Get titles of Non-Java Windows in Java

Question

Window.getWindows();

Doesn't get all the open windows, just the java ones. Is there a way to get all the windows the operating system has open. I'm making a java taskbar.

Answer 1

You can now use Foreign Memory API in pure-Java code to call into native Windows APIs to read the titles of windows.

This example works in JDK22 with code bindings generated by a compatible jextract, and is the equivalent to Hovercraft Full Of Eels answer:

/** 
 * Run with Windows JDK22:
 * jdk-22\bin\java --enable-native-access=ALL-UNNAMED -cp your.jar EnumWindowsDemo
 */
public class EnumWindowsDemo {
    public static void main(String[] args) {
        // TODO: You could use someParam as a way to detect a specific hWnd inside the callback
        long someParam = 0x1234;

        final int isRunning = User32_h.TRUE();
        final int maxChars = 1024;

        System.out.format("EnumWindows Panama demo Java %s%n", System.getProperty("java.runtime.version"));

        HashMap<Long,String> windowNames = new HashMap<>();
        try(Arena arena = Arena.ofConfined()) {
            MemorySegment lpString = arena.allocate(JAVA_CHAR, maxChars);

            // Windows callback WNDENUMPROC(MemorySegment hwnd, long lParam)
            WNDENUMPROC.Function callback = (hWnd, lParam) -> {

                // https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getwindowtextw
                int used = User32_h.GetWindowTextW(hWnd, lpString, maxChars-1);
                if (used > 0) {
                    assert used < maxChars;
                    String text = lpString.getString(0, StandardCharsets.UTF_16LE);
                    System.out.format("hWnd:%x lParam:%x => `%s`%n", hWnd.address(), lParam, text);
                    windowNames.put(hWnd.address(), text);
                }
                return isRunning;
            };

            final MemorySegment ewProc = WNDENUMPROC.allocate(callback, arena);

            // https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-enumwindows
            // Note that someParam is passed to the callback as lParam value
            int rc = User32_h.EnumWindows(ewProc, someParam);
            System.out.format("EnumWindows rc:%s%n", rc == 0 ? "FAIL":"OK");
            if (rc == 0) {
                throw new RuntimeException("EnumWindows failed rc="+rc);
            }
        }
        System.out.format("Found windowNames: %s%n", windowNames.toString());
    }
}

The standalone jextract is used to generate bindings for calling native code. You need to create a header User32.h referencing suitable Windows API headers:

echo #include ^<shlobj_core.h^> > User32.h

Run jextract to generate bindings for a suitable set of Windows APIs:

set JEXTRACT_ARGS=--source -luser32 User32.h -t somepackage --output java
set JEXTRACT_SYMBOLS=--include-macro TRUE --include-function EnumWindows --include-function GetWindowTextW --include-function GetWindowTextLengthW --include-typedef  WNDENUMPROC
jextract %JEXTRACT_ARGS% %JEXTRACT_SYMBOLS%

You can also save symbols to a file for easy editing, and re-generate by replacing use of %JEXTRACT_SYMBOLS% above by @your.symbols.

See also:

• EnumWindows
• GetWindowTextW

Answer 2

Addition to @Hovercraft Full Of Eels, if you also have windows which are titled with unreadable characters and get "?" printed for those characters, try to:

• convert byte[] to string by using different encoding (as I understood)

see below:

String wText = Native.toString(windowText, "windows-1254").trim();

Possible encodings instead of "windows-1254 (which worked well for Turkish characters)":

• UTF-8
• UTF-16
• ASCII
• ISO-8859-1
• ...

Answer 3

There is no solution using core Java, but the problem can be solved using JNI or for an easier time, JNA. As noted in the comments, no solution (that I know of) will be platform independent.

For instance, this demo program uses JNA to enumerate all Windows with title in a Windows platform, but will also include windows that are not top-level windows and even windows that aren't visible:

import java.util.ArrayList;
import java.util.List;

import com.sun.jna.Native;
import com.sun.jna.Pointer;
import com.sun.jna.win32.StdCallLibrary;

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

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

      boolean EnumWindows(WNDENUMPROC lpEnumFunc, Pointer userData);
      int GetWindowTextA(Pointer hWnd, byte[] lpString, int nMaxCount);
      Pointer GetWindow(Pointer hWnd, int uCmd);
   }

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

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

      return windowNames;
   }

   public static void main(String[] args) {
      List<String> winNameList = getAllWindowNames();
      for (String winName : winNameList) {
         System.out.println(winName);
      }
   }


}