# 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](/styleguide/java/java.md#IntDef-Instead-of-Enum) * 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: ```cpp // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.chrome // GENERATED_JAVA_CLASS_NAME_OVERRIDE: FooBar // GENERATED_JAVA_PREFIX_TO_STRIP: BAR_ enum SomeEnum { BAR_A, BAR_B, BAR_C = BAR_B, }; ``` 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_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: ```java package org.chromium.chrome; import androidx.annotation.IntDef; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; @IntDef({ FooBar.A, FooBar.B, FooBar.C }) @Retention(RetentionPolicy.SOURCE) public @interface FooBar { int A = 0; int B = 1; int C = 1; } ``` ## Formatting Notes * Handling long package names: ```cpp // 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: ```cpp // GENERATED_JAVA_ENUM_PACKAGE: org.foo enum NotificationActionType { BUTTON, TEXT }; ``` * Multi-line enums should have one enum entry per line, like this: ```cpp // GENERATED_JAVA_ENUM_PACKAGE: org.foo enum NotificationActionType { BUTTON, TEXT }; ``` * Multi-line enum entries are allowed but should be formatted like this: ```cpp // GENERATED_JAVA_ENUM_PACKAGE: org.foo enum NotificationActionType { LongKeyNumberOne, LongKeyNumberTwo, ... LongKeyNumberThree = LongKeyNumberOne | LongKeyNumberTwo | ... }; ``` * Preserving comments ```cpp // GENERATED_JAVA_ENUM_PACKAGE: org.chromium enum CommentEnum { // This comment will be preserved. ONE, TWO, // This comment will NOT be preserved. THREE } ``` ```java ... 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: ```shell $ 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: ```shell $ 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 * [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) and [Tests](https://cs.chromium.org/chromium/src/build/android/gyp/java_cpp_enum_tests.py?dr=C&q=java_cpp_enum_tests&sq=package:chromium&l=1) * [GN template](https://cs.chromium.org/chromium/src/build/config/android/rules.gni?q=java_cpp_enum.py&sq=package:chromium&dr=C&l=458)