Android: add java_cpp_features GN rule

This adds the java_cpp_features GN rule to autogenerate Java String
constants representing C++ features. This refactors parts of
java_cpp_strings so java_cpp_features can share it.

This aims to address the most common syntaxes for declaring C++
features (string literal names, "brace" and "equals brace"
initialization, with & without the "base::" namespace).

Design: http://go/autogen-java-features
Bug: 1060097
Fixed: 1091031
Test: vpython build/android/gpy/java_cpp_strings_tests.py
Test: vpython build/android/gpy/java_cpp_features_tests.py
Test: tools/md_browser/md_browser.py docs/android_accessing_cpp_features_in_java.md
Change-Id: I5311f72f8837c122186148cf183f4219087df96a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412840
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812889}

PRESUBMIT.py

@@ -1345,6 +1345,7 @@ _GENERIC_PYDEPS_FILES = [
     'build/android/gyp/ijar.pydeps',
     'build/android/gyp/jacoco_instr.pydeps',
     'build/android/gyp/java_cpp_enum.pydeps',
+    'build/android/gyp/java_cpp_features.pydeps',
     'build/android/gyp/java_cpp_strings.pydeps',
     'build/android/gyp/jetify_jar.pydeps',
     'build/android/gyp/jinja_template.pydeps',

build/android/gyp/java_cpp_enum.py

@@ -333,9 +333,8 @@ def DoGenerate(source_paths):
                       '"// GENERATED_JAVA_ENUM_PACKAGE: foo"?' %
                       source_path)
     for enum_definition in enum_definitions:
-      package_path = enum_definition.enum_package.replace('.', os.path.sep)
-      file_name = enum_definition.class_name + '.java'
-      output_path = os.path.join(package_path, file_name)
+      output_path = java_cpp_utils.GetJavaFilePath(enum_definition.enum_package,
+                                                   enum_definition.class_name)
       output = GenerateOutput(source_path, enum_definition)
       yield output_path, output
 

build/android/gyp/java_cpp_features.py

+#!/user/bin/env python
+#
+# Copyright 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import argparse
+import os
+import re
+import sys
+import zipfile
+
+from util import build_utils
+from util import java_cpp_utils
+
+
+class FeatureParserDelegate(java_cpp_utils.CppConstantParser.Delegate):
+  # Ex. 'const base::Feature kConstantName{"StringNameOfTheFeature", ...};'
+  # would parse as:
+  #   ExtractConstantName() -> 'ConstantName'
+  #   ExtractValue() -> '"StringNameOfTheFeature"'
+  FEATURE_RE = re.compile(r'\s*const (?:base::)?Feature\s+k(\w+)\s*(?:=\s*)?{')
+  VALUE_RE = re.compile(r'\s*("(?:\"|[^"])*")\s*,')
+
+  def ExtractConstantName(self, line):
+    match = FeatureParserDelegate.FEATURE_RE.match(line)
+    return match.group(1) if match else None
+
+  def ExtractValue(self, line):
+    match = FeatureParserDelegate.VALUE_RE.search(line)
+    return match.group(1) if match else None
+
+  def CreateJavaConstant(self, name, value, comments):
+    return java_cpp_utils.JavaString(name, value, comments)
+
+
+def _GenerateOutput(template, source_paths, template_path, features):
+  description_template = """
+    // This following string constants were inserted by
+    //     {SCRIPT_NAME}
+    // From
+    //     {SOURCE_PATHS}
+    // Into
+    //     {TEMPLATE_PATH}
+
+"""
+  values = {
+      'SCRIPT_NAME': java_cpp_utils.GetScriptName(),
+      'SOURCE_PATHS': ',\n    //     '.join(source_paths),
+      'TEMPLATE_PATH': template_path,
+  }
+  description = description_template.format(**values)
+  native_features = '\n\n'.join(x.Format() for x in features)
+
+  values = {
+      'NATIVE_FEATURES': description + native_features,
+  }
+  return template.format(**values)
+
+
+def _ParseFeatureFile(path):
+  with open(path) as f:
+    feature_file_parser = java_cpp_utils.CppConstantParser(
+        FeatureParserDelegate(), f.readlines())
+  return feature_file_parser.Parse()
+
+
+def _Generate(source_paths, template_path):
+  with open(template_path) as f:
+    lines = f.readlines()
+
+  template = ''.join(lines)
+  package, class_name = java_cpp_utils.ParseTemplateFile(lines)
+  output_path = java_cpp_utils.GetJavaFilePath(package, class_name)
+
+  features = []
+  for source_path in source_paths:
+    features.extend(_ParseFeatureFile(source_path))
+
+  output = _GenerateOutput(template, source_paths, template_path, features)
+  return output, output_path
+
+
+def _Main(argv):
+  parser = argparse.ArgumentParser()
+
+  parser.add_argument('--srcjar',
+                      required=True,
+                      help='The path at which to generate the .srcjar file')
+
+  parser.add_argument('--template',
+                      required=True,
+                      help='The template file with which to generate the Java '
+                      'class. Must have "{NATIVE_FEATURES}" somewhere in '
+                      'the template.')
+
+  parser.add_argument('inputs',
+                      nargs='+',
+                      help='Input file(s)',
+                      metavar='INPUTFILE')
+  args = parser.parse_args(argv)
+
+  with build_utils.AtomicOutput(args.srcjar) as f:
+    with zipfile.ZipFile(f, 'w', zipfile.ZIP_STORED) as srcjar:
+      data, path = _Generate(args.inputs, args.template)
+      build_utils.AddToZipHermetic(srcjar, path, data=data)
+
+
+if __name__ == '__main__':
+  _Main(sys.argv[1:])

build/android/gyp/java_cpp_features.pydeps

+# Generated by running:
+#   build/print_python_deps.py --root build/android/gyp --output build/android/gyp/java_cpp_features.pydeps build/android/gyp/java_cpp_features.py
+../../gn_helpers.py
+java_cpp_features.py
+util/__init__.py
+util/build_utils.py
+util/java_cpp_utils.py

build/android/gyp/java_cpp_features_tests.py

+#!/usr/bin/env python
+
+# Copyright 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+"""Tests for java_cpp_features.py.
+
+This test suite contains various tests for the C++ -> Java base::Feature
+generator.
+"""
+
+import unittest
+
+import java_cpp_features
+from util import java_cpp_utils
+
+
+class _TestFeaturesParser(unittest.TestCase):
+  def testParseComments(self):
+    test_data = """
+/**
+ * This should be ignored as well.
+ */
+
+// Comment followed by a blank line.
+
+// Comment followed by unrelated code.
+int foo() { return 3; }
+
+// Real comment.
+const base::Feature kSomeFeature{"SomeFeature",
+                                 base::FEATURE_DISABLED_BY_DEFAULT};
+
+// Real comment that spans
+// multiple lines.
+const base::Feature kSomeOtherFeature{"SomeOtherFeature",
+                                      base::FEATURE_ENABLED_BY_DEFAULT};
+
+// Comment followed by nothing.
+""".split('\n')
+    feature_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_features.FeatureParserDelegate(), test_data)
+    features = feature_file_parser.Parse()
+    self.assertEqual(2, len(features))
+    self.assertEqual('SOME_FEATURE', features[0].name)
+    self.assertEqual('"SomeFeature"', features[0].value)
+    self.assertEqual(1, len(features[0].comments.split('\n')))
+    self.assertEqual('SOME_OTHER_FEATURE', features[1].name)
+    self.assertEqual('"SomeOtherFeature"', features[1].value)
+    self.assertEqual(2, len(features[1].comments.split('\n')))
+
+  def testWhitespace(self):
+    test_data = """
+// 1 line
+const base::Feature kShort{"Short", base::FEATURE_DISABLED_BY_DEFAULT};
+
+// 2 lines
+const base::Feature kTwoLineFeatureA{"TwoLineFeatureA",
+                                     base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kTwoLineFeatureB{
+    "TwoLineFeatureB", base::FEATURE_DISABLED_BY_DEFAULT};
+
+// 3 lines
+const base::Feature kFeatureWithAVeryLongNameThatWillHaveToWrap{
+    "FeatureWithAVeryLongNameThatWillHaveToWrap",
+    base::FEATURE_DISABLED_BY_DEFAULT};
+""".split('\n')
+    feature_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_features.FeatureParserDelegate(), test_data)
+    features = feature_file_parser.Parse()
+    self.assertEqual(4, len(features))
+    self.assertEqual('SHORT', features[0].name)
+    self.assertEqual('"Short"', features[0].value)
+    self.assertEqual('TWO_LINE_FEATURE_A', features[1].name)
+    self.assertEqual('"TwoLineFeatureA"', features[1].value)
+    self.assertEqual('TWO_LINE_FEATURE_B', features[2].name)
+    self.assertEqual('"TwoLineFeatureB"', features[2].value)
+    self.assertEqual('FEATURE_WITH_A_VERY_LONG_NAME_THAT_WILL_HAVE_TO_WRAP',
+                     features[3].name)
+    self.assertEqual('"FeatureWithAVeryLongNameThatWillHaveToWrap"',
+                     features[3].value)
+
+  def testCppSyntax(self):
+    test_data = """
+// Mismatched name
+const base::Feature kMismatchedFeature{"MismatchedName",
+    base::FEATURE_DISABLED_BY_DEFAULT};
+
+namespace myfeature {
+// In a namespace
+const base::Feature kSomeFeature{"SomeFeature",
+                                 base::FEATURE_DISABLED_BY_DEFAULT};
+}
+
+// Defined with equals sign
+const base::Feature kFoo = {"Foo", base::FEATURE_DISABLED_BY_DEFAULT};
+
+// Build config-specific base::Feature
+#if defined(OS_ANDROID)
+const base::Feature kAndroidOnlyFeature{"AndroidOnlyFeature",
+                                        base::FEATURE_DISABLED_BY_DEFAULT};
+#endif
+
+// Value depends on build config
+const base::Feature kMaybeEnabled{"MaybeEnabled",
+#if defined(OS_ANDROID)
+    base::FEATURE_DISABLED_BY_DEFAULT
+#else
+    base::FEATURE_ENABLED_BY_DEFAULT
