你问：

Would you please give me some more details on how to do it with JNA
sorry,I’m writing for windows

对于Windows,您将与user32.dll库进行交互.像这样的东西对我有用：

User32.java

import com.sun.jna.Native;
import com.sun.jna.Pointer;
import com.sun.jna.platform.win32.BaseTSD.LONG_PTR;
import com.sun.jna.platform.win32.WinDef.HWND;
import com.sun.jna.platform.win32.WinDef.RECT;
import com.sun.jna.win32.StdCallLibrary;

/**
 * JNA interface with Window's user32.dll
 * 
 * @author Pete S
 * 
 */
public interface User32 extends StdCallLibrary {
   User32 INSTANCE = (User32) Native.loadLibrary("user32",User32.class);

   interface WNDENUMPROC extends StdCallCallback {
      boolean callback(Pointer hWnd,Pointer arg);
   }

   public static final int GW_OWNER = 4; // used with GetWindow to get win owner
   public static final int GW_HWNDNEXT = 2; // used with GetNextWindow
   public static final int GA_ROOT = 2; // used with GetAncestor
   public static final int GWL_EXSTYLE = -20; // used with GetWindowLong
   public static final long WS_EX_APPWINDOW = 0x00040000L;
   public static final Pointer HWND_TOP = new Pointer(0L); // used with
                                                           // SetWindowPos

   boolean EnumWindows(WNDENUMPROC lpEnumFunc,Pointer userData);

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

   int SetForegroundWindow(Pointer hWnd);

   Pointer GetForegroundWindow();

   boolean GetWindowRect(Pointer hWnd,RECT rect);

   boolean SetWindowPos(Pointer hWnd,Pointer hWndInsertAfter,int x,int y,int cx,int cy,int uFlags);

   boolean MoveWindow(Pointer hWnd,int nWidth,int nHeight,boolean bRepaint);

   boolean IsWindow(Pointer hWnd);

   Pointer GetWindow(Pointer hWnd,int uCmd);

   LONG_PTR GetWindowLongPtr(HWND hWnd,int nIndex);

   Pointer GetParent(Pointer hWnd);

   Pointer GetAncestor(Pointer hWnd,int gaFlags);

   boolean IsWindowVisible(Pointer hWnd);
}

JnaUtil.java

import java.awt.AWTException;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.image.BufferedImage;
import java.util.ArrayList;
import java.util.List;

import javax.swing.ImageIcon;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.SwingUtilities;

import com.sun.jna.Native;
import com.sun.jna.Pointer;
import com.sun.jna.platform.win32.BaseTSD.LONG_PTR;
import com.sun.jna.platform.win32.WinDef.HWND;
import com.sun.jna.platform.win32.WinDef.RECT;

/**
 * static methods to allow Java to call Windows code. user32.dll code is as
 * specified in the JNA interface User32.java
 * 
 * @author Pete S
 * 
 */
public class JnaUtil {
   private static final User32 user32 = User32.INSTANCE;
   private static Pointer callBackHwnd;

   public static boolean windowExists(final String startOfWindowName) {
      return !user32.EnumWindows(new User32.WNDENUMPROC() {
         @Override
         public boolean callback(Pointer hWnd,Pointer userData) {
            byte[] windowText = new byte[512];
            user32.GetWindowTextA(hWnd,windowText,512);
            String wText = Native.toString(windowText).trim();

            if (!wText.isEmpty() && wText.startsWith(startOfWindowName)) {
               return false;
            }
            return true;
         }
      },null);
   }

   public static List<String> getAllWindowNames() {
      final List<String> windowNames = new ArrayList<String>();
      user32.EnumWindows(new User32.WNDENUMPROC() {

         @Override
         public boolean callback(Pointer hWnd,Pointer arg) {
            byte[] windowText = new byte[512];
            user32.GetWindowTextA(hWnd,512);
            String wText = Native.toString(windowText).trim();
            if (!wText.isEmpty()) {
               windowNames.add(wText);
            }
            return true;
         }
      },null);

      return windowNames;
   }

   public static boolean windowExists(Pointer hWnd) {
      return user32.IsWindow(hWnd);
   }

   public static Pointer getWinHwnd(final String startOfWindowName) {
      callBackHwnd = null;

      user32.EnumWindows(new User32.WNDENUMPROC() {
         @Override
         public boolean callback(Pointer hWnd,512);
            String wText = Native.toString(windowText).trim();

            if (!wText.isEmpty() && wText.startsWith(startOfWindowName)) {
               callBackHwnd = hWnd;
               return false;
            }
            return true;
         }
      },null);
      return callBackHwnd;
   }

   public static boolean setForegroundWindow(Pointer hWnd) {
      return user32.SetForegroundWindow(hWnd) != 0;
   }

   public static Pointer getForegroundWindow() {
      return user32.GetForegroundWindow();
   }

   public static String getForegroundWindowText() {
      Pointer hWnd = getForegroundWindow();
      int nMaxCount = 512;
      byte[] lpString = new byte[nMaxCount];
      int getWindowTextResult = user32
            .GetWindowTextA(hWnd,lpString,nMaxCount);
      if (getWindowTextResult == 0) {
         return "";
      }

      return Native.toString(lpString);
   }

   public static boolean isForegroundWindow(Pointer hWnd) {
      return user32.GetForegroundWindow().equals(hWnd);
   }

   public static boolean setForegroundWindow(String startOfWindowName) {
      Pointer hWnd = getWinHwnd(startOfWindowName);
      return user32.SetForegroundWindow(hWnd) != 0;
   }

