java[8], Modula-3[12], and C\#[6] provide a different kind of "finalization" facility: Cleanup methods may be run when the garbage collector discovers a heap object to be otherwise inaccessible. Unlike C++ destructors, such methods run in a separate thread at some much less well-defined time.Languages like Java[8], Modula-3[12], and C\#[6] provide a different kind of "finalization" facility: Cleanup methods may be run when the garbage collector discovers a heap object to be otherwise inaccessible. Unlike C++ destructors, such methods run in a separate thread at some much less well-defined time.We argue that these are fundamentally different, and potentially complementary, language facilities. We also try to resolve some common misunderstandings about finalization in the process. In particular:

• The asynchronous nature of finalizers is not just an accident of implementation or a shortcoming of tracing collectors; it is necessary for correctness of client code, fundamentally affects how finalizers must be written, and how finalization facilities should be presented to the user.
• An object may legitimately be finalized while one of its methods are still running. This should and can be addressed by the language specification amd client code.

REFERENCES

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INDEX TERMS

Primary Classification:
  D. Software
  D.3 PROGRAMMING LANGUAGES
      D.3.3 Language Constructs and Features
          Subjects: Dynamic storage management

Additional Classification:
  D. Software
  D.4 OPERATING SYSTEMS
      D.4.1 Process Management
          Subjects: Threads
      D.4.2 Storage Management
          Subjects: Garbage collection

General Terms:
Design, Languages

Keywords:
deadlock, destructor, finalization, garbage collection, synchronization, thread