JavaMemoryModel: Efficient implementation of strong class initialization safety

From: Bill Pugh (
Date: Mon Aug 09 1999 - 14:26:30 EDT

I assume that the OS must support a mechanism to perform a global
memory barrier: something initiated by one thread, that forces all
threads to see the writes performed by the initiating thread. A local
memory barrier only forces a processor to become consistent with
global memory, not other processors.

Definition of strong class initialization safety -
        that require a class to be initialized.

        Make clear that if you check that a class is initialized, seeing
        that the class has been initialized by another class must involve
        seeing all writes done by the thread that initialized the class.

Efficient implementation

        After class initializer is run, mark class as PBND
        (prepared-but-not-distributed). Before creating an instance of a class
        that is PBND, do a global memory barrier. When doing a global memory
        barrier, update all classes that are PBND to PAD

        If a class initialization check sees that a class is PAD, then no
        memory barrier is needed. If it is PBND, then we can either do
        a local memory barrier, or do a global memory barrier (changing the
        status of the class (and others)).

        If while a thread is initializing a class Foo, it sees a recursive
        attempt to create a non-thread local instance of the class,
then it must
        do a global memory barrier before the instance is created, but still
        mark the class as PBND after the initializer runs. This barrier for
        recursive creation only has to be done once (it guarantees that the
        recursive creation can't allow another thread to see the class data
        structures in a corrupted form.

        Note: when doing a garbage collection, we can also change all classes
        that are marked PBND to PAD.

        Note: This handles the case where multiple classes are initialized
        before an instance is created (e.g., superclasses) without requiring
        a memory barrier for each, and also avoids
        doing global memory barriers for abstract classes and interfaces.
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