HADOOP-17947. Provide alternative to Guava VisibleForTesting

[virajjasani]

Description of PR

Provide alternative to Guava VisibleForTesting so that we can get rid of Guava dependency to represent testing scope for a class/method/field.

[virajjasani]

FYI @amahussein @aajisaka @tasanuma @steveloughran. Once this PR gets in, will update PR #3503 to just use this @interface.
Thanks

[steveloughran]

If we are going to change the javadocs, maybe make clear this is a guava replacement and what guarantees we offer (none). Here's my draft

  /**
   * Annotates a program element that exists, or is more widely visible than
   * otherwise necessary, for the benefits of testability.
   * More specifically <i>testability within the hadoop-* modules</i>.
   * This gives the implicit scope and stability of:
   * <pre>
   * @InterfaceAudience.Private
   * @InterfaceStability.Unstable
   * </pre>
   * If external modules need to access/override these methods, then
   * they MUST be rescoped as public/limited private.
   */

Now, should we actually move this to the package org.apache.hadoop.classification in the module hadoop-annotations? It is a classification, after all. I think from a consistency perspective, it makes sense

[virajjasani]

@steveloughran Javadoc changes, specifically the relation with IA.Private and IS.Unstable makes the doc look better.

Now, should we actually move this to the package org.apache.hadoop.classification in the module hadoop-annotations? It is a classification, after all. I think from a consistency perspective, it makes sense

This is interesting thought. I think it makes sense. In fact, we also have another advantage here - all modules have dependency on hadoop-annotations but not on hadoop-common so all hadoop-* modules can use it. It's worth moving VFT to hadoop-annotations.

[tasanuma]

Adjust to the VFT in Guava, we may want to annotate it with the following annotations.

@Retention(RetentionPolicy.CLASS)
@Target({ElementType.TYPE, ElementType.METHOD})
@Documented

[virajjasani]

Adjust to the VFT in Guava, we may want to annotate it with the following annotations.

@Retention(RetentionPolicy.CLASS)
@Target({ElementType.TYPE, ElementType.METHOD})
@Documented

Thanks @tasanuma. I tried to adjust it to the one defined here https://github.com/google/guava/blob/master/guava/src/com/google/common/annotations/VisibleForTesting.java
But now I see that GwtCompatible annotation itself has the Retention and Target annotations so they should be applied to VFT as well. Let me make this change.

[tasanuma]

@virajjasani Thanks for updating it. LGTM.

[steveloughran]

+1 from me

[tasanuma]

Merged it. Thanks for your contribution, @virajjasani. Thanks for your review, @steveloughran.