# Gerrit Code Review - Crafting Changes

version v3.4.4

Here are some hints as to what approvers may be looking for before approving or submitting changes to the Gerrit project. Let’s start with the simple nit picky stuff. You are likely excited that your code works; help us share your excitement by not distracting us with the simple stuff. Thanks to Gerrit, problems are often highlighted and we find it hard to look beyond simple spacing issues. Blame it on our short attention spans, we really do want your code.

## Branch

Gerrit provides support for more than one version, which naturally raises the question of which branch you should start your contribution on. There are no hard and fast rules, but below we try to outline some guidelines:

- Genuinely new and/or disruptive features, should generally start on `master`. Also consider submitting a design doc beforehand to allow discussion by the ESC and the community.

- Improvements of existing features should also generally go into `master`. But we understand that if you cannot run `master`, it might take a while until you could benefit from it. In that case, start on the newest `stable-*` branch that you can run.

- Bug-fixes should generally at least cover the oldest affected and still supported version. If you’re affected and run an even older version, you’re welcome to upload to that older version, even if it is no longer officially supported, bearing in mind that verification and release may happen only once merged upstream.

Regardless of the above, changes might get moved to a different branch before being submitted or might get cherry-picked/re-merged to a different branch even after they’ve landed.

For each of the above items, you’ll find ad-hoc exceptions. The point is: We’d much rather see your code and fixes than not see them.

## Commit Message

It is essential to have a good commit message if you want your change to be reviewed.

- Keep lines no longer than 72 chars

- Start with a short one line summary

- Followed by a blank line

- Followed by one or more explanatory paragraphs

- Use the present tense (fix instead of fixed)

- Use the past tense when describing the status before this commit

- Include a `Bug: Issue <#>` line if fixing a Gerrit issue, or a `Feature: Issue <#>` line if implementing a feature request.

- Include a `Change-Id` line

### Setting up Vim for Git commit message

Git uses Vim as the default commit message editor. Put this into your `$HOME/.vimrc` file to configure Vim for Git commit message formatting and writing:

    " Enable spell checking, which is not on by default for commit messages.
    au FileType gitcommit setlocal spell

    " Reset textwidth if you've previously overridden it.
    au FileType gitcommit setlocal textwidth=72

### A sample good Gerrit commit message:

    Add sample commit message to guidelines doc

    The original patch set for the contributing guidelines doc did not
    include a sample commit message, this new patchset does.  Hopefully this
    makes things a bit clearer since examples can sometimes help when
    explanations don't.

    Note that the body of this commit message can be several paragraphs, and
    that I word wrap it at 72 characters.  Also note that I keep the summary
    line under 50 characters since it is often truncated by tools which
    display just the git summary.

    Bug: Issue 98765605
    Change-Id: Ic4a7c07eeb98cdeaf44e9d231a65a51f3fceae52

The `Change-Id` line is, as usual, created by a local git hook. To install it, simply copy it from the checkout and make it executable:

    cp ./gerrit-server/src/main/resources/com/google/gerrit/server/tools/root/hooks/commit-msg .git/hooks/
    chmod +x .git/hooks/commit-msg

If you are working on core plugins, you will also need to install the same hook in the submodules:

    export hook=$(pwd)/.git/hooks/commit-msg
    git submodule foreach 'cp -p "$hook" "$(git rev-parse --git-dir)/hooks/"'

To set up git’s remote for easy pushing, run the following:

    git remote add gerrit https://gerrit.googlesource.com/gerrit

The HTTPS access requires proper username and password; this can be obtained by clicking the 'Obtain Password' link on the HTTP Password tab of the user settings page.

Alternately, you may use the git-review tool to submit changes to Gerrit. If you do, it will set up the Change-Id hook and `gerrit` remote for you. You will still need to do the HTTP access step.

## Style

This project has a policy of Eclipse’s warning free code. Eclipse configuration is added to git and we expect the changes to be warnings free.

We do not ask you to use Eclipse for editing, obviously. We do ask you to provide Eclipse’s warning free patches only. If for some reasons, you are not able to set up Eclipse and verify, that your patch hasn’t introduced any new Eclipse warnings, mention this in a comment to your change, so that reviewers will do it for you. Yes, the way to go is to extend gerrit CI to take care of this, but it’s not yet implemented.

Gerrit follows the Google Java Style.

To format Java source code, Gerrit uses the `google-java-format` tool (version 1.7), and to format Bazel BUILD, WORKSPACE and .bzl files the `buildifier` tool (version 4.0.0). Unused dependencies are found and removed using the `unused_deps` build tool, a sibling of `buildifier`.

These tools automatically apply format according to the style guides; this streamlines code review by reducing the need for time-consuming, tedious, and contentious discussions about trivial issues like whitespace.

You may download and run `google-java-format` on your own, or you may run `./tools/setup_gjf.sh` to download a local copy and set up a wrapper script. If you run your own copy, please use the same version, as there may be slight differences between versions.

