Getting active window information in Java

Save yourself some pain and use JNA. You will need to download jna.jar and jna-platform.jar for the Win32 API. The pinvoke wiki and MSDN are useful for finding the right system calls.

Anyway, here is the code to print the title and process of the currently active window.

import static enumeration.EnumerateWindows.Kernel32.*;
import static enumeration.EnumerateWindows.Psapi.*;
import static enumeration.EnumerateWindows.User32DLL.*;
import com.sun.jna.Native;
import com.sun.jna.Pointer;
import com.sun.jna.platform.win32.WinDef.HWND;
import com.sun.jna.ptr.PointerByReference;

public class EnumerateWindows {
    private static final int MAX_TITLE_LENGTH = 1024;

    public static void main(String[] args) throws Exception {
        char[] buffer = new char[MAX_TITLE_LENGTH * 2];
        GetWindowTextW(GetForegroundWindow(), buffer, MAX_TITLE_LENGTH);
        System.out.println("Active window title: " + Native.toString(buffer));

        PointerByReference pointer = new PointerByReference();
        GetWindowThreadProcessId(GetForegroundWindow(), pointer);
        Pointer process = OpenProcess(PROCESS_QUERY_INFORMATION | PROCESS_VM_READ, false, pointer.getValue());
        GetModuleBaseNameW(process, null, buffer, MAX_TITLE_LENGTH);
        System.out.println("Active window process: " + Native.toString(buffer));
    }

    static class Psapi {
        static { Native.register("psapi"); }
        public static native int GetModuleBaseNameW(Pointer hProcess, Pointer hmodule, char[] lpBaseName, int size);
    }

    static class Kernel32 {
        static { Native.register("kernel32"); }
        public static int PROCESS_QUERY_INFORMATION = 0x0400;
        public static int PROCESS_VM_READ = 0x0010;
        public static native int GetLastError();
        public static native Pointer OpenProcess(int dwDesiredAccess, boolean bInheritHandle, Pointer pointer);
    }

    static class User32DLL {
        static { Native.register("user32"); }
        public static native int GetWindowThreadProcessId(HWND hWnd, PointerByReference pref);
        public static native HWND GetForegroundWindow();
        public static native int GetWindowTextW(HWND hWnd, char[] lpString, int nMaxCount);
    }
}