java 将画布内容保存到本地图片（简单画图板） - 好文

概述：一个简单的画图板，实现将画布内容保存为本地图片

1、手绘图形

    可以画任意图形，即在鼠标拖动下实现任意线段的绘制。

    基本原理是扩展swing中的JPanel类（也可以用Java的Canvas类，但两者还是有些区别的）
，同时添加对鼠标的监听。任意的线段绘制，可以看做是一点一点的很小的线段叠加而成，就是点构成线么，即在鼠标拖动监听器里每次采集点，然后跟上一次的点构成线，同时每次都repaint，而在paint方法里就要实现对对所有点的一次绘制。

    在这里用到的鼠标监听事件是鼠标的拖动，要实现 MouseMotionListener 这个接口。

完整代码：
import java.awt.BasicStroke; import java.awt.Canvas; import java.awt.Color;
import java.awt.Graphics; import java.awt.Graphics2D; import
java.awt.Rectangle; import java.awt.event.MouseEvent; import
java.awt.event.MouseListener; import java.awt.event.MouseMotionListener; import
java.util.ArrayList; import javax.swing.BorderFactory; import
javax.swing.JPanel; public class MyCanvas extends JPanel implements
MouseListener, MouseMotionListener{ private int width = 300; private int height
= 300; private int x = 0;      private int y = 0; public ArrayList<Integer[]>
myList = new ArrayList<Integer[]>(); public MyCanvas() { super();
this.setSize(width, height); this.setBounds(new Rectangle(0,0,450,300));
this.setBorder(BorderFactory.createLineBorder(Color.GRAY));
this.setBackground(Color.WHITE); this.setVisible(true);
this.addMouseListener(this); this.addMouseMotionListener(this); } public void
paint(Graphics graphics){ super.paint(graphics); Graphics2D g2d =
(Graphics2D)graphics; g2d.setBackground(Color.WHITE);
g2d.setColor(Color.BLACK); g2d.setStroke(new BasicStroke(10)); for(int
i=0;i<myList.size();i++){ g2d.drawLine(myList.get(i)[0],myList.get(i)[1],
myList.get(i)[2],myList.get(i)[3]); } g2d.dispose(); } @Override public void
mouseDragged(MouseEvent e) {     myList.add(new
Integer[]{this.x,this.y,e.getX(),e.getY()});     this.x = e.getX();     this.y
= e.getY();     this.repaint(); } @Override public void mouseMoved(MouseEvent
e) { // TODO Auto-generated method stub } @Override public void
mouseClicked(MouseEvent e) { // TODO Auto-generated method stub } @Override
public void mousePressed(MouseEvent e) { // TODO Auto-generated method stub
    this.x = e.getX();     this.y = e.getY(); //首次的点击采集点 } @Override public
void mouseReleased(MouseEvent e) { // TODO Auto-generated method stub }
@Override public void mouseEntered(MouseEvent e) { // TODO Auto-generated
method stub } @Override public void mouseExited(MouseEvent e) { // TODO
Auto-generated method stub } }

在这里遇到一个小问题，本来是用继承Canvas类来做的，但是画布上写东西的时候会不停地闪烁，而且之后图片也一直保存不下来，一直没找到问题所在，换成JPanel就好了，个人猜测是组件在调用repaint（）方法时的问题，在这里不多说。屏幕闪烁可以用双缓冲解决。

2、保存图片到本地

    主要用到awt的image类以及IO流。

    主要步骤：新建一个BufferedImage
，然后利用graphics在其上在画一遍存在于canvas上的内容，然后将BufferedImage保存于本地即可，利用ImageIO里的静态方法write（）。

部分代码：

        Dimension imagesize = this.canvas.getSize(); BufferedImage image = new
BufferedImage(imagesize.width,imagesize.height,BufferedImage.TYPE_INT_RGB);
Graphics2D graphics = image.createGraphics(); this.canvas.paint(graphics);
graphics.dispose();         File f=new File("src/temp/1.jpg"); if( !f.exists()
)         {     f.createNewFile();     System.out.println(123); }
ImageIO.write(myImage, "jpg",f); return null;
3、调整保存图片的大小

因为笔者需要用到固定大小的图片，所以对图片进行了调整，之后保存。

完整代码：
import java.awt.BasicStroke; import java.awt.Color; import java.awt.Dimension;
import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.Image;
import java.awt.Rectangle; import java.awt.event.ActionEvent; import
java.awt.event.ActionListener; import java.awt.image.BufferedImage; import
java.io.File; import java.io.FileOutputStream; import java.io.IOException;
import javax.imageio.ImageIO; import javax.swing.JButton; import
javax.swing.JFrame; public class MainofFrame extends JFrame{ private int width
= 450; private int height = 450; private MyCanvas canvas = null; public
MainofFrame() { super(); this.setTitle("paint"); this.setSize(width, height);
this.setResizable(false); this.setLocation(400, 180);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); this.setLayout(null);
this.canvas = new MyCanvas(); this.add(canvas); this.save = new JButton();
this.save.setText("save"); this.save.setBounds(40, 360, 80, 30);
this.save.addActionListener(new ActionListener() { @Override public void
actionPerformed(ActionEvent e) { try { savePic(); } catch (IOException e1) {
e1.printStackTrace(); } } }); this.add(save); this.setVisible(true); } public
String savePic() throws IOException{ Dimension imagesize =
this.canvas.getSize(); BufferedImage image = new
BufferedImage(imagesize.width,imagesize.height,BufferedImage.TYPE_INT_RGB);
Graphics2D graphics = image.createGraphics();
this.canvas.paint(graphics);         graphics.dispose();                 Image
newImage = image.getScaledInstance(28, 28, Image.SCALE_SMOOTH);
        BufferedImage myImage = new
BufferedImage(28,28,BufferedImage.TYPE_INT_RGB);         Graphics graphics1 =
myImage.getGraphics();          graphics1.drawImage(newImage, 0, 0, null);
        graphics1.dispose();//对图片调整大小                 File f=new
File("src/temp/1.jpg"); if( !f.exists() ) { f.createNewFile();
System.out.println(123); } ImageIO.write(myImage, "jpg",f); return null; } }
正在学习中，如果有错误希望帮忙矫正，十分感谢！

    +（在eclipse上复制粘贴过来的，不知道代码为什么排版这么乱，还望谅解。。。）

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