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From "Bill Havanki (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-2497) InstanceFactory
Date Thu, 20 Mar 2014 01:18:42 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-2497?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13941269#comment-13941269
] 

Bill Havanki commented on ACCUMULO-2497:
----------------------------------------

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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