I wrote a simple java applet program to scan local port all the time,
the following is the code.
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import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
import java.net.*;

public class SimpleScannerApplet extends JApplet {

// Declare the GUI components used by the applet
JTextField portTextField = new JTextField("8080",5);
JTextArea resultsTextArea = new JTextArea(2,60);
JButton scanButton = new JButton("scane");
JLabel portLabel = new JLabel("port number: ");
JLabel resultsLabel = new JLabel("result: ");
JPanel mainPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
JPanel topPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
JPanel bottomPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));

public SimpleScannerApplet() {
// Layout the applet's components
Border etched = BorderFactory.createEtchedBorder();
// Add an etched border to the main panel
mainPanel.setBorder(BorderFactory.createTitledBorder(etched, "local prot scanner"));
// Host and port fields in top panel
topPanel.add(portLabel);
topPanel.add(portTextField);
// Scan button and result field in bottom panel
bottomPanel.add(scanButton);
bottomPanel.add(resultsLabel);
bottomPanel.add(resultsTextArea);
// Add top and bottom panels to main panel
mainPanel.add(topPanel);
mainPanel.add(bottomPanel);
// Set up the button's event handler
scanButton.addActionListener(new Scanner());
// Add the main panel to the applet's content pane
getContentPane().add(mainPanel);
}

// The Scan button's event handler does the actual scanning
private class Scanner implements ActionListener {

// Declare fