在写GUI程序时，经常需要读取一些比较大的文件，此时可能需要几分钟的时间。我们希望界面尽量友好，使用户随时知道读取文件、处理的进度。在大多数Windows程序中，大家对进度条都已经非常熟悉，Java中也有现成的类可以非常方便地完成此项功能－－ProgressMonitorInputStream。下面是一个小的程序，你可以编译运行。运行后点击窗口中的“Press me”按钮，将弹出一个窗口，其中包含一个进度条，显示读取当前目录中名为bigfile.dat的文件的进度。还包含一个“取消”按钮，可以随时中止读取线程。
import java.awt.FlowLayout;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.io.FileInputStream;import java.io.InputStream;import javax.swing.JButton;import javax.swing.JFrame;import javax.swing.ProgressMonitorInputStream;public class Test { public static void main(String[] args) { // create a test frame with a "press me" button final JFrame f = new JFrame("Sample"); f.getContentPane().setLayout(new FlowLayout()); JButton b = new JButton("Press me"); f.getContentPane().add(b); f.pack(); // set up the file read action b.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { // when button is pressed, start a new thread // to read the file. A new thread is needed because we // need to free the GUI update thread to paint the // progress monitor new Thread() { public void run() { try { // open the file, wrapping it in a ProgressMonitorInputStream InputStream in = new FileInputStream("bigfile.dat"); ProgressMonitorInputStream pm = new ProgressMonitorInputStream(f,"Reading the big file",in); // read the file. If it´s taking too long, the progress // monitor will appear. The amount of time is roughly // 1/100th of the estimated read time (based on how long // it took to read the first 1/100th of the file.) // Note that by default, the dialog won´t appear unless // the overall estimate is over 2 seconds. int c; while((c=pm.read()) != -1) { // do something } pm.close(); // needs better error handling, of course... } catch(Exception ex) { ex.printStackTrace(); } } }.start(); }}); // display the frame f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); f.setVisible(true); }}