+#endif
+};
+""".split('\n')
+    feature_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_features.FeatureParserDelegate(), test_data)
+    features = feature_file_parser.Parse()
+    self.assertEqual(5, len(features))
+    self.assertEqual('MISMATCHED_FEATURE', features[0].name)
+    self.assertEqual('"MismatchedName"', features[0].value)
+    self.assertEqual('SOME_FEATURE', features[1].name)
+    self.assertEqual('"SomeFeature"', features[1].value)
+    self.assertEqual('FOO', features[2].name)
+    self.assertEqual('"Foo"', features[2].value)
+    self.assertEqual('ANDROID_ONLY_FEATURE', features[3].name)
+    self.assertEqual('"AndroidOnlyFeature"', features[3].value)
+    self.assertEqual('MAYBE_ENABLED', features[4].name)
+    self.assertEqual('"MaybeEnabled"', features[4].value)
+
+  def testNotYetSupported(self):
+    # Negative test for cases we don't yet support, to ensure we don't misparse
+    # these until we intentionally add proper support.
+    test_data = """
+// Not currently supported: name depends on C++ directive
+const base::Feature kNameDependsOnOs{
+#if defined(OS_ANDROID)
+    "MaybeName1",
+#else
+    "MaybeName2",
+#endif
+    base::FEATURE_DISABLED_BY_DEFAULT};
+
+// Not currently supported: feature named with a constant instead of literal
+const base::Feature kNamedAfterConstant{kNamedStringConstant,
+                                        base::FEATURE_DISABLED_BY_DEFAULT};
+""".split('\n')
+    feature_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_features.FeatureParserDelegate(), test_data)
+    features = feature_file_parser.Parse()
+    self.assertEqual(0, len(features))
+
+  def testTreatWebViewLikeOneWord(self):
+    test_data = """
+const base::Feature kSomeWebViewFeature{"SomeWebViewFeature",
+                                        base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kWebViewOtherFeature{"WebViewOtherFeature",
+                                         base::FEATURE_ENABLED_BY_DEFAULT};
+const base::Feature kFeatureWithPluralWebViews{
+    "FeatureWithPluralWebViews",
+    base::FEATURE_ENABLED_BY_DEFAULT};
+""".split('\n')
+    feature_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_features.FeatureParserDelegate(), test_data)
+    features = feature_file_parser.Parse()
+    self.assertEqual('SOME_WEBVIEW_FEATURE', features[0].name)
+    self.assertEqual('"SomeWebViewFeature"', features[0].value)
+    self.assertEqual('WEBVIEW_OTHER_FEATURE', features[1].name)
+    self.assertEqual('"WebViewOtherFeature"', features[1].value)
+    self.assertEqual('FEATURE_WITH_PLURAL_WEBVIEWS', features[2].name)
+    self.assertEqual('"FeatureWithPluralWebViews"', features[2].value)
+
+  def testSpecialCharacters(self):
+    test_data = r"""
+const base::Feature kFeatureWithEscapes{"Weird\tfeature\"name\n",
+                                        base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kFeatureWithEscapes2{
+    "Weird\tfeature\"name\n",
+    base::FEATURE_ENABLED_BY_DEFAULT};
+""".split('\n')
+    feature_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_features.FeatureParserDelegate(), test_data)
+    features = feature_file_parser.Parse()
+    self.assertEqual('FEATURE_WITH_ESCAPES', features[0].name)
+    self.assertEqual(r'"Weird\tfeature\"name\n"', features[0].value)
+    self.assertEqual('FEATURE_WITH_ESCAPES2', features[1].name)
+    self.assertEqual(r'"Weird\tfeature\"name\n"', features[1].value)
+
+  def testNoBaseNamespacePrefix(self):
+    test_data = """
+const Feature kSomeFeature{"SomeFeature", FEATURE_DISABLED_BY_DEFAULT};
+""".split('\n')
+    feature_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_features.FeatureParserDelegate(), test_data)
+    features = feature_file_parser.Parse()
+    self.assertEqual('SOME_FEATURE', features[0].name)
+    self.assertEqual('"SomeFeature"', features[0].value)
+
+
+if __name__ == '__main__':
+  unittest.main()

build/android/gyp/java_cpp_strings.py

@@ -14,130 +14,20 @@ from util import build_utils
 from util import java_cpp_utils
 
