In Java, the Java Native Access (JNA) library allows us to interact with native code written in C or C++. One useful feature of JNA is the ability to use callbacks and function pointers to call native code functions from Java. In this blog post, we will explore how callbacks and function pointers work in Java JNA.

Callbacks in JNA

Callbacks in JNA allow us to pass a Java function as a parameter to a native code function. The native code function can then call this Java function at an appropriate time. This allows us to handle events or notifications from the native code within our Java application.

To define a callback function in Java JNA, we need to create a Java interface that extends the Callback interface provided by JNA. This interface should define a method that matches the signature of the native code function we want to call.

import com.sun.jna.Callback;

public interface MyCallback extends Callback {
    void callbackMethod(int param1, String param2);
}

In this example, we have defined a MyCallback interface with a single method callbackMethod that takes an integer and a string as parameters. This method will be called by the native code.

Next, we need to pass an instance of MyCallback to the native code function. We can do this by creating an instance of MyCallback using the Native.load method provided by JNA. The Native.load method loads the native library and maps the specified interface to a native function pointer.

import com.sun.jna.Library;
import com.sun.jna.Native;

public interface MyLibrary extends Library {
    MyLibrary INSTANCE = Native.load("mylib", MyLibrary.class);

    void registerCallback(MyCallback callback);
}

In this example, we have created a MyLibrary interface that extends the Library interface provided by JNA. This interface contains the registerCallback method, which takes an instance of MyCallback.

To use the callback, we can then create an instance of MyCallback and pass it to the registerCallback method.

MyCallback callback = new MyCallback() {
    @Override
    public void callbackMethod(int param1, String param2) {
        // Handle the callback from the native code
        System.out.println("Received callback: " + param1 + ", " + param2);
    }
};

MyLibrary.INSTANCE.registerCallback(callback);

In this example, we create an anonymous instance of MyCallback and override the callbackMethod to handle the callback from the native code. We then register the callback with the native code using the registerCallback method.

Function Pointers in JNA

Function pointers in JNA allow us to obtain a reference to a native code function and call it from Java code. This can be useful when we need to invoke a specific function dynamically at runtime.

To use function pointers in JNA, we need to define an interface that extends the Library interface and contains a declaration for the native code function.

import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Pointer;
import com.sun.jna.ptr.PointerByReference;

public interface MyLibrary extends Library {
    MyLibrary INSTANCE = Native.load("mylib", MyLibrary.class);

    void myFunction(int param);
    void setCallbackFunction(PointerByReference functionPointer);
}

In this example, we have defined the MyLibrary interface, which extends the Library interface. It declares the myFunction, which is the native code function we want to call, and the setCallbackFunction, which takes a PointerByReference parameter and sets the function pointer for the callback.

To use the function pointer, we can create a Function object using the Function.getFunction method provided by JNA. The Function object represents the native code function and can be passed to the setCallbackFunction method.

Function callbackFunction = new Function() {
    public void invoke(int param) {
        // Handle the callback function
        System.out.println("Invoked callback function with param: " + param);
    }
};

Pointer functionPointer = callbackFunction.getFunctionPointer();
PointerByReference functionPointerRef = new PointerByReference(functionPointer);

MyLibrary.INSTANCE.setCallbackFunction(functionPointerRef);

In this example, we create an anonymous Function object and override the invoke method to handle the callback function. We then obtain the function pointer using the getFunctionPointer method and pass it to the setCallbackFunction method.

That’s it! Now we can use callbacks and function pointers in Java JNA to interact with native code and handle events or invoke specific functions dynamically. These features provide powerful capabilities for integrating Java with native code in a seamless manner.

#java #JNA #callbacks #functionpointers