Introduction

Experimental bindings generator for Java bindings through dart:ffi and JNI.

jnigen scans compiled JAR files or Java source code to generate a description of the API, then uses it to generate Dart annd C bindings. The Dart bindings call the C bindings, which in-turn call the Java functions through JNI. Shared functionality and base classes are provided through the support library, package:jni.

The configuration for binding generation is usually provided through YAML.

Three configuration details are needed to generate the bindings. Everything else is optional:

• Inputs: input can be Java source files (source_path), or compiled classes / JARs (class_path). Some maven / gradle based tooling is also provided to simplify obtaining dependencies.

• Outputs: Output can be generated in package-structured (one file per class) or single file bindings. Target path to write C and Dart bindings needs to be specified.

• Classes: Specify which classes or packages you need bindings for. Specifying a package includes all classes inside it recursively.

Check out the examples to see some sample configurations.

C code is always generated into a directory with it's own build configuration. It's built as a separate dynamic library.

Lastly, dart_only bindings mode is also available as a proof-of-concept. It does not need intermediate C bindings, only a dependency on the support library package:jni.

Example

It's possible to generate bindings for JAR libraries, or Java source files.

Here's a simple example Java file, in a Flutter Android app.

package com.example.in_app_java;

import android.app.Activity;
import android.widget.Toast;
import androidx.annotation.Keep;

@Keep
public abstract class AndroidUtils {
  // Hide constructor
  private AndroidUtils() {}

  public static void showToast(Activity mainActivity, CharSequence text, int duration) {
    mainActivity.runOnUiThread(() -> Toast.makeText(mainActivity, text, duration).show());
  }
}

This produces the following boilerplate:

Dart Bindings:

/// Some boilerplate is omitted for clarity.
final ffi.Pointer<T> Function<T extends ffi.NativeType>(String sym) jniLookup =
    ProtectedJniExtensions.initGeneratedLibrary("android_utils");

/// from: com.example.in_app_java.AndroidUtils
class AndroidUtils extends jni.JObject {
  AndroidUtils.fromRef(ffi.Pointer<ffi.Void> ref) : super.fromRef(ref);

  static final _showToast = jniLookup<
          ffi.NativeFunction<
              jni.JniResult Function(ffi.Pointer<ffi.Void>,
                  ffi.Pointer<ffi.Void>, ffi.Int32)>>("AndroidUtils__showToast")
      .asFunction<
          jni.JniResult Function(
              ffi.Pointer<ffi.Void>, ffi.Pointer<ffi.Void>, int)>();

  /// from: static public void showToast(android.app.Activity mainActivity, java.lang.CharSequence text, int duration)
  static void showToast(
          jni.JObject mainActivity, jni.JObject text, int duration) =>
      _showToast(mainActivity.reference, text.reference, duration).check();
}

C Bindings:

// Some boilerplate is omitted for clarity.

// com.example.in_app_java.AndroidUtils
jclass _c_AndroidUtils = NULL;

jmethodID _m_AndroidUtils__showToast = NULL;
FFI_PLUGIN_EXPORT
JniResult AndroidUtils__showToast(jobject mainActivity,
                                  jobject text,
                                  int32_t duration) {
  load_env();
  load_class_gr(&_c_AndroidUtils, "com/example/in_app_java/AndroidUtils");
  if (_c_AndroidUtils == NULL)
    return (JniResult){.result = {.j = 0}, .exception = check_exception()};
  load_static_method(_c_AndroidUtils, &_m_AndroidUtils__showToast, "showToast",
                     "(Landroid/app/Activity;Ljava/lang/CharSequence;I)V");
  if (_m_AndroidUtils__showToast == NULL)
    return (JniResult){.result = {.j = 0}, .exception = check_exception()};
  (*jniEnv)->CallStaticVoidMethod(jniEnv, _c_AndroidUtils,
                                  _m_AndroidUtils__showToast, mainActivity,
                                  text, duration);
  return (JniResult){.result = {.j = 0}, .exception = check_exception()};
}

