Using JNA to get GetForegroundWindow();

Question

I asked a similar question on a previous thread (https://stackoverflow.com/questions/5206633/java-find-out-what-application-window-is-in-focus) but I was guided to use JNI, and I'm not having much success with it... I've read some tutorials and while some work fine, others don't I still can't get the information I need, which is the title of the window on the foreground.

Now I'm looking into JNA but I can't figure out how to access GetForegroundWindow() ... I think I can print the text once I get the handle to the window using this code (found on another thread):

import com.sun.jna.*;
import com.sun.jna.win32.*;

public class jnatest {
    public interface User32 extends StdCallLibrary {
        User32 INSTANCE = (User32) Native.loadLibrary("user32", User32.class);

        int GetWindowTextA(PointerType hWnd, byte[] lpString, int nMaxCount);
    }

    public static void main(){
        byte[] windowText = new byte[512];

        PointerType hwnd = //GetForegroundWindow() (?)...
        User32.INSTANCE.GetWindowTextA(hwnd, windowText, 512);
        System.out.println(Native.toString(windowText));

    }
}

Any suggestions? Thanks!

Answer 1

How about simply adding a method call to match the native GetForegroundWindow to your interface, something like so:

import com.sun.jna.*;
import com.sun.jna.platform.win32.WinDef.HWND;
import com.sun.jna.win32.*;

public class JnaTest {
   public interface User32 extends StdCallLibrary {
      User32 INSTANCE = (User32) Native.loadLibrary("user32", User32.class);
      HWND GetForegroundWindow();  // add this
      int GetWindowTextA(PointerType hWnd, byte[] lpString, int nMaxCount);
   }

   public static void main(String[] args) throws InterruptedException {
      byte[] windowText = new byte[512];

      PointerType hwnd = User32.INSTANCE.GetForegroundWindow(); // then you can call it!
      User32.INSTANCE.GetWindowTextA(hwnd, windowText, 512);
      System.out.println(Native.toString(windowText));
   }
}

Answer 2

If getting the window title is all you want to do, you don't have to explicitly load the user32 library. JNA comes with it, in the platform.jar file (at least in v3.4 it does).

I got this working here:

import com.sun.jna.Native;
import com.sun.jna.platform.win32.WinDef.HWND;
import com.sun.jna.platform.win32.User32;

public class JnaApp {

    public static void main(String[] args) {
        System.out.println("title is " + getActiveWindowTitle());
    }

    private static String getActiveWindowTitle() {
        HWND fgWindow = User32.INSTANCE.GetForegroundWindow();
        int titleLength = User32.INSTANCE.GetWindowTextLength(fgWindow) + 1;
        char[] title = new char[titleLength];
        User32.INSTANCE.GetWindowText(fgWindow, title, titleLength);
        return Native.toString(title);
    }

}

See more on User32's Javadoc. Its got almost all the functions in the user32 library.