## Code Rules

### Final

When to use `final` modifier and when not (in new code):

Always:

- final fields: marking fields as final forces them to be initialized in the constructor or at declaration

- final static fields: clearly communicates the intent

- to use final variables in inner anonymous classes

Optional:

- final classes: use when appropriate, e.g. API restriction

- final methods: similar to final classes

Never:

- local variables: it clutters the code, and makes the code less readable. When copying old code to new location, finals should be removed

- method parameters: similar to local variables

### Optional / Nullable

Recommended:

- Optionals in arguments are discouraged (use @Nullable instead)

- Return types should be objects or Optionals of objects, but not null/nullable

## Code Organization

Do your best to organize classes and methods in a logical way. Here are some guidelines that Gerrit uses:

- Ensure a standard copyright header is included at the top of any new files (copy it from another file, update the year).

- Always place loggers first in your class!

- Define any static interfaces next in your class.

- Define non static interfaces after static interfaces in your class.

- Next you should define static types, static members, and static methods, in decreasing order of visibility (public to private).

- Finally instance types, instance members, then constructors, and then instance methods.

- Some common exceptions are private helper static methods, which might appear near the instance methods which they help (but may also appear at the top).

- Getters and setters for the same instance field should usually be near each other barring a good reason not to.

- If you are using assisted injection, the factory for your class should be before the instance members.

- Annotations should go before language keywords (`final`, `private`, etc)
  Example: `@Assisted @Nullable final type varName`

- Prefer to open multiple AutoCloseable resources in the same try-with-resources block instead of nesting the try-with-resources blocks and increasing the indentation level more than necessary.

Wow that’s a lot! But don’t worry, you’ll get the habit and most of the code is organized this way already; so if you pay attention to the class you are editing you will likely pick up on it. Naturally new classes are a little harder; you may want to come back and consult this section when creating them.

## Design

Here are some design level objectives that you should keep in mind when coding:

- Most client pages should perform only one RPC to load so as to keep latencies down. Exceptions would apply to RPCs which need to load large data sets if splitting them out will help the page load faster. Generally page loads are expected to complete in under 100ms. This will be the case for most operations, unless the data being fetched is not using Gerrit’s caching infrastructure. In these slower cases, it is worth considering mitigating this longer load by using a second RPC to fill in this data after the page is displayed (or alternatively it might be worth proposing caching this data).

- `@Inject` should be used on constructors, not on fields. The current exceptions are the ssh commands, these were implemented earlier in Gerrit’s development. To stay consistent, new ssh commands should follow this older pattern; but eventually these should get converted to eliminate this exception.

- Don’t leave repository objects (git or schema) open. Use a try-with-resources statement to ensure that repository objects get closed after use.

- Don’t leave UI components, which can cause new actions to occur, enabled during RPCs which update Git repositories, including NoteDb. This is to prevent people from submitting actions more than once when operating on slow links. If the action buttons are disabled, they cannot be resubmitted and the user can see that Gerrit is still busy.

## Tests

- Tests for new code will greatly help your change get approved.

## Javadoc

- Javadocs for new code (especially public classes and public/protected methods) will greatly help your change get approved.

## Change Size/Number of Files Touched

And finally, I probably cannot say enough about change sizes. Generally, smaller is better, hopefully within reason. Do try to keep things which will be confusing on their own together, especially if changing one without the other will break something!

- If a new feature is implemented and it is a larger one, try to identify if it can be split into smaller logical features; when in doubt, err on the smaller side.

- Separate bug fixes from feature improvements. The bug fix may be an easy candidate for approval and should not need to wait for new features to be approved. Also, combining the two makes reviewing harder since then there is no clear line between the fix and the feature.

- Separate supporting refactoring from feature changes. If your new feature requires some refactoring, it helps to make the refactoring a separate change which your feature change depends on. This way, reviewers can easily review the refactor change as a something that should not alter the current functionality, and feel more confident they can more easily spot errors this way. Of course, it also makes it easier to test and locate later on if an unfortunate error does slip in. Lastly, by not having to see refactoring changes at the same time, it helps reviewers understand how your feature changes the current functionality.

- Separate logical features into separate changes. This is often the hardest part. Here is an example: when adding a new ability, make separate changes for the UI and the ssh commands if possible.

- Do only what the commit message describes. In other words, things which are not strictly related to the commit message shouldn’t be part of a change, even trivial things like externalizing a string somewhere or fixing a typo. This helps keep `git blame` more useful in the future and it also makes `git revert` more useful.

- Use topics to link your separate changes together.

## Opportunistic Refactoring

Opportunistic Refactoring is a terminology used by Martin Fowler also known as the "boy scout rule" of the software developer: "always leave the code behind in a better state than you found it."

In practice, this rule means you should not add technical debt in the code while implementing a new feature or fixing a bug. If you or a reviewer find an opportunity to clean up the code during implementation or review of your change, take the time to do a little cleanup to improve the overall code base.

When approaching refactoring, keep in mind that changes should do one thing (see change size section above). If a change you’re making requires cleanup/refactoring, it is best to do that cleanup in a preparatory and separate change. Likewise, if during review for a functional change, an opportunity for cleanup/refactoring is discovered, then it is preferable to do the cleanup first in a separate change so as to improve the reviewability of the functional change.

Reviewers should keep in mind the scope of the change under review and ensure suggested refactoring is aligned with that scope.

Part of Gerrit Code Review