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https://issues.apache.org/jira/browse/ACCUMULO-2497?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13941269#comment-13941269
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Bill Havanki commented on ACCUMULO-2497:
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A. That would work, except it would require a change to the code under test to
{{new}} the different class, and then a change back to the "real" one later. In
contrast, code using a factory can just offer a setter for which factory to
use: in production a factory for real objects is used, while under test one for
mocks is used. That way, the same code can be used in test and production,
untouched.
{code}
// for code example above
void setInstanceFactory(InstanceFactory ifactory) {
this.ifactory = ifactory;
}
{code}
B. In order for the code to be testable there needs to be hooks for things like
mocks to be injected ... so-called "design for testability". There's no easy
way to hook into {{Instance instance = new ActualInstance()}}, because you
can't override a constructor. A factory field that can be set is an easy hook
to use. Now, the factory _implementation_ that provides a mock object
definitely belongs in test code only, but since the factory interface must be
in the code under test, the interface can't be.
{code}
testedObject.setInstanceFactory(new RealInstanceFactory()); // for reals
testedObject.setInstanceFactory(new TestInstanceFactory(mockInstance)); // for
test
{code}
The downside is that there is this {{setInstanceFactory}} method that has to at
least be package-visible, which is why these testability methods are usually
documented with "only use for testing" or annotated @VisibleForTesting or
something like that. It can get messy.
> InstanceFactory
> ---------------
>
> Key: ACCUMULO-2497
> URL: https://issues.apache.org/jira/browse/ACCUMULO-2497
> Project: Accumulo
> Issue Type: Improvement
> Components: client
> Reporter: Bill Havanki
> Assignee: Bill Havanki
> Priority: Minor
> Labels: factory, static, testability
> Fix For: 1.6.1, 1.7.0
>
>
> Working on ACCUMULO-2470 (unit tests for server/base) is made difficult by
> the heavy dependence on static method calls in our architecture. This ticket
> is for introducing an {{InstanceFactory}} interface that will greatly ease
> testing of code that currently retrieves instances via static methods (e.g.,
> {{HdfsZooInstance.getInstance()}}).
> Limitations:
> * Work here should be limited to creating {{InstanceFactory}} and its
> implementations, and not integrating it into existing code (yet).
> * Work is limited to 1.6 and later. To avoid perturbing the 1.6.0 release no
> new code may be introduced into the public API ({{Instance}} is in the public
> API).
> * A refactoring of the client API is being considered (see ACCUMULO-2128), so
> this change should not further complicate - and should hopefully help - that
> future effort.
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