 
-class _String(object):
-
-  def __init__(self, name, value, comments):
-    self.name = java_cpp_utils.KCamelToShouty(name)
-    self.value = value
-    self.comments = '\n'.join('    ' + x for x in comments)
-
-  def Format(self):
-    return '%s\n    public static final String %s = %s;' % (
-        self.comments, self.name, self.value)
-
-
-def ParseTemplateFile(lines):
-  package_re = re.compile(r'^package (.*);')
-  class_re = re.compile(r'.*class (.*) {')
-  package = ''
-  class_name = ''
-  for line in lines:
-    package_line = package_re.match(line)
-    if package_line:
-      package = package_line.groups()[0]
-    class_line = class_re.match(line)
-    if class_line:
-      class_name = class_line.groups()[0]
-      break
-  return package, class_name
-
-
-# TODO(crbug.com/937282): It should be possible to parse a file for more than
-# string constants. However, this currently only handles extracting string
-# constants from a file (and all string constants from that file). Work will
-# be needed if we want to annotate specific constants or non string constants
-# in the file to be parsed.
-class StringFileParser(object):
-  SINGLE_LINE_COMMENT_RE = re.compile(r'\s*(// [^\n]*)')
+class StringParserDelegate(java_cpp_utils.CppConstantParser.Delegate):
   STRING_RE = re.compile(r'\s*const char k(.*)\[\]\s*=')
   VALUE_RE = re.compile(r'\s*("(?:\"|[^"])*")\s*;')
 
-  def __init__(self, lines, path=''):
-    self._lines = lines
-    self._path = path
-    self._in_string = False
-    self._in_comment = False
-    self._package = ''
-    self._current_comments = []
-    self._current_name = ''
-    self._current_value = ''
-    self._strings = []
-
-  def _ExtractVariable(self, line):
-    match = StringFileParser.STRING_RE.match(line)
+  def ExtractConstantName(self, line):
+    match = StringParserDelegate.STRING_RE.match(line)
     return match.group(1) if match else None
 
-  def _ExtractValue(self, line):
-    match = StringFileParser.VALUE_RE.search(line)
+  def ExtractValue(self, line):
+    match = StringParserDelegate.VALUE_RE.search(line)
     return match.group(1) if match else None
 
-  def _Reset(self):
-    self._current_comments = []
-    self._current_name = ''
-    self._current_value = ''
-    self._in_string = False
-    self._in_comment = False
-
-  def _AppendString(self):
-    self._strings.append(
-        _String(self._current_name, self._current_value,
-                self._current_comments))
-    self._Reset()
-
-  def _ParseValue(self, line):
-    current_value = self._ExtractValue(line)
-    if current_value is not None:
-      self._current_value = current_value
-      self._AppendString()
-    else:
-      self._Reset()
-
-  def _ParseComment(self, line):
-    comment_line = StringFileParser.SINGLE_LINE_COMMENT_RE.match(line)
-    if comment_line:
-      self._current_comments.append(comment_line.groups()[0])
-      self._in_comment = True
-      self._in_string = True
-      return True
-    else:
-      self._in_comment = False
-      return False
-
-  def _ParseString(self, line):
-    current_name = self._ExtractVariable(line)
-    if current_name is not None:
-      self._current_name = current_name
-      current_value = self._ExtractValue(line)
-      if current_value is not None:
-        self._current_value = current_value
-        self._AppendString()
-      else:
-        self._in_string = True
-      return True
-    else:
-      self._in_string = False
-      return False
-
-  def _ParseLine(self, line):
-    if not self._in_string:
-      if not self._ParseString(line):
-        self._ParseComment(line)
-      return
-
-    if self._in_comment:
-      if self._ParseComment(line):
-        return
-      if not self._ParseString(line):
-        self._Reset()
-      return
-
-    if self._in_string:
-      self._ParseValue(line)
-
-  def Parse(self):
-    for line in self._lines:
-      self._ParseLine(line)
-    return self._strings
+  def CreateJavaConstant(self, name, value, comments):
+    return java_cpp_utils.JavaString(name, value, comments)
 
 
 def _GenerateOutput(template, source_paths, template_path, strings):
@@ -166,7 +56,9 @@ def _GenerateOutput(template, source_paths, template_path, strings):
 
 def _ParseStringFile(path):
   with open(path) as f:
-    return StringFileParser(f.readlines(), path).Parse()
+    string_file_parser = java_cpp_utils.CppConstantParser(
+        StringParserDelegate(), f.readlines())
+  return string_file_parser.Parse()
 
 
 def _Generate(source_paths, template_path):
@@ -174,10 +66,8 @@ def _Generate(source_paths, template_path):
     lines = f.readlines()
 
   template = ''.join(lines)
-  package, class_name = ParseTemplateFile(lines)
-  package_path = package.replace('.', os.path.sep)
-  file_name = class_name + '.java'
-  output_path = os.path.join(package_path, file_name)
+  package, class_name = java_cpp_utils.ParseTemplateFile(lines)
+  output_path = java_cpp_utils.GetJavaFilePath(package, class_name)
   strings = []
   for source_path in source_paths:
     strings.extend(_ParseStringFile(source_path))
@@ -189,17 +79,15 @@ def _Generate(source_paths, template_path):
 def _Main(argv):
   parser = argparse.ArgumentParser()
 
