有时候，需要创建一个特殊形状的panel。这个panel 的形状需要自己绘制，也可以使用现有的layout manager,但是可供现有的layout manager管理的面积会发生变化，这个需要通过getInsets通知layout manager.

import java.awt.BasicStroke;

import java.awt.Color;

import java.awt.Graphics;

import java.awt.Graphics2D;

import java.awt.Insets;

import java.awt.RenderingHints;

import javax.swing.JPanel;

/**

* This class represents a Rounded Border JPanel.

*/

public class ArrowPanel extends JPanel {

private static final long serialVersionUID = 1L;

//笔画的粗细

/** Stroke size. it is recommended to set it to 1 for better view */

protected int strokeSize = 1;

/** Color of shadow */

protected Color shadowColor = Color.black;

/** Sets if it drops shadow */

protected boolean shady = true;

/** Sets if it has an High Quality view */

protected boolean highQuality = true;

//阴影和边界的距离。

/** Distance between border of shadow and border of opaque panel */

protected int shadowGap = 4;

//阴影的偏移量。

/** The offset of shadow.  */

protected int shadowOffset = 3;

/** The transparency value of shadow. ( 0 - 255) */

protected int shadowAlpha = 150;

//这里设定箭头的五个顶点。

//x,y坐标

// Points of the Polygon.

protected int xArrowPoints[]={0,0,0,0,0};

protected int yArrowPoints[]={0,0,0,0,0};

public ArrowPanel() {

super();

setOpaque(false);

}

// 更改Layout Manager的insets的大小，这个很重要，可以通知layout manager 该箭头形状的

//panel的到底那些部分可以用于绘图和布局子组件。

// New size of insets for the layout manager.

@Override

public Insets getInsets() {

Insets insets = (Insets) super.getInsets().clone();

// Return new insets.

insets.top  += strokeSize;

insets.left += strokeSize;

insets.bottom += strokeSize + shadowGap;

insets.right += strokeSize + shadowGap + getHeight() /2;

return insets;

}

// 绘制这个特殊的panel

// Paint the panel.

@Override

protected void paintComponent(Graphics g) {

super.paintComponent(g);

int width = getWidth();

int height = getHeight();

int shadowGap = this.shadowGap;

Color shadowColorA = new Color(shadowColor.getRed(), shadowColor.getGreen(), shadowColor.getBlue(), shadowAlpha);

Graphics2D graphics = (Graphics2D) g;

//Sets antialiasing if HQ.

if (highQuality) {

graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

}

//绘制阴影

//Draws shadow borders if any.

if (shady) {

graphics.setColor(shadowColorA);

//计算阴影的形状。

calcArrowPoints(shadowOffset,shadowOffset,

width - strokeSize - shadowOffset,

height - strokeSize - shadowOffset);

graphics.fillPolygon(xArrowPoints, yArrowPoints, 5);

} else {

shadowGap = 1;

}

//绘制透明图层。

//Draws the arrow opaque panel with borders.

// fill the arrow.

graphics.setColor(getBackground());

calcArrowPoints(0,0,

width - shadowGap,

height - shadowGap);

graphics.fillPolygon(xArrowPoints, yArrowPoints, 5);

// 绘制这个panel本身。

//Draw the arrow panel border.

graphics.setColor(getForeground());

graphics.setStroke(new BasicStroke(strokeSize));

calcArrowPoints(0,0,

width - shadowGap,

height - shadowGap);

graphics.drawPolygon(xArrowPoints, yArrowPoints, 5);

//Sets strokes to default, is better.

graphics.setStroke(new BasicStroke());

}

/**

* Check if component has High Quality enabled.

*

* @returnTRUEif it is HQ ;FALSEOtherwise

*/

public boolean isHighQuality() {

return highQuality;

}

/**

* Sets whether this component has High Quality or not

*

* @param highQuality ifTRUE, set this component to HQ

*/

public void setHighQuality(boolean highQuality) {

this.highQuality = highQuality;

}

/**

* Returns the Color of shadow.

*

* @return a Color object.

*/

public Color getShadowColor() {

return shadowColor;

}

/**

* Sets the Color of shadow

*

* @param shadowColor Color of shadow

*/

public void setShadowColor(Color shadowColor) {

this.shadowColor = shadowColor;

}

/**

* Check if component drops shadow.

*

* @returnTRUEif it drops shadow ;FALSEOtherwise

*/

public boolean isShady() {

return shady;

}

/**

* Sets whether this component drops shadow

*

* @param shady ifTRUE, it draws shadow

*/

public void setShady(boolean shady) {

this.shady = shady;

}

/**

* Returns the size of strokes.

*

* @return the value of size.

*/

public float getStrokeSize() {

return strokeSize;

}

/**

* Sets the stroke size value.

*

* @param strokeSize stroke size value

*/

public void setStrokeSize(int strokeSize) {

this.strokeSize = strokeSize;

}

/**

* Get the value of shadowOffset

*

* @return the value of shadowOffset

*/

public int getShadowOffset() {

return shadowOffset;

}

/**

* Set the value of shadowOffset

*

* @param shadowOffset new value of shadowOffset

*/

public void setShadowOffset(int shadowOffset) {

if (shadowOffset >= 1) {

this.shadowOffset = shadowOffset;

} else {

this.shadowOffset = 1;

}

}

/**

* Get the value of shadowGap

*

* @return the value of shadowGap

*/

public int getShadowGap() {

return shadowGap;

}

/**

* Set the value of shadowGap

*

* @param shadowGap new value of shadowGap

*/

public void setShadowGap(int shadowGap) {

if (shadowGap >= 1) {

this.shadowGap = shadowGap;

} else {

this.shadowGap = 1;

}

}

/**

* Get the value of shadowAlpha

*

* @return the value of shadowAlpha

*/

public int getShadowAlpha() {

return shadowAlpha;

}

/**

* Set the value of shadowAlpha

*

* @param shadowAlpha new value of shadowAlpha

*/

public void setShadowAlpha(int shadowAlpha) {

if (shadowAlpha >= 0 && shadowAlpha <= 255) {

this.shadowAlpha = shadowAlpha;

} else {

this.shadowAlpha = 255;

}

}

//计算箭头形状的五个顶点。

/**

* Calculate the value of Arrow points.

*

* @param iStartX: start X of  Arrow points.

* iStartY: start Y of  Arrow points.

* iWidth:  width of Arrow.

* iHeight: height of Arrow.

*

*/

public void calcArrowPoints(int iStartX, int iStartY, int iWidth, int iHeight) {

// Point 0;

xArrowPoints[0] = iStartX;

yArrowPoints[0] = iStartY;

// Point 1;

xArrowPoints[1] = iStartX+iWidth-iHeight/2;

yArrowPoints[1] = iStartY;

// Point 2;

xArrowPoints[2] = iStartX+iWidth;

yArrowPoints[2] = iStartY+iHeight/2;

// Point 3;

xArrowPoints[3] = iStartX+iWidth-iHeight/2;

yArrowPoints[3] = iStartY+iHeight;

// Point 4;

xArrowPoints[4] = iStartX;

yArrowPoints[4] = iStartY+iHeight;

}

}

效果：