How to Detect workstation/System Screen Lock/unlock in windows OS using java?

You can do it in pure Java using JNA. Add jna.jar and jna-platform.jar to your project. And in this file com.sun.jna.platform.win32.Win32WindowDemo there is a full example of lock and unlock listener and much more. Here is the necessary code from thah Win32WindowDemo:

public class WorkstationLockListening implements WindowProc
{

    /**
     * Instantiates a new win32 window test.
     */
    public WorkstationLockListening()
    {
        // define new window class
        final WString windowClass = new WString("MyWindowClass");
        final HMODULE hInst = Kernel32.INSTANCE.GetModuleHandle("");

        WNDCLASSEX wClass = new WNDCLASSEX();
        wClass.hInstance = hInst;
        wClass.lpfnWndProc = WorkstationLockListening.this;
        wClass.lpszClassName = windowClass;

        // register window class
        User32.INSTANCE.RegisterClassEx(wClass);
        getLastError();

        // create new window
        final HWND hWnd = User32.INSTANCE.CreateWindowEx(User32.WS_EX_TOPMOST, windowClass, "'TimeTracker hidden helper window to catch Windows events", 0, 0, 0, 0, 0, null, // WM_DEVICECHANGE contradicts parent=WinUser.HWND_MESSAGE
                null, hInst, null);

        getLastError();
        System.out.println("window sucessfully created! window hwnd: " + hWnd.getPointer().toString());

        Wtsapi32.INSTANCE.WTSRegisterSessionNotification(hWnd, Wtsapi32.NOTIFY_FOR_THIS_SESSION);

        MSG msg = new MSG();
        while (User32.INSTANCE.GetMessage(msg, hWnd, 0, 0) != 0)
        {
            User32.INSTANCE.TranslateMessage(msg);
            User32.INSTANCE.DispatchMessage(msg);
        }

            /// This code is to clean at the end. You can attach it to your custom application shutdown listener
            Wtsapi32.INSTANCE.WTSUnRegisterSessionNotification(hWnd);
            User32.INSTANCE.UnregisterClass(windowClass, hInst);
            User32.INSTANCE.DestroyWindow(hWnd);
            System.out.println("program exit!");
    }

    /*
     * (non-Javadoc)
     * 
     * @see com.sun.jna.platform.win32.User32.WindowProc#callback(com.sun.jna.platform .win32.WinDef.HWND, int, com.sun.jna.platform.win32.WinDef.WPARAM, com.sun.jna.platform.win32.WinDef.LPARAM)
     */
    public LRESULT callback(HWND hwnd, int uMsg, WPARAM wParam, LPARAM lParam)
    {
        switch (uMsg)
        {
            case WinUser.WM_DESTROY:
            {
                User32.INSTANCE.PostQuitMessage(0);
                return new LRESULT(0);
            }
            case WinUser.WM_SESSION_CHANGE:
            {
                this.onSessionChange(wParam, lParam);
                return new LRESULT(0);
            }
            default:
                return User32.INSTANCE.DefWindowProc(hwnd, uMsg, wParam, lParam);
        }
    }

    /**
     * Gets the last error.
     * 
     * @return the last error
     */
    public int getLastError()
    {
        int rc = Kernel32.INSTANCE.GetLastError();

        if (rc != 0)
            System.out.println("error: " + rc);

        return rc;
    }

    /**
     * On session change.
     * 
     * @param wParam
     *            the w param
     * @param lParam
     *            the l param
     */
    protected void onSessionChange(WPARAM wParam, LPARAM lParam)
    {
        switch (wParam.intValue())
        {
            case Wtsapi32.WTS_SESSION_LOCK:
            {
                this.onMachineLocked(lParam.intValue());
                break;
            }
            case Wtsapi32.WTS_SESSION_UNLOCK:
            {
                this.onMachineUnlocked(lParam.intValue());
                break;
            }
        }
    }

    /**
     * On machine locked.
     * 
     * @param sessionId
     *            the session id
     */
    protected void onMachineLocked(int sessionId)
    {
        System.out.println("Machine locked right now!");
    }

    /**
     * On machine unlocked.
     * 
     * @param sessionId
     *            the session id
     */
    protected void onMachineUnlocked(int sessionId)
    {
        System.out.println("Machine unlocked right now!");
    }
}

We have solved this problem in Google Group Workstation Lock / Unlock listener. You can find there my own implementation but this code right here is much better! Enjoy 🙂