Implementing native methods

Java Native Interface (JNI) with CheerpJ

With CheerpJ, it is possible to implement Java ‘native’ methods (that would normally be implemented in C/C++ or other AOT-compiled language) in JavaScript, similarly to what would be done in regular Java using the Java Native Interface (JNI).

As an example, consider the following Java class:

package com.example;

public class TestClass {

  public static native void nativeAlert(String str);

  public static void main(String[] args) {
        nativeAlert("Hello from Java!");
  }

}

To provide an implementation of nativeAlert, pass it to the cheerpjInit function as a property of the natives object:

await cheerpjInit({
  natives: {
    async Java_TestClass_nativeAlert(lib, str) {
      window.alert(str);
    },
  },
});
await cheerpjRunMain("TestClass", "/app/");

The name of methods in the natives object must be in the form Java_<fully-qualified-class-name>_<method-name>.

The lib parameter is a CJ3Library. It can be used to access other classes and methods of the library.

Parameters and return values of JNI calls are automatically converted between JavaScript and Java types.