here - Stefan-Marr.de

6.2. Case Studies
Immutability In an attempt to go beyond the guarantees provided by Clojure’s
agent implementation, Sec. 5.3.2 discussed how immutability can be
guaranteed.
Ad Hoc Solution Enforcing immutability on top of a VM that neither supports
it directly nor provides mechanisms for managing mutation is a challenge.
While Java’s SecurityManager can provide a partial solution, it cannot
be used to enforce immutability for public object fields (cf. Sec. 3.3.5). Thus,
an ad hoc solution would require implementing managed mutation including
support for handling primitives. This could potentially be done using AOP
for instance with AspectJ, 3 or program transformation as it is used for the implementation
of the OMOP (cf. Sec. 7.1). Thus, it takes additional mechanisms
to realize immutability on top of a JVM or Smalltalk for instance.
OMOP-based Solution With the OMOP’s support for managed state,
the foundation is already available, and an ImmutableDomain such as given
in Lst. 6.3 can be defined. The ImmutableDomain adapts field access operations
by customizing #write:toField:of:. This prevents any mutation from
succeeding for any object, i. e., any object that is owned by the immutable
domain. Note that on line 24 of Lst. 6.1, the ImmutableDomain adopts the return
value of an update function. Furthermore, note that it handles the issue
of reflection by customizing not only the writing of fields but also all
primitive operations, which includes the primitives used for reflection, e. g.,
#priminstVarAt:put:on:.
Handled Challenges With this case study, the evaluation showed solutions
for two of the common implementation challenges (cf. Tab. 6.1). It uses the
OMOP’s mechanisms to intercept all mutating operations, i. e., writing to object
fields and primitives performing mutation. This is an example of a solution
to provide immutability also against reflective operations when desired,
and it shows how the problems of defining state access policies can be solved.
Furthermore, this case study demonstrates that the OMOP satisfies the requirements
for managed state, because it reifies all state access and enables
its customization.
3 http://www.eclipse.org/aspectj/
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