diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 20faecf..2856f3f 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -2,6 +2,16 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
+from os import path
+from subprocess import Popen, PIPE, STDOUT
+
+FMT_CMD = path.join(
+    'third_party',
+    'google-java-format',
+    'google-java-format-google-java-format-1.7',
+    'scripts',
+    'google-java-format-diff.py')
+
 def CheckDoNotMerge(input_api, output_api):
   for l in input_api.change.FullDescriptionText().splitlines():
     if l.lower().startswith('do not merge'):
@@ -9,7 +19,36 @@
       return [output_api.PresubmitPromptWarning(msg, [])]
   return []
 
-def CheckChangeOnUpload(input_api, output_api):
+def CheckFormatting(input_api, output_api):
   results = []
+  for f in input_api.AffectedFiles():
+    path = f.LocalPath()
+    if not path.endswith('.java'):
+      continue
+    proc = Popen(FMT_CMD, stdin=PIPE, stdout=PIPE, stderr=STDOUT)
+    (stdout, stderr) = proc.communicate(input=f.GenerateScmDiff())
+    if len(stdout) > 0:
+      results.append(output_api.PresubmitError(stdout))
+  if len(results) > 0:
+    results.append(output_api.PresubmitError(
+        """Please fix the formatting by running:
+
+  git diff -U0 $(git cl upstream) | %s -p1 -i
+
+or bypass the checks with:
+
+  cl upload --bypass-hooks
+  """ % FMT_CMD))
+  return results
+
+def CheckChange(input_api, output_api):
+  results = []
+  results.extend(CheckFormatting(input_api, output_api))
   results.extend(CheckDoNotMerge(input_api, output_api))
   return results
+
+def CheckChangeOnCommit(input_api, output_api):
+  return CheckChange(input_api, output_api)
+
+def CheckChangeOnUpload(input_api, output_api):
+  return CheckChange(input_api, output_api)