The YAML configuration used to generate the above code looks like this:

android_sdk_config:
  add_gradle_deps: true

output:
  c:
    library_name: android_utils
    path: src/android_utils/
  dart:
    path: lib/android_utils.dart
    structure: single_file

source_path:
  - 'android/app/src/main/java'
classes:
  - 'com.example.in_app_java.AndroidUtils'

The complete example can be found in jnigen/example/in_app_java, which adds few more classes to demonstrate using classes from gradle JAR and source dependencies.

Supported platforms

Platform	Dart Standalone	Flutter
Android	n/a	Supported
Linux	Supported	Supported
Windows	Supported	Supported
MacOS	Supported	Not Yet

On Android, the flutter application runs embedded in Android JVM. On other platforms, a JVM needs to be explicitly spawned using Jni.spawn. package:jni provides the infrastructure for initializing and managing the JNI on both Android and Non-Android platforms.

Java features support

Currently basic features of the Java language are supported in the bindings. Each Java class is mapped to a Dart class. Bindings are generated for methods, constructors and fields. Exceptions thrown in Java are rethrown in Dart with stack trace from Java.

More advanced features such as callbacks are not supported yet. Support for these features is tracked in the issue tracker.

Note on Dart (standalone) target

package:jni is an FFI plugin containing native code, and any bindings generated from jnigen contains native code too.

On Flutter targets, native libraries are built automatically and bundled. On standalone platforms, no such infrastructure exists yet. As a stopgap solution, running dart run jni:setup in a target directory builds all JNI native dependencies of the package into build/jni_libs.

By default jni:setup goes through pubspec configuration and builds all JNI dependencies of the project. It can be overridden to build a custom directory using -s switch, which can be useful when output configuration for C bindings does not follow standard FFI plugin layout.

The build directory has to be passed to Jni.spawn call. It's assumed that all dependencies are built into the same target directory, so that once JNI is initialized, generated bindings can load their respective C libraries automatically.

Requirements

SDK

Flutter SDK is required.

Dart standalone target is supported, but due to some problems with pubspec format, the dart command must be from Flutter SDK and not Dart SDK. See dart-lang/pub#3563.

Java tooling

Along with JDK, maven (mvn command) is required. On windows, it can be installed using a package manager such as chocolatey or scoop.

On windows, append the path of jvm.dll in your JDK installation to PATH.

For example, on Powershell:

$env:Path += ";${env:JAVA_HOME}\bin\server".

If JAVA_HOME not set, find the java.exe executable and set the environment variable in Control Panel. If java is installed through a package manager, there may be a more automatic way to do this. (Eg: scoop reset).

C/C++ tooling

CMake and a standard C toolchain are required to build package:jni and C bindings generated by jnigen.

It's recommended to have clang-format installed for formatting the generated C bindings. On Windows, it's part of LLVM installation. On most Linux distributions it is available as a separate package. On MacOS, it can be installed using Homebrew.

FAQs

I am getting ClassNotFoundError at runtime.

jnigen does not handle getting the classes into application. It has to be done by target-specific mechanism. Such as adding a gradle dependency on Android, or manually providing classpath to Jni.spawn on desktop / standalone targets.

On Android, proguard prunes classes which it deems inaccessible. Since JNI class lookup happens in runtime, this leads to ClassNotFound errors in release mode even if the dependency is included in gradle. in_app_java example discusses two mechanisms to prevent this: using Keep annotation (androidx.annotation.Keep) for the code written in the application itself, and proguard-rules file for external libraries.

Lastly, some libraries such as java.awt do not exist in android. Attempting to use libraries which depend on them can also lead to ClassNotFound errors.

jnigen is not finding classes.