-  parser.add_argument(
-      '--srcjar',
-      required=True,
-      help='When specified, a .srcjar at the given path is '
-      'created instead of individual .java files.')
+  parser.add_argument('--srcjar',
+                      required=True,
+                      help='The path at which to generate the .srcjar file')
 
-  parser.add_argument(
-      '--template',
-      required=True,
-      help='Can be used to provide a context into which the'
-      'new string constants will be inserted.')
+  parser.add_argument('--template',
+                      required=True,
+                      help='The template file with which to generate the Java '
+                      'class. Must have "{NATIVE_STRINGS}" somewhere in '
+                      'the template.')
 
   parser.add_argument(
       'inputs', nargs='+', help='Input file(s)', metavar='INPUTFILE')

build/android/gyp/java_cpp_strings_tests.py

@@ -12,6 +12,7 @@ This test suite contains various tests for the C++ -> Java string generator.
 import unittest
 
 import java_cpp_strings
+from util import java_cpp_utils
 
 
 class _TestStringsParser(unittest.TestCase):
@@ -36,7 +37,9 @@ const char kAnotherSwitch[] = "another-value";
 
 // Comment followed by nothing.
 """.split('\n')
-    strings = java_cpp_strings.StringFileParser(test_data).Parse()
+    string_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_strings.StringParserDelegate(), test_data)
+    strings = string_file_parser.Parse()
     self.assertEqual(2, len(strings))
     self.assertEqual('A_SWITCH', strings[0].name)
     self.assertEqual('"a-value"', strings[0].value)
@@ -82,7 +85,9 @@ const char kConcatenateMultipleStringLiterals[] =
     "first line"
     "second line";
 """.split('\n')
-    strings = java_cpp_strings.StringFileParser(test_data).Parse()
+    string_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_strings.StringParserDelegate(), test_data)
+    strings = string_file_parser.Parse()
     self.assertEqual(9, len(strings))
     self.assertEqual('A_STRING', strings[0].name)
     self.assertEqual('"a-value"', strings[0].value)
@@ -113,7 +118,9 @@ const char kSomeWebViewSwitch[] = "some-webview-switch";
 const char kWebViewOtherSwitch[] = "webview-other-switch";
 const char kSwitchWithPluralWebViews[] = "switch-with-plural-webviews";
 """.split('\n')
-    strings = java_cpp_strings.StringFileParser(test_data).Parse()
+    string_file_parser = java_cpp_utils.CppConstantParser(
+        java_cpp_strings.StringParserDelegate(), test_data)
+    strings = string_file_parser.Parse()
     self.assertEqual('SOME_WEBVIEW_SWITCH', strings[0].name)
     self.assertEqual('"some-webview-switch"', strings[0].value)
     self.assertEqual('WEBVIEW_OTHER_SWITCH', strings[1].name)
@@ -123,16 +130,10 @@ const char kSwitchWithPluralWebViews[] = "switch-with-plural-webviews";
 
   def testTemplateParsing(self):
     test_data = """
-// Copyright {YEAR} The Chromium Authors. All rights reserved.
+// Copyright 2019 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// This file is autogenerated by
-//     {SCRIPT_NAME}
-// From
-//     {SOURCE_PATH}, and
-//     {TEMPLATE_PATH}
-
 package my.java.package;
 
 public any sort of class MyClass {{
@@ -141,7 +142,7 @@ public any sort of class MyClass {{
 
 }}
 """.split('\n')
-    package, class_name = java_cpp_strings.ParseTemplateFile(test_data)
+    package, class_name = java_cpp_utils.ParseTemplateFile(test_data)
     self.assertEqual('my.java.package', package)
     self.assertEqual('MyClass', class_name)
 

build/android/gyp/util/java_cpp_utils.py

@@ -13,6 +13,12 @@ def GetScriptName():
   return os.path.basename(os.path.abspath(sys.argv[0]))
 
 
+def GetJavaFilePath(java_package, class_name):
+  package_path = java_package.replace('.', os.path.sep)
+  file_name = class_name + '.java'
+  return os.path.join(package_path, file_name)
+
+
 def KCamelToShouty(s):
   """Convert |s| from kCamelCase or CamelCase to SHOUTY_CASE.
 
