Add gjf code style pre commit hook support.

README.md

@@ -73,6 +73,17 @@ See the [Usage](https://github.com/uber/okbuck/blob/master/Usage.md) page for co
 
 To speed up your builds even more, you can use an implementation of [Buck's HTTP Cache API](https://github.com/uber/buck-http-cache) to take advantage of building once and using the same build artifacts on all machines.
 
+## Code Style
+We use [google-java-format](https://github.com/google/google-java-format) to format Java files.
+You can either install the IntelliJ plugin or use a pre commit hook to automatically re-format files.
+
+To add a pre commit hook:
+
+- Build `google-java-format` jar following instructions from [github page](https://github.com/google/google-java-format).
+- Copy `config/git-hooks/pre-commit.template` to `.git/hooks/pre-commit`
+- Make the script executable: `chmod +x .git/hooks/pre-commit`
+- Replace `<<PATH_TO_JAR>>` in `.git/hooks/pre-commit` with the location of google-java-format jar.
+
 ## Contributors
 
 We'd love for you to contribute to our open source projects. Before we can accept your contributions, we kindly ask you to sign our [Uber Contributor License Agreement](https://docs.google.com/a/uber.com/forms/d/1pAwS_-dA1KhPlfxzYLBqK6rsSWwRwH95OCCZrcsY5rk/viewform).

pre-commit.template

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+REPO_ROOT_DIR="$(git rev-parse --show-toplevel)"
+
+FORMAT_JAR_LOCATION="<<PATH_TO_JAR>>/google-java-format-1.5-all-deps.jar"
+
+for staged_file in $(git diff --cached --name-only); do
+  [ -f "${staged_file}" ] || continue
+
+  "${REPO_ROOT_DIR}/scripts/check-java-file-format" "${FORMAT_JAR_LOCATION}" "${staged_file}"
+  git add "${staged_file}"
+done

tooling/check-java-file-format

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+JAR_PATH=$1
+FILE_PATH=$2
+
+# Test if the file is Java file
+echo "${FILE_PATH}" | grep -Eqi "\.java$" || exit 1
+
+# Try to format the file
+java -jar "${JAR_PATH}" -i "${FILE_PATH}"
+
+exit 0