Ensure you are providing correct source and class paths, and they follow standard directory structure. If your class name is com.abc.MyClass, MyClass must be in com/abc/MyClass.java relative to one of the source paths, or com/abc/MyClass.class relative to one of the class paths specified in YAML.

If the classes are in JAR file, make sure to provide path to JAR file itself, and not the containing directory.

jnigen is unable to parse sources.

If the errors are similar to symbol not found, ensure all dependencies of the source are available. If such dependency is compiled, it can be included in class_path.

How are classes mapped into bindings?

Each Java class generates a subclass of JObject class, which wraps a jobject reference in JNI. Nested classes use _ as separator, Example.NestedClass will be mapped to Example_NestedClass.

Does JObject hold a local or global reference? Does it need to be manually deleted?

Each Java object returned into Dart creates a JNI global reference. Reference deletion is taken care of by NativeFinalizer and that's usually sufficient.

It's a good practice to keep the interface between languages sparse. However, if there's a need to create several references (Eg: in a loop), you can use FFI Arena mechanism (using function) and deletedIn method, or manually delete the object using delete method.

Should I use jnigen over Method channels?

This is currently an experimental package. Many features are missing, and it's rough around the edges. You're welcome to try it and give feedback.

YAML Configuration Reference

Keys ending with a colon (:) denote subsections.

The typical invocation with YAML configuration is

dart run jnigen --config jnigen.yaml

Any configuration can be overridden through command line using -D or --override switch. For example -Dlog_level=warning or -Dsummarizer.backend=asm. (Use . to separate subsection and property name).

A * denotes required configuration.

Configuration property	Type / Values	Description
preamble	Text	Text to be pasted in the start of each generated file.
source_path	List of directory paths	Directories to search for source files. Note: source_path for dependencies downloaded using maven_downloads configuration is added automatically without the need to specify here.
class_path	List of directory / JAR paths	Classpath for API summary generation. This should include any JAR dependencies of the source files in source_path.
classes *	List of qualified class / package names	List of qualified class / package names. source_path will be scanned assuming the sources follow standard java-ish hierarchy. That is a.b.c either maps to a directory a/b/c or a class file a/b/c.java.
suspend_fun_to_async	True/False	Converting Kotlin's suspend functions to Dart's async functions. Defaults to False.
output:	(Subsection)	This subsection will contain configuration related to output files.
output: >> bindings_type	c_based (default) or dart_only	Binding generation strategy. Trade-offs are explained at the end of this document.
output: >> c:	(Subsection)	This subsection specified C output configuration. Required if bindings_type is c_based.
output: >> c: >> path *	Directory path	Directory to write C bindings. Usually src/ in case of an FFI plugin template.
output: >> c: >> subdir	Directory path	If specified, C bindings will be written to subdir resolved relative to path. This is useful when bindings are supposed to be under source's license, and written to a subdirectory such as third_party.
output: >> c: >> library_name *	Identifier (snake_case)	Name for generated C library.
output: >> dart:	(Subsection)	This subsection specifies Dart output configuration.
output: >> dart: >> structure	package_structure / single_file	Whether to map resulting dart bindings to file-per-class source layout, or write all bindings to single file.
output: >> dart: >> path *	Directory path or File path	Path to write Dart bindings. Should end in .dart for single_file configurations, and end in / for package_structure (default) configuration.
maven_downloads:	(Subsection)	This subsection will contain configuration for automatically downloading Java dependencies (source and JAR) through maven.
maven_downloads: >> source_deps	List of maven package coordinates	Source packages to download and unpack using maven. The names should be valid maven artifact coordinates. (Eg: org.apache.pdfbox:pdfbox:2.0.26). The downloads do not include transitive dependencies.
maven_downloads: >> source_dir	Path	Directory in which maven sources are extracted. Defaults to mvn_java. It's not required to list this explicitly in source_path.
maven_downloads: >> jar_only_deps	List of maven package coordinates	JAR dependencies to download which are not mandatory transitive dependencies of source_deps. Often, it's required to find and include optional dependencies so that entire source is valid for further processing.
maven_downloads: >> jar_dir	Path	Directory to store downloaded JARs. Defaults to mvn_jar.
log_level	Logging level	Configure logging level. Defaults to info.
android_sdk_config:	(Subsection)	Configuration for autodetection of Android dependencies and SDK. Note that this is more experimental than others, and very likely subject to change.
android_sdk_config: >> add_gradle_deps	Boolean	If true, run a gradle stub during jnigen invocation, and add Android compile classpath to the classpath of jnigen. This requires a release build to have happened before, so that all dependencies are cached appropriately.
android_sdk_config: >> android_example	Directory path	In case of an Android plugin project, the plugin itself cannot be built and add_gradle_deps is not directly feasible. This property can be set to relative path of package example app (usually example/ so that gradle dependencies can be collected by running a stub in this directory. See notification_plugin example for an example.
summarizer:	(Subsection)	Configuration specific to summarizer component, which builds API descriptions from Java sources or JAR files.
summarizer: >> backend	auto, doclet or asm	Specifies the backend to use in API summary generation. doclet uses OpenJDK Doclet API to build summary from sources. asm uses ASM library to build summary from classes in class_path JARs. auto attempts to find the class in sources, and falls back to using ASM.
summarizer: >> extra_args (DEV)	List of CLI arguments	Extra arguments to pass to summarizer JAR.
exclude:	(Subsection)	Exclude methods or fields using regex filters. It's generally useful to exclude problematic fields or methods which, with current binding generation, can lead to syntax errors
exclude: >> methods	List of methods in classBinaryName#methodName format where classBinaryName is same as qualified name, but $ preceding a nested class instead of .. Example: com.example.MyClass or com.example.MyClass$NestedClass	Methods to exclude.
exclude: >> fields	List of fields in classBinaryName#fieldName format	Fields to exclude.