   public static Rectangle getWindowRect(Pointer hWnd) throws JnaUtilException {
      if (hWnd == null) {
         throw new JnaUtilException(
               "Failed to getWindowRect since Pointer hWnd is null");
      }
      Rectangle result = null;
      RECT rect = new RECT();
      boolean rectOK = user32.GetWindowRect(hWnd,rect);
      if (rectOK) {
         int x = rect.left;
         int y = rect.top;
         int width = rect.right - rect.left;
         int height = rect.bottom - rect.top;
         result = new Rectangle(x,y,width,height);
      }

      return result;
   }

   /**
    * set window at x and y position with w and h width. Set on top of z-order
    * 
    * @param hWnd
    * @param x
    * @param y
    * @param w
    * @param h
    * @return boolean -- did it work?
    */
   public static boolean setWindowPos(Pointer hWnd,int w,int h) {
      int uFlags = 0;
      return user32.SetWindowPos(hWnd,User32.HWND_TOP,x,w,h,uFlags);
   }

   public static boolean moveWindow(Pointer hWnd,int nHeight) {
      boolean bRepaint = true;
      return user32.MoveWindow(hWnd,nWidth,nHeight,bRepaint );
   }

   public static Rectangle getWindowRect(String startOfWindowName)
         throws JnaUtilException {
      Pointer hWnd = getWinHwnd(startOfWindowName);
      if (hWnd != null) {
         return getWindowRect(hWnd);
      } else {
         throw new JnaUtilException("Failed to getWindowRect for \""
               + startOfWindowName + "\"");
      }
   }

   public static Pointer getWindow(Pointer hWnd,int uCmd) {
      return user32.GetWindow(hWnd,uCmd);
   }

   public static String getWindowText(Pointer hWnd) {
      int nMaxCount = 512;
      byte[] lpString = new byte[nMaxCount];
      int result = user32.GetWindowTextA(hWnd,nMaxCount);
      if (result == 0) {
         return "";
      }
      return Native.toString(lpString);
   }

   public static Pointer getOwnerWindow(Pointer hWnd) {
      return user32.GetWindow(hWnd,User32.GW_OWNER);
   }

   public static String getOwnerWindow(String childTitle) {
      Pointer hWnd = getWinHwnd(childTitle);
      Pointer parentHWnd = getOwnerWindow(hWnd);
      if (parentHWnd == null) {
         return "";
      }
      return getWindowText(parentHWnd);

   }

   public static Pointer getNextWindow(Pointer hWnd) {
      if (hWnd == null) {
         return null;
      }

      return user32.GetWindow(hWnd,User32.GW_HWNDNEXT);
   }

   public static boolean isWindowVisible(Pointer hWnd) {
      return user32.IsWindowVisible(hWnd);
   }

   public static Pointer getParent(Pointer hWnd) {
      return user32.GetParent(hWnd);
   }

   public static Pointer getRoot(Pointer hWnd) {
      return user32.GetAncestor(hWnd,User32.GA_ROOT);
   }

   public static LONG_PTR getWindowLongPtr(Pointer hWndP,int nIndex) {
      HWND hwnd = new HWND(hWndP);
      return user32.GetWindowLongPtr(hwnd,nIndex);
   }

   // main method to test the library
   public static void main(String[] args) throws InterruptedException {
      List<String> winNameList = getAllWindowNames();
      for (String winName : winNameList) {
         System.out.println(winName);
      }

      String[] testStrs = { "Untitled-Notepad","Untitled - Notepad","Untitled  -  Notepad","Java-Epic","Java - Epic","Fubars rule!","The First Night","New Tab","Citrix X","EHR PROD - SVC" };
      for (String testStr : testStrs) {
         Pointer hWnd = getWinHwnd(testStr);
         boolean isWindow = windowExists(hWnd);
         System.out.printf("%-22s %5b %16s %b%n",testStr,windowExists(testStr),hWnd,isWindow);
      }

      String notePad = "Untitled - Notepad";
      Pointer hWnd = getWinHwnd(notePad);
      System.out
            .println("is it foreground window? " + isForegroundWindow(hWnd));
      boolean foo = setForegroundWindow(notePad);
      System.out.println("foregroundwindow: " + foo);
      Thread.sleep(400);
      System.out
            .println("is it foreground window? " + isForegroundWindow(hWnd));
      Thread.sleep(1000);
      System.out.println("here A");
      try {
         Rectangle rect = getWindowRect(notePad);
         System.out.println("rect: " + rect);
         Robot robot = new Robot();
         System.out.println("here B");

         BufferedImage img = robot.createScreenCapture(rect);
         System.out.println("here C,img is " + img);
         Thread.sleep(500);
         ImageIcon icon = new ImageIcon(img);
         System.out.println("here D. icon is null? " + icon);
         Thread.sleep(500);
         final JLabel label = new JLabel(icon);
         System.out.println("here E. label is null? " + label);
         Thread.sleep(500);
         SwingUtilities.invokeLater(new Runnable() {
            public void run() {
               System.out.println("here F");               
               JOptionPane.showMessageDialog(null,label);
               System.out.println("here G");
            }
         });

      } catch (AWTException e) {
         e.printStackTrace();
      } catch (JnaUtilException e) {
         e.printStackTrace();
      }
   }

}

JnaUtilException.java

public class JnaUtilException extends Exception {
   private static final long serialVersionUID = 1L;

   public JnaUtilException(String text) {
      super(text);
   }

}