Add Mouse Wheel support to Swing Widgets

Introduction

There are at least five Request for Enhancement's (RFE) in the JavaSoft bug database related to Mouse Wheel support in Java. One of the RFE's BugID #4202656 has 281 votes from developers requesting Sun for a fix. Sun has finally agreed to support this feature in JDK 1.4 codenamed Merlin accroding to the BugID #4289845 in its bug database.

This article is intended for those who don't want to wait till then or cannot move to a new JDK whenever Sun releases them. It shows how you can add support for Mouse Wheel with a little JNI code.

Searching for a solution

The first step was to develop a sample where this functionality can be tested. A sample from http://java.sun.com/docs/books/tutorial/uiswing/components/scrollpane.html#update looked ideal with a few modifications. The screenshot of this sample is shown below.

Game Plan

Given the information above, implementation was kind of straight forward. To enable mouse wheel scrolling in a Swing widget, the WM_MOUSEWHEEL messages have to intercepted in the parent frame and somehow posted to the widget. Here's a simple plan to do this.

• Step 1: Get the HWND of the parent JFrameEx.
• Step 2: Subclass the HWND using SetWindowLong(hwnd, GWL_WNDPROC,(LONG)FrameWindowProc);
• Step 3: When FrameWindowProc is called with a WM_MOUSEWHEEL, notify the java JFrameEx object.
• Step 4: In the JFrameEx's notifyMouseWheel function, figure out which scrollpane should receive the message and call the setValue of it's vertical scrollbar

    // Returns the HWND for canvas. 
	// This is undocumented, but works on JDK1.1.8, JDK1.2.2 and JDK1.3 
    public int getHWND() 
    {
        DrawingSurfaceInfo drawingSurfaceInfo;
        Win32DrawingSurface win32DrawingSurface;
        int hwnd = 0;
     
        // Get the drawing surface 
        drawingSurfaceInfo =
            ((DrawingSurface)(getPeer())).getDrawingSurfaceInfo();
        
        if (null != drawingSurfaceInfo) {
            drawingSurfaceInfo.lock();
            // Get the Win32 specific information 
            win32DrawingSurface = 
                 (Win32DrawingSurface)drawingSurfaceInfo.getSurface();
            hwnd = win32DrawingSurface.getHWnd();
            drawingSurfaceInfo.unlock();
        }
        return hwnd;
    }

Step 2: Create a native entry point (using JNI) where we can call SetWindowLong. Here's the code snippet from MouseWheel.CPP.

// JNI native entry point for subclassing the window
JNIEXPORT void JNICALL Java_JFrameEx_setHook
  (JNIEnv *pEnv, jobject f, jint hwnd)
{
	// ensure that the java object can be called from any thread.
	jobject frame = pEnv->NewGlobalRef(f);

	WNDPROC oldProc = (WNDPROC)::SetWindowLong((HWND)hwnd, GWL_WNDPROC, (LONG)FrameWindowProc);

	// store the java object
	setFrame((HWND)hwnd, frame);
	// store the old window proc
	setProc ((HWND)hwnd, oldProc);
}

Step 3: Here's the code snippet from MouseWheel.CPP which deals with the WM_MOUSEWHEEL message.

// Window Proc for subclassing.
LRESULT CALLBACK FrameWindowProc(
  HWND hwnd,      // handle to window
  UINT uMsg,      // message identifier
  WPARAM wParam,  // first message parameter
  LPARAM lParam   // second message parameter
)
{
	// Get the WindowProc for this window.
	WNDPROC oldProc = getProc((HWND)hwnd);

	// When we get a mouse wheel message
	if(uMsg == WM_MOUSEWHEEL)
	{
		MSG *pMsg = new MSG;
		pMsg->hwnd = hwnd;
		pMsg->message = uMsg;
		pMsg->wParam = wParam;
		pMsg->lParam = lParam;

		// send it to the java code from a new thread.
		_beginthread(StartNotifyThread, 0, pMsg);
		return 0;
	}
	return ::CallWindowProc(oldProc, hwnd, uMsg, wParam, lParam);
}

// Helper Thread for notification.
void WINAPIV StartNotifyThread(LPVOID lpVoid)
{
	MSG *pMsg = (MSG *)lpVoid;

	// This is the java object that needs to be notified.
	jobject frame = getFrame(pMsg->hwnd);

	// extract info from the wparam and lparam.
	WPARAM fwKeys = LOWORD(pMsg->wParam);
	short zDelta  = HIWORD(pMsg->wParam);

	LPARAM xPos = GET_X_LPARAM(pMsg->lParam); 
	LPARAM yPos = GET_Y_LPARAM(pMsg->lParam); 

	// call the helper function.
	notifyMouseWheel(frame,fwKeys,zDelta,xPos,yPos);

	delete pMsg;
}

// This helper is the one that actually invokes the java JFrameEx's notifyMouseWheel method
void notifyMouseWheel(jobject frame,short fwKeys,short zDelta,long xPos, long yPos)
{
	HMODULE hMod	= NULL;
	JavaVM *vmBuf	= NULL;
	JNIEnv *pEnv	= NULL;
	jsize bufLen	= 1;
	jint nVMs		= 0;

	// Get the Java VM.
	pJNI_GetCreatedJavaVMs pFunc = getJavaVM();
	jint nRet = (*pFunc)(&vmBuf,bufLen,&nVMs);

	// Attach this thread.
	vmBuf->AttachCurrentThread((void **)&pEnv,NULL);

	// Inform the java object that the child has been created.
	jclass cls = pEnv->GetObjectClass(frame);
	jmethodID mid = pEnv->GetMethodID(cls, "notifyMouseWheel", "(SSJJ)V");
	if (mid == 0)
		return;
	pEnv->CallVoidMethod(frame, mid, (jshort)fwKeys, (jshort)zDelta, (jlong)xPos, (jlong)yPos);

	// Detach this thread.
	vmBuf->DetachCurrentThread();
}

Step 4: Here's the code snippet from JFrameEx.java which is called from the previous step. The only trick here is that we need to get the scrollpane of the widget. From the documentation of JScrollPane we see that we have to call widget.getParent().getParent() to get the widget's scrollpane.

// this is the function which serves as a call back when 
	// a mouse wheel movement is detected.
	public void notifyMouseWheel(short fwKeys,short zDelta,long xPos, long yPos)
	{
		// Convert screen coordinates to component specific offsets.
		Point p = new Point((int)xPos,(int)yPos);
		SwingUtilities.convertPointFromScreen(p,this);

		// Find the embedded Swing component which should receive the scroll messages
		Component c = SwingUtilities.getDeepestComponentAt(this, p.x, p.y);
		try
		{
			// Get the scroll pane for the widget.
			JScrollPane scrollPane = (JScrollPane) c.getParent().getParent();

			// Get the vertical scrollbar for the scroll pane.
			JScrollBar scrollBar = scrollPane.getVerticalScrollBar();

			// Get the current value and set the new value depending on 
			// the direction of the mouse wheel.
			int nValue = scrollBar.getValue();
			nValue = nValue + ((zDelta > 0) ? 5 : -5);
			scrollBar.setValue(nValue);
		} catch (Exception e){
			// Ignore exceptions.
		}
	}

That's it folks.

Notes

• There is a Build.bat in the zip file which can be use to build the sources. please edit this file to set the location of your VC++ and JDK 1.2.2 directories
• To run the code use "java ScrollDemo2"