It's possible to use the programmatic API instead of YAML.

• Create a tool script. (Eg: tool/generate_jni_bindings.dart)
• import package:jnigen/jnigen.dart
• construct a Config object and pass it to generateJniBindings function. The parameters are similar to the ones described above.

Pure dart Bindings

It's possible to generate bindings that do not rely on an intermediate layer of C code. Bindings will still depend on package:jni and its support library written in C. But this approach avoids large C bindings.

To enable pure dart bindings, specify

output:
	bindings_type: dart_only

Any C output configuration will be ignored.

However, pure dart bindings will require additional allocations and check runtimeType of the arguments. This will be the case until Variadic arguments land in Dart FFI.

Android core libraries

These days, Android projects depend heavily on AndroidX and other libraries downloaded via gradle. We have a tracking issue to improve detection of android SDK and dependencies. (#31). Currently we can fetch the JAR dependencies of an android project, by running a gradle stub, if android_sdk_config >> add_gradle_deps is specified.

But core libraries (the android.** namespace) are not downloaded through gradle. The core libraries are shipped as stub JARs with the Android SDK. ($SDK_ROOT/platforms/android-$VERSION/android-stubs-src.jar).

Currently we don't have an automatic mechanism for using these. You can unpack this JAR manually into some directory and provide it as a source path.

However there are 2 caveats to this caveat.

• SDK stubs after version 28 are incomplete. OpenJDK Doclet API we use to generate API summaries will error on incomplete sources.
• The API can't process the java.** namespaces in the Android SDK stubs, because it expects a module layout. So if you want to generate bindings for, say, java.lang.Math, you cannot use the Android SDK stubs. OpenJDK sources can be used instead.

The JAR files ($SDK_ROOT/platforms/android-$VERSION/android.jar) can be used instead. But compiled JARs do not include JavaDoc and method parameter names. This JAR is automatically included by Gradle when android_sdk_config >> add_gradle_deps is specified.

Contributing

See the wiki for architecture-related documents.