Java interop using package:jnigen

Dart mobile, command-line, and server apps running on the Dart Native platform, on Android, Windows, macOS, and Linux can use package:jni and package:jnigen to call Java and Kotlin APIs.

package:jni allows Dart code to interact with Java through JNI. However, doing so involves a lot of boilerplate code, so you can use package:jnigen to automatically generate the Dart bindings for a given Java API.

You can compile Kotlin to Java bytecode, allowing package:jnigen to generate bindings for Kotlin as well.

Simple Java example

This guide walks you through an example that uses package:jnigen to generate bindings for a simple class.

Prerequisites

• JDK
• Maven
• (Optional) clang-format to format the generated C bindings

Configure jnigen

First, add package:jni as a dependency and package:jnigen as a dev dependency.

$ dart pub add jni dev:jnigen

Next, create a top-level file called jnigen.yaml. This file contains the configuration for generating the bindings.

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

source_path:
  - 'java/'
classes:
  - 'dev.dart.Example'

path specifies the path for the generated c and dart bindings.

source_path specifies the path of the Java source file that you want to generate bindings for, and classes specifies the Java class.

java/dev/dart/Example.java contains a very simple class, which has a public static method called sum:

package dev.dart;

public class Example {
  public static int sum(int a, int b) {
    return a + b;
  }
}

Generate the Dart bindings

To generate the Dart (and C) bindings, run jnigen and specify the config file using the --config option:

$ dart run jnigen --config jnigen.yaml

In this example, this generates lib/example.dart, just as you specified in jnigen.yaml.

This file contains a class called Example, which has a static method called sum, just like the Java file.

Use the bindings

Now you’re ready to load and interact with the generated library. The example app, bin/sum.dart, gets two numbers as arguments and prints their sum. Using the Example.sum method is identical to Java.

// a and b are integer arguments
print(Example.sum(a, b));

Run the example

Before running the example, you must build the dynamic libraries for jni and the generated C files. The Java sources also must be compiled. To do so, run:

$ dart run jni:setup -p jni -s src/example
$ javac java/dev/dart/Example.java

Now you can run the example:

$ dart run jnigen_example:sum 17 25

Which outputs 42!

More examples

The following are some more comprehensive examples of using package:jnigen: