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Accessing C++ Enums In Java

[TOC]

Introduction

Accessing C++ enums in Java is implemented via a Python script which analyzes the C++ enum and spits out the corresponding Java class. The enum needs to be annotated in a particular way. By default, the generated class name will be the same as the name of the enum. If all the names of the enum values are prefixed with the MACRO_CASED_ name of the enum those prefixes will be stripped from the Java version.

Features

  • Customize the package name of the generated class using the GENERATED_JAVA_ENUM_PACKAGE directive (required)
  • Customize the class name using the GENERATED_JAVA_CLASS_NAME_OVERRIDE directive (optional)
  • Strip enum entry prefixes to make the generated classes less verbose using the GENERATED_JAVA_PREFIX_TO_STRIP directive (optional)
  • Follows best practices by using IntDef Instead of Enum
  • Generate the flag attribute using the GENERATED_JAVA_IS_FLAG directive (optional)
  • Copies comments that directly precede enum entries into the generated Java class

Usage

  1. Add directives to your C++ enum. Only the GENERATED_JAVA_ENUM_PACKAGE directive is required:

    // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.chrome
// GENERATED_JAVA_CLASS_NAME_OVERRIDE: FooBar
// GENERATED_JAVA_PREFIX_TO_STRIP: BAR_
// GENERATED_JAVA_IS_FLAG: true
enum SomeEnum {
  BAR_A = 1 << 0,
  BAR_B = 1 << 1,
  BAR_C = BAR_B,
};

  2. Add a new build target and add it to the srcjar_deps of an android_library target:

    if (is_android) {
  import("//build/config/android/rules.gni")
}

if (is_android) {
  java_cpp_enum("java_enum_srcjar") {
    # External code should depend on ":foo_java" instead.
    visibility = [ ":*" ]
    sources = [
      # Include the .h or .cc file(s) which defines the enum(s).
      "base/android/native_foo_header.h",
    ]
  }

  # If there's already an android_library target, you can add
  # java_enum_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_enum_srcjar" ]

    # Important: the generated enum uses the @IntDef annotation provided by
    # this dependency.
    deps = [ "//third_party/androidx:androidx_annotation_annotation_java" ]
  }
}

  3. The generated file org/chromium/chrome/FooBar.java would contain:

    package org.chromium.chrome;

import androidx.annotation.IntDef;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@IntDef(flag = true, value = {
    FooBar.A, FooBar.B, FooBar.C
})
@Retention(RetentionPolicy.SOURCE)
public @interface FooBar {
  int A = 1 << 0;
  int B = 1 << 1;
  int C = 1 << 1;
}

Formatting Notes

  • Handling long package names:

    // GENERATED_JAVA_ENUM_PACKAGE: (
//   org.chromium.chrome.this.package.is.too.long.to.fit.on.a.single.line)

  • Enum entries

    • Single line enums should look like this:

      // GENERATED_JAVA_ENUM_PACKAGE: org.foo
enum NotificationActionType { BUTTON, TEXT };

    • Multi-line enums should have one enum entry per line, like this:

      // GENERATED_JAVA_ENUM_PACKAGE: org.foo
enum NotificationActionType {
  BUTTON,
  TEXT
};

    • Multi-line enum entries are allowed but should be formatted like this:

      // GENERATED_JAVA_ENUM_PACKAGE: org.foo
enum NotificationActionType {
  LongKeyNumberOne,
  LongKeyNumberTwo,
  ...
  LongKeyNumberThree =
      LongKeyNumberOne | LongKeyNumberTwo | ...
};

  • Preserving comments

    // GENERATED_JAVA_ENUM_PACKAGE: org.chromium
enum CommentEnum {
  // This comment will be preserved.
  ONE,
  TWO, // This comment will NOT be preserved.
  THREE
}
    ...
public @interface CommentEnum {
  ...
  /**
   * This comment will be preserved.
   */
  int ONE = 0;
  int TWO = 1;
  int THREE = 2;
}

Troubleshooting

Symbol not found/could not resolve IntDef

You may see an error like this when compiling:

$ autoninja -C out/Default base/foo_java
util.build_utils.CalledProcessError: Command failed: ...
org/chromium/chrome/FooBar.java:13: error: symbol not found androidx.annotation.IntDef
Hint: Add "//third_party/androidx:androidx_annotation_annotation_java" to deps of //base/foo_java
import androidx.annotation.IntDef;
       ^
org/chromium/chrome/FooBar.java:18: error: could not resolve IntDef
@IntDef({
^

The fix is to add "//third_party/androidx:androidx_annotation_annotation_java" to the deps of the android_library. Note: do not add this to the java_cpp_enum target by mistake, otherwise you'll see a new error:

$ autoninja -C out/Default base/foo_java
[0/1] Regenerating ninja files
ERROR at //base/BUILD.gn:194:12: Assignment had no effect.
    deps = [ "//third_party/androidx:androidx_annotation_annotation_java" ]
           ^--------------------------------------------------------------
You set the variable "deps" here and it was unused before it went
out of scope.
...

See also

Code