Getting active window information in Java

Question

I am trying to upgrade my application in Java to work only if a window of process with certain name is active. I have found out that this is possible by using JNI, but I have no idea how exactly to do that. I just could not find any description or example that would explain it. My question is - how to get process name of currently active window in Windows (via JNI, or anything else - I accept any another solution)?

Answer 1

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);
    }
}

Answer 2

This code works with JNA 4.0

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

// see http://java-native-access.github.io/jna/4.0/javadoc/

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];
        HWND hwnd = User32.INSTANCE.GetForegroundWindow();
        User32.INSTANCE.GetWindowText(hwnd, buffer, MAX_TITLE_LENGTH);
        System.out.println("Active window title: " + Native.toString(buffer));
        RECT rect = new RECT();
        User32.INSTANCE.GetWindowRect(hwnd, rect);
        System.out.println("rect = " + rect);
    }
 }

Answer 3

Simply

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

public class WinSysUser32HWND {
    public interface User32 extends StdCallLibrary {
        User32 INSTANCE = (User32) Native.loadLibrary("user32", User32.class);
        HWND GetForegroundWindow();
        void GetWindowTextW(HWND hWnd, char[] lpString, int nMaxCount);
    }

    public static void main(String[] args) {
        char[] buffer = new char[1024 * 2];
        User32.INSTANCE.GetWindowTextW(User32.INSTANCE.GetForegroundWindow(), buffer, 1024);
        System.out.println("Active window title: " + Native.toString(buffer));
    }
}