如果对JFrame调用了方法setUndecorated(true);，那么窗体将没有标题栏，以及标题栏上的关闭、最大化、最小化菜单了。
这是要实现对鼠标的监控就必须自己手动去添加鼠标监听。下面代码就是实现对鼠标的监听，从而改变窗体的大小。（这里没有实现添加关闭、最大化、最小化按钮）
/*** 监听鼠标事件,实现鼠标对窗体大小的改变* * @author mengke* @email wqjsir@* @version 1.0* @date -2-25 下午10:45:48*/private class MouseInputHandler extends MouseInputAdapter {private static final int CORNER_DRAG_WIDTH = 10;private static final int BORDER_DRAG_THICKNESS = 5;/** 是否可移动窗体 */private boolean isMovingWindow;private boolean dragging;private int dragCursor;private int dragOffsetX;private int dragOffsetY;private int dragWidth;private int dragHeight;private MouseInputHandler() {}/*** 只处理鼠标左键按下操作*/@Overridepublic void mousePressed(MouseEvent e) {if (!SwingUtilities.isLeftMouseButton(e)) {return;}Point dragWindowOffset = e.getPoint();Window win = (Window) e.getSource();this.dragging = true;if (win != null) {/** 将窗体置前 */win.toFront();}Frame frame = (win instanceof Frame) ? (Frame) win : null;int frameState = frame != null ? frame.getExtendedState(): Frame.NORMAL;if (((frame != null) && ((frameState & Frame.MAXIMIZED_BOTH) == 0))) {this.isMovingWindow = true;this.dragOffsetX = dragWindowOffset.x;this.dragOffsetY = dragWindowOffset.y;}if (!this.isMovingWindow && frame != null && frame.isResizable()&& ((frameState & Frame.MAXIMIZED_BOTH) == 0)) {this.dragOffsetX = dragWindowOffset.x;this.dragOffsetY = dragWindowOffset.y;}this.dragWidth = win.getWidth();this.dragHeight = win.getHeight();this.dragCursor = getCursor(win, e);}@Overridepublic void mouseReleased(MouseEvent e) {if (!SwingUtilities.isLeftMouseButton(e)) {return;}if ((this.dragCursor != Cursor.DEFAULT_CURSOR)&& (!JRoundFrame.this.isValid())) {JRoundFrame.this.validate();}this.dragging = false;this.isMovingWindow = false;this.dragCursor = Cursor.DEFAULT_CURSOR;mouseMoved(e);}@Overridepublic void mouseMoved(MouseEvent e) {Window win = (Window) e.getSource();Frame frame = (win instanceof Frame) ? (Frame) win : null;int cursor = getCursor(win, e);if ((cursor != 0)&& (win.getBounds().contains(e.getLocationOnScreen()))&& ((frame != null) && (frame.isResizable()) && ((frame.getExtendedState() & Frame.MAXIMIZED_BOTH) == 0))) {win.setCursor(Cursor.getPredefinedCursor(cursor));} else {win.setCursor(Cursor.getDefaultCursor());}}@Overridepublic void mouseExited(MouseEvent e) {if (!this.dragging) {Window win = (Window) e.getSource();win.setCursor(Cursor.getDefaultCursor());}}@Overridepublic void mouseDragged(MouseEvent e) {if (!SwingUtilities.isLeftMouseButton(e)) {if (JRoundFrame.this.getBounds().contains(e.getPoint())) {mouseMoved(e);} else {mouseExited(e);}return;}Window win = (Window) e.getSource();Point point = e.getPoint();if (this.isMovingWindow && this.dragCursor == Cursor.DEFAULT_CURSOR) {Point eventLocationOnScreen = e.getLocationOnScreen();win.setLocation(eventLocationOnScreen.x - this.dragOffsetX,eventLocationOnScreen.y - this.dragOffsetY);} else if (this.dragCursor != Cursor.DEFAULT_CURSOR) {Rectangle rect = win.getBounds();Rectangle startBounds = new Rectangle(rect);Dimension min = win.getMinimumSize();Dimension max = win.getMaximumSize();switch (this.dragCursor) {case Cursor.E_RESIZE_CURSOR:adjust(rect, min, max, 0, 0, point.x+ (this.dragWidth - this.dragOffsetX) - rect.width,0);break;case Cursor.S_RESIZE_CURSOR:adjust(rect, min, max, 0, 0, 0, point.y+ (this.dragHeight - this.dragOffsetY)- rect.height);break;case Cursor.N_RESIZE_CURSOR:adjust(rect, min, max, 0, point.y - this.dragOffsetY, 0,-(point.y - this.dragOffsetY));break;case Cursor.W_RESIZE_CURSOR:adjust(rect, min, max, point.x - this.dragOffsetX, 0,-(point.x - this.dragOffsetX), 0);break;case Cursor.NE_RESIZE_CURSOR:adjust(rect, min, max, 0, point.y - this.dragOffsetY,point.x + (this.dragWidth - this.dragOffsetX)- rect.width, -(point.y - this.dragOffsetY));break;case Cursor.SE_RESIZE_CURSOR:adjust(rect, min, max, 0, 0, point.x+ (this.dragWidth - this.dragOffsetX) - rect.width,point.y + (this.dragHeight - this.dragOffsetY)- rect.height);break;case Cursor.NW_RESIZE_CURSOR:adjust(rect, min, max, point.x - this.dragOffsetX, point.y- this.dragOffsetY, -(point.x - this.dragOffsetX),-(point.y - this.dragOffsetY));break;case Cursor.SW_RESIZE_CURSOR:adjust(rect, min, max, point.x - this.dragOffsetX, 0,-(point.x - this.dragOffsetX), point.y+ (this.dragHeight - this.dragOffsetY)- rect.height);break;}if (!rect.equals(startBounds)) {win.setBounds(rect);if (Toolkit.getDefaultToolkit().isDynamicLayoutActive()) {win.validate();JRoundFrame.this.getRootPane().repaint();}}}}/*** 鼠标左键点击事件*/@Overridepublic void mouseClicked(MouseEvent e) {if (!SwingUtilities.isLeftMouseButton(e)) {return;}Window win = (Window) e.getSource();Frame frame = (win instanceof Frame) ? (Frame) win : null;Point point = e.getPoint();int clickCount = e.getClickCount();if ((clickCount % 2 == 0)&& (point.x <= 22)&& (point.y <= 22)&& (JRoundFrame.this.getRootPane().getWindowDecorationStyle() != 0)) {JRoundFrame.this.dispatchEvent(new WindowEvent(JRoundFrame.this, WindowEvent.WINDOW_CLOSING));return;}if (frame == null) {return;}int state = frame.getExtendedState();Window fullWin = GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().getFullScreenWindow();if ((frame.isResizable()) && (frame != fullWin)&& (clickCount % 2 == 0)) {if ((state & Frame.MAXIMIZED_BOTH) != 0) {frame.setExtendedState(state & 0xFFFFFFF9);} else {frame.setExtendedState(state | Frame.MAXIMIZED_BOTH);}}}private void adjust(Rectangle bounds, Dimension min, Dimension max,int deltaX, int deltaY, int deltaWidth, int deltaHeight) {bounds.x += deltaX;bounds.y += deltaY;bounds.width += deltaWidth;bounds.height += deltaHeight;if (min != null) {if (bounds.width < min.width) {int correction = min.width - bounds.width;if (deltaX != 0) {bounds.x -= correction;}bounds.width = min.width;}if (bounds.height < min.height) {int correction = min.height - bounds.height;if (deltaY != 0) {bounds.y -= correction;}bounds.height = min.height;}}if (max != null) {if (bounds.width > max.width) {int correction = max.width - bounds.width;if (deltaX != 0) {bounds.x -= correction;}bounds.width = max.width;}if (bounds.height > max.height) {int correction = max.height - bounds.height;if (deltaY != 0) {bounds.y -= correction;}bounds.height = max.height;}}}private int getCursor(Window win, MouseEvent e) {int winWidth = win.getBounds().width;int winHeight = win.getBounds().height;Point p = e.getPoint();// e.getLocationOnScreen();if (win.getBounds().contains(e.getLocationOnScreen())) {// 先判断四个角if (p.x - CORNER_DRAG_WIDTH < 0 && p.y - CORNER_DRAG_WIDTH < 0) {return Cursor.NW_RESIZE_CURSOR;} else if (p.x - (winWidth - CORNER_DRAG_WIDTH) > 0&& p.y - CORNER_DRAG_WIDTH < 0) {return Cursor.NE_RESIZE_CURSOR;} else if (p.x - CORNER_DRAG_WIDTH < 0&& p.y - (winHeight - CORNER_DRAG_WIDTH) > 0) {return Cursor.SW_RESIZE_CURSOR;} else if (p.x - (winWidth - CORNER_DRAG_WIDTH) > 0&& p.y - (winHeight - CORNER_DRAG_WIDTH) > 0) {return Cursor.SE_RESIZE_CURSOR;}// 判断是否在中间部分if ((p.x > BORDER_DRAG_THICKNESS && p.x < winWidth- BORDER_DRAG_THICKNESS)&& (p.y > BORDER_DRAG_THICKNESS && p.y < winHeight- BORDER_DRAG_THICKNESS)) {return Cursor.DEFAULT_CURSOR;} else {if (p.x - BORDER_DRAG_THICKNESS < 0) {return Cursor.W_RESIZE_CURSOR;} else if (p.y - BORDER_DRAG_THICKNESS < 0) {return Cursor.N_RESIZE_CURSOR;} else if (winWidth - BORDER_DRAG_THICKNESS > p.x) {return Cursor.S_RESIZE_CURSOR;} else {return Cursor.E_RESIZE_CURSOR;}}}return Cursor.DEFAULT_CURSOR;}}
其中JRoundFrame为继承JFrame类的类。在该类的构造方法中添加该鼠标监听，就能实现鼠标对该窗体的大小的改变了。