@@ -32,3 +38,159 @@ def KCamelToShouty(s):
   # Add _ between lower -> upper transitions.
   s = re.sub(r'([^A-Z_0-9])([A-Z])', r'\1_\2', s)
   return s.upper()
+
+
+class JavaString(object):
+  def __init__(self, name, value, comments):
+    self.name = KCamelToShouty(name)
+    self.value = value
+    self.comments = '\n'.join('    ' + x for x in comments)
+
+  def Format(self):
+    return '%s\n    public static final String %s = %s;' % (
+        self.comments, self.name, self.value)
+
+
+def ParseTemplateFile(lines):
+  package_re = re.compile(r'^package (.*);')
+  class_re = re.compile(r'.*class (.*) {')
+  package = ''
+  class_name = ''
+  for line in lines:
+    package_line = package_re.match(line)
+    if package_line:
+      package = package_line.groups()[0]
+    class_line = class_re.match(line)
+    if class_line:
+      class_name = class_line.groups()[0]
+      break
+  return package, class_name
+
+
+# TODO(crbug.com/937282): Work will be needed if we want to annotate specific
+# constants in the file to be parsed.
+class CppConstantParser(object):
+  """Parses C++ constants, retaining their comments.
+
+  The Delegate subclass is responsible for matching and extracting the
+  constant's variable name and value, as well as generating an object to
+  represent the Java representation of this value.
+  """
+  SINGLE_LINE_COMMENT_RE = re.compile(r'\s*(// [^\n]*)')
+
+  class Delegate(object):
+    def ExtractConstantName(self, line):
+      """Extracts a constant's name from line or None if not a match."""
+      raise NotImplementedError()
+
+    def ExtractValue(self, line):
+      """Extracts a constant's value from line or None if not a match."""
+      raise NotImplementedError()
+
+    def CreateJavaConstant(self, name, value, comments):
+      """Creates an object representing the Java analog of a C++ constant.
+
+      CppConstantParser will not interact with the object created by this
+      method. Instead, it will store this value in a list and return a list of
+      all objects from the Parse() method. In this way, the caller may define
+      whatever class suits their need.
+
+      Args:
+        name: the constant's variable name, as extracted by
+          ExtractConstantName()
+        value: the constant's value, as extracted by ExtractValue()
+        comments: the code comments describing this constant
+      """
+      raise NotImplementedError()
+
+  def __init__(self, delegate, lines):
+    self._delegate = delegate
+    self._lines = lines
+    self._in_variable = False
+    self._in_comment = False
+    self._package = ''
+    self._current_comments = []
+    self._current_name = ''
+    self._current_value = ''
+    self._constants = []
+
+  def _ExtractVariable(self, line):
+    match = StringFileParser.STRING_RE.match(line)
+    return match.group(1) if match else None
+
+  def _ExtractValue(self, line):
+    match = StringFileParser.VALUE_RE.search(line)
+    return match.group(1) if match else None
+
+  def _Reset(self):
+    self._current_comments = []
+    self._current_name = ''
+    self._current_value = ''
+    self._in_variable = False
+    self._in_comment = False
+
+  def _AppendConstant(self):
+    self._constants.append(
+        self._delegate.CreateJavaConstant(self._current_name,
+                                          self._current_value,
+                                          self._current_comments))
+    self._Reset()
+
+  def _ParseValue(self, line):
+    current_value = self._delegate.ExtractValue(line)
+    if current_value is not None:
+      self._current_value = current_value
+      self._AppendConstant()
+    else:
+      self._Reset()
+
+  def _ParseComment(self, line):
+    comment_line = CppConstantParser.SINGLE_LINE_COMMENT_RE.match(line)
+    if comment_line:
+      self._current_comments.append(comment_line.groups()[0])
+      self._in_comment = True
+      self._in_variable = True
+      return True
+    else:
+      self._in_comment = False
+      return False
+
+  def _ParseVariable(self, line):
+    current_name = self._delegate.ExtractConstantName(line)
+    if current_name is not None:
+      self._current_name = current_name
+      current_value = self._delegate.ExtractValue(line)
+      if current_value is not None:
+        self._current_value = current_value
+        self._AppendConstant()
+      else:
+        self._in_variable = True
+      return True
+    else:
+      self._in_variable = False
+      return False
+
+  def _ParseLine(self, line):
+    if not self._in_variable:
+      if not self._ParseVariable(line):
+        self._ParseComment(line)
+      return
+
+    if self._in_comment:
+      if self._ParseComment(line):
+        return
+      if not self._ParseVariable(line):
+        self._Reset()
+      return
+
+    if self._in_variable:
+      self._ParseValue(line)
+
+  def Parse(self):
+    """Returns a list of objects representing C++ constants.
+
+    Each object in the list was created by Delegate.CreateJavaValue().
+    """
+    for line in self._lines:
+      self._ParseLine(line)
+    return self._constants

