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https://issues.apache.org/jira/browse/ACCUMULO-2497?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13940927#comment-13940927
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Bill Havanki commented on ACCUMULO-2497:
----------------------------------------
This enables easier mocking of Instance objects for testing. For example, take
this code:
{code}
public void doSomethingWithInstance() {
Instance instance = HdfsZooInstance.getInstance();
X x = instance.foo();
Y y = instance.bar();
Z z = y.baz();
// ...
}
{code}
If I want to test this method, I would normally need to deal with a real,
actual {{HdfsZooInstance}} object, and I'd probably have to do all sorts of
things to my environment to make sure it works. (Also, the {{x}}, {{y}}, and
{{z}} objects would be real, making things even more complex.) I'd rather slip
in a mock {{Instance}} object that I can control. That isolates the test to
only having to deal with the method. I also won't need any special environment.
In general it's hard to mock static method calls, because most mocking
frameworks use class extension to create mock instances. (Ditto for mocking
final classes.) The PowerMock test framework does let you mock static method
calls, but still it's considered a bad pattern. A better way is to use a
factory class which can return your mock objects.
{code}
InstanceFactory ifactory = new HdfsZooInstanceFactory();
public void doSomethingWithInstance() {
Instance instance = ifactory.getInstance();
X x = instance.foo();
Y y = instance.bar();
Z z = y.baz();
// ...
}
{code}
Normally the code uses a "real" factory, but for testing the factory is swapped
for one that returns mocks. The method under test doesn't even know the
difference. Also, the method won't accidentally rely on the object being a
particular implementation.
In general, this is just dependency injection, so a further benefit is that if
a new, better instance implementation comes along, it can be swapped in
trivially by setting a new factory, instead of by editing static method calls.
> 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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