动态改变scroll pane的client的大小，并达到其滚动条出现当client超出我们设定的view port大小时。

package com.han;

import java.awt.BorderLayout;

import java.awt.Color;

import java.awt.Dimension;

import java.awt.Graphics;

import java.awt.Graphics2D;

import java.awt.GridLayout;

import java.awt.Shape;

import java.awt.event.MouseAdapter;

import java.awt.event.MouseEvent;

import java.awt.geom.Ellipse2D;

import java.util.ArrayList;

import javax.swing.JFrame;

import javax.swing.JLabel;

import javax.swing.JPanel;

import javax.swing.JScrollPane;

import javax.swing.SwingUtilities;

/**

* This application aims to dynamically change the client's size in a scroll

* pane.

*

* @author HAN

*

*/

@SuppressWarnings("serial")

public class ScrollDemo2 extends JPanel {

/**

* We can also use Vector to store a variable array. But the overhead will

* be more than ArrayList. So the difference between them is thread-safe or

* not.

*/

private ArrayList circles;

private final int CIRCLE_DIAMETER = 100;

private Dimension area;// indicate area taken up by graphics.

private Color[] colors = { Color.BLACK, Color.BLUE, Color.CYAN,

Color.DARK_GRAY, Color.GRAY, Color.GREEN, Color.LIGHT_GRAY,

Color.MAGENTA, Color.ORANGE, Color.PINK, Color.RED, Color.YELLOW };

ScrollDemo2() {

// The constructor serves also as a content pane. This is a common

// trick: invoke the constructor with parameter from a constructor

// without parameter.

super(new BorderLayout());

// Allocate memory for member variables if necessary.

circles = new ArrayList();

area = new Dimension(0, 0);

// Create "labelPanel".

JPanel labelPanel = new JPanel(new GridLayout(0, 1));

labelPanel

.add(new JLabel("Click left mouse button to place a circle."));

labelPanel.add(new JLabel(

"Click right mouse button to clear drawing area."));

// Create "drawingPane".

final JPanel drawingPane = new JPanel() {

@Override

protected void paintComponent(Graphics g) {

// Paint first the default behavior of the JPanel painting,

// which includes fill the whole area with a background color we

// set. This arrives to the effect that "clear the panel", which

// can also be realized by setColor followed by fillRect.

super.paintComponent(g);

// g.setColor(getBackground());

// g.fillRect(0, 0, getWidth(), getHeight());

Graphics2D g2 = (Graphics2D) g;

// For showing always all drawn circles even if you resize the

// window, we are forced to draw all circles in the "current"

// painting.

for (int i = 0; i < circles.size(); i++) {

g2.setColor(colors[i % colors.length]);

g2.fill(circles.get(i));

}

}

};

drawingPane.setOpaque(true);

// Set the background property, which you can get by getBackground.

drawingPane.setBackground(Color.WHITE);

drawingPane.addMouseListener(new MouseAdapter() {

@Override

public void mouseReleased(MouseEvent e) {

boolean changed = false;

int x = e.getX() - CIRCLE_DIAMETER / 2;

int y = e.getY() - CIRCLE_DIAMETER / 2;

if (x < 0)

x = 0;

if (y < 0)

y = 0;

if (SwingUtilities.isLeftMouseButton(e)) {

circles.add(new Ellipse2D.Double(x, y, CIRCLE_DIAMETER,

CIRCLE_DIAMETER));

// Here we can use two repaint methods: the one with

// parameter and the other without.

// repaint(clip) will repaint updates only in the desired

// region (as clip), which can have a more efficient

// performance than use directly repaint().

drawingPane.repaint(x, y, CIRCLE_DIAMETER, CIRCLE_DIAMETER);

// drawingPane.repaint();

// Update the area taken up by graphics.

int thisWidth = x + CIRCLE_DIAMETER;

if (thisWidth > area.width) {

area.width = thisWidth;

changed = true;

}

int thisHeight = y + CIRCLE_DIAMETER;

if (thisHeight > area.height) {

area.height = thisHeight;

changed = true;

}

} else if (SwingUtilities.isRightMouseButton(e)) {

circles.clear();

// Because in this case we want to change the whole painting

// area, use the repaint() directly.

drawingPane.repaint();

area.width = 0;

area.height = 0;

changed = true;

}

if (changed) {

// The setPreferredSize method is to be used in a

// combination way with the revalidate() method.

drawingPane.setPreferredSize(area);

// Let the scroll pane know to update itself especially its

// scroll bars.

drawingPane.revalidate();

}

}

});

// Put the drawingPane to a scroll pane.

JScrollPane scrollPane = new JScrollPane(drawingPane);

// setPreferredSize for the first "pack()" use.

scrollPane.setPreferredSize(new Dimension(200, 200));

// Lay out the content pane.

add(labelPanel, BorderLayout.NORTH);

add(scrollPane, BorderLayout.CENTER);

}

/**

* For thread safety, this method should be invoked from the

* event-dispatching thread.

*/

private static void createAndShowGUI() {

// Create and set up the window.

JFrame frame = new JFrame("ScrollDemo2");

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

// Create and set up the content pane.

JPanel contentPane = new ScrollDemo2();

contentPane.setOpaque(true);// Content pane must be opaque.

frame.setContentPane(contentPane);

// Display the window.

frame.pack();

frame.setVisible(true);

}

public static void main(String[] args) {

// Schedule a job for the event-dispatching thread:

// creating and showing this application's GUI.

SwingUtilities.invokeLater(new Runnable() {

@Override

public void run() {

createAndShowGUI();

}

});

}

}