build/config/android/rules.gni

@@ -607,6 +607,81 @@ if (enable_java_templates) {
     }
   }
 
+  # Declare a target for generating Java classes with string constants matching
+  # those found in C++ base::Feature declarations, using a python script.
+  #
+  # This target will create a single .srcjar. Adding this target to an
+  # android_library target's srcjar_deps will make the generated java files be
+  # included in that library's final outputs.
+  #
+  # Variables
+  #   sources: list of files to be processed by the script. For each
+  #            base::Feature in the source files, the script will add a
+  #            corresponding Java string for that feature's name to the
+  #            specified template file.
+  # Example
+  #   java_cpp_features("foo_features") {
+  #     sources = [
+  #       "src/foo_features.cc",
+  #     ]
+  #     template = "src/templates/FooFeatures.java.tmpl
+  #   }
+  #
+  # foo_features.cc:
+  #
+  # // A feature.
+  # const base::Feature kSomeFeature{"SomeFeature",
+  #                                  base::FEATURE_DISABLED_BY_DEFAULT};
+  #
+  # FooFeatures.java.tmpl
+  #
+  # // Copyright $YEAR The Chromium Authors. All rights reserved.
+  # // Use of this source code is governed by a BSD-style license that can be
+  # // found in the LICENSE file.
+  #
+  # package my.java.package;
+  #
+  # public final class FooFeatures {{
+  #     // ...snip...
+  # {NATIVE_STRINGS}
+  #     // ...snip...
+  #     // Do not instantiate this class.
+  #     private FooFeatures() {{}}
+  # }}
+  #
+  # result:
+  #   A FooFeatures.java file, defining a class named FooFeatures in the package
+  #   my.java.package.
+  template("java_cpp_features") {
+    set_sources_assignment_filter([])
+    action_with_pydeps(target_name) {
+      forward_variables_from(invoker,
+                             [
+                               "sources",
+                               "testonly",
+                               "visibility",
+                             ])
+
+      # The sources aren't compiled so don't check their dependencies.
+      check_includes = false
+      script = "//build/android/gyp/java_cpp_features.py"
+
+      _srcjar_path = "${target_gen_dir}/${target_name}.srcjar"
+      _rebased_srcjar_path = rebase_path(_srcjar_path, root_build_dir)
+      _rebased_sources = rebase_path(invoker.sources, root_build_dir)
+      _rebased_template = rebase_path(invoker.template, root_build_dir)
+
+      args = [
+        "--srcjar=$_rebased_srcjar_path",
+        "--template=$_rebased_template",
+      ]
+      args += _rebased_sources
+      sources += [ invoker.template ]
+
+      outputs = [ _srcjar_path ]
+    }
+  }
+
   # Declare a target for processing a Jinja template.
   #
   # Variables

docs/android_accessing_cpp_enums_in_java.md

@@ -146,6 +146,10 @@ class
     }
     ```
 
+## See also
+* [Accessing C++ Switches In Java](android_accessing_cpp_switches_in_java.md)
+* [Accessing C++ Features In Java](android_accessing_cpp_features_in_java.md)
+
 ## Code
 * [Generator
 code](https://cs.chromium.org/chromium/src/build/android/gyp/java_cpp_enum.py?dr=C&sq=package:chromium)

docs/android_accessing_cpp_features_in_java.md

+# Accessing C++ Features In Java
+
+[TOC]
+
+## Introduction
+
+Accessing C++ `base::Features` in Java is implemented via a Python script which
+analyzes the `*_features.cc` file and generates the corresponding Java class,
+based on a template file. The template file must be specified in the GN target.
+This outputs Java String constants which represent the name of the
+`base::Feature`.
+
+## Usage
+
+1. Create a template file (ex. `FooFeatures.java.tmpl`). Change "Copyright
+   2020" to be whatever the year is at the time of writing (as you would for any
+   other file).
+   ```java
+    // Copyright 2020 The Chromium Authors. All rights reserved.
+    // Use of this source code is governed by a BSD-style license that can be
+    // found in the LICENSE file.
+
+    package org.chromium.foo;
+
+    // Be sure to escape any curly braces in your template by doubling as
+    // follows.
+    /**
+     * Contains features that are specific to the foo project.
+     */
+    public final class FooFeatures {{
+
+    {NATIVE_FEATURES}
+
+        // Prevents instantiation.
+        private FooFeatures() {{}}
+    }}
+   ```
+
+2. Add a new build target and add it to the `srcjar_deps` of an
+   `android_library` target:
+
+    ```gn
+    if (is_android) {
+      import("//build/config/android/rules.gni")
+    }
+
+    if (is_android) {
+      java_cpp_features("java_features_srcjar") {
+        # External code should depend on ":foo_java" instead.
+        visibility = [ ":*" ]
+        sources = [
+          "//base/android/foo_features.cc",
+        ]
+        template = "//base/android/java_templates/FooFeatures.java.tmpl"
+      }
+
+      # If there's already an android_library target, you can add
+      # java_features_srcjar to that target's srcjar_deps. Otherwise, the best
+      # practice is to create a new android_library just for this target.
+      android_library("foo_java") {
+        srcjar_deps = [ ":java_features_srcjar" ]
+      }
+    }
+    ```
+
+3. The generated file `out/Default/gen/.../org/chromium/foo/FooFeatures.java`
+   would contain:
+
+    ```java
+    // Copyright $YEAR The Chromium Authors. All rights reserved.
+    // Use of this source code is governed by a BSD-style license that can be
+    // found in the LICENSE file.
+
+    package org.chromium.foo;
+
+    // Be sure to escape any curly braces in your template by doubling as
+    // follows.
+    /**
+     * Contains features that are specific to the foo project.
+     */
+    public final class FooFeatures {
+
+        // This following string constants were inserted by
+        //     java_cpp_features.py
+        // From
+        //     ../../base/android/foo_features.cc
+        // Into
+        //     ../../base/android/java_templates/FooFeatures.java.tmpl
+
+        // Documentation for the C++ Feature is copied here.
+        public static final String SOME_FEATURE = "SomeFeature";
+
+        // ...snip...
+
+        // Prevents instantiation.
+        private FooFeatures() {}
+    }
+    ```
+
+### Troubleshooting
+
+The script only supports limited syntaxes for declaring C++ base::Features. You
+may see an error like the following during compilation:
+
+```
+...
+org/chromium/foo/FooFeatures.java:41: error: duplicate declaration of field: MY_FEATURE
+    public static final String MY_FEATURE = "MyFeature";
+```
+
+This can happen if you've re-declared a feature for mutually-exclsuive build
+configs (ex. the feature is enabled-by-default for one config, but
+disabled-by-default for another). Example:
+
+```c++
+#if defined(...)
+const base::Feature kMyFeature{
+    "MyFeature", base::FEATURE_ENABLED_BY_DEFAULT};
+#else
+const base::Feature kMyFeature{
+    "MyFeature", base::FEATURE_DISABLED_BY_DEFAULT};
+#endif
+```
+
+The `java_cpp_features` rule doesn't know how to evaluate C++ preprocessor
+directives, so it generates two identical Java fields (which is what the
+compilation error is complaining about). Fortunately, the workaround is fairly
+simple. Rewrite the definition to only use directives around the enabled state:
+
+```c++
+const base::Feature kMyFeature{
+    "MyFeature",
+#if defined(...)
+    base::FEATURE_ENABLED_BY_DEFAULT
+#else
+    base::FEATURE_DISABLED_BY_DEFAULT
+#endif
+};
+
+```
+
+## Checking if a Feature is enabled
+
+The standard pattern is to create a `FooFeatureList.java` class with an
+`isEnabled()` method (ex.
+[`ContentFeatureList`](/content/public/android/java/src/org/chromium/content_public/browser/ContentFeatureList.java)).
+This should call into C++ (ex.
+[`content_feature_list`](/content/browser/android/content_feature_list.cc)),
+where a subset of features are exposed via the `kFeaturesExposedToJava` array.
+You can either add your `base::Feature` to an existing `feature_list` or create
+a new `FeatureList` class if no existing one is suitable. Then you can check the
+enabled state like so:
+
+```java
+// It's OK if ContentFeatureList checks FooFeatures.*, so long as
+// content_feature_list.cc exposes `kMyFeature`.
+if (ContentFeatureList.isEnabled(FooFeatures.MY_FEATURE)) {
+    // ...
+}
+```
+
+At the moment, `base::Features` must be explicitly exposed to Java this way, in
+whichever layer needs to access their state. See https://crbug.com/1060097.
+
+## See also
+* [Accessing C++ Enums In Java](android_accessing_cpp_enums_in_java.md)
+* [Accessing C++ Switches In Java](android_accessing_cpp_switches_in_java.md)
+
+## Code
+* [Generator code](/build/android/gyp/java_cpp_features.py) and
+  [Tests](/build/android/gyp/java_cpp_features_tests.py)
+* [GN template](/build/config/android/rules.gni)

docs/android_accessing_cpp_switches_in_java.md

@@ -10,9 +10,11 @@ template file. The template file must be specified in the GN target.
 
 ## Usage
 
-1. Create a template file (ex. `FooSwitches.java.tmpl`)
+1. Create a template file (ex. `FooSwitches.java.tmpl`). Change "Copyright
+   2020" to be whatever the year is at the time of writing (as you would for any
+   other file).
    ```java
-    // Copyright $YEAR The Chromium Authors. All rights reserved.
+    // Copyright 2020 The Chromium Authors. All rights reserved.
     // Use of this source code is governed by a BSD-style license that can be
     // found in the LICENSE file.
 
@@ -93,6 +95,10 @@ template file. The template file must be specified in the GN target.
     }
     ```
 
+## See also
+* [Accessing C++ Enums In Java](android_accessing_cpp_enums_in_java.md)
+* [Accessing C++ Features In Java](android_accessing_cpp_features_in_java.md)
+
 ## Code
 * [Generator
 code](https://cs.chromium.org/chromium/src/build/android/gyp/java_cpp_strings.py?dr=C&sq=package:chromium)