JTextArea 实现行号

package graphic; import java.awt.Color; import java.awt.FontMetrics; import java.awt.Component; import java.awt.Graphics; import java.awt.Insets; import javax.swing.JTextArea; import javax.swing.border.AbstractBorder; public class LineNumberBorder extends AbstractBorder { public LineNumberBorder(){ } /*Insets 对象是容器边界的表示形式。 它指定容器必须在其各个边缘留出的空间。 */ //此方法在实例化时自动调用 //此方法关系到边框是否占用组件的空间 public Insets getBorderInsets(Component c) { return getBorderInsets(c,new Insets(0,0,0,0)); } public Insets getBorderInsets(Component c, Insets insets) { if(c instanceof JTextArea){ int width=lineNumberWidth((JTextArea)c); insets.left=width; } return insets; } public boolean isBorderOpaque() { return false; } //边框的绘制方法 //此方法必须实现 public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) { //获得当前剪贴区域的边界矩形。 java.awt.Rectangle clip=g.getClipBounds(); FontMetrics fm=g.getFontMetrics(); int fontHeight=fm.getHeight(); //starting location at the "top" of the page... // y is the starting baseline for the font... int ybaseline=y+fm.getAscent(); // now determine if it is the "top" of the page...or somewhere else int startingLineNumber=(clip.y/fontHeight)+1; if(startingLineNumber!=1){ ybaseline=y+startingLineNumber*fontHeight- (fontHeight-fm.getAscent()); } int yend=ybaseline+height; if(yend>(y+height)){ yend=y+height; } JTextArea jta=(JTextArea)c; int lineWidth=lineNumberWidth(jta); int lnxStart=x+lineWidth; g.setColor(Color.blue); // loop until out of the "visible" region... int length=(""+Math.max(jta.getRows(), jta.getLineCount()+1)).length(); //绘制行号 while(ybaseline<yend) { String label = padLabel(startingLineNumber, length, true); g.drawString(label, lnxStart- fm.stringWidth(label), ybaseline); ybaseline+=fontHeight; startingLineNumber++; } } //寻找适合的数字宽度 private int lineNumberWidth(JTextArea jta){ int lineCount=Math.max(jta.getRows(), jta.getLineCount()); return jta.getFontMetrics(jta.getFont()).stringWidth(lineCount+" "); } private static String padLabel(int lineNumber, int length, boolean addSpace) { StringBuffer buffer=new StringBuffer(); buffer.append(lineNumber); for(int count=(length-buffer.length());count>0;count--){ buffer.insert(0, ' '); } if(addSpace){ buffer.append(' '); } return buffer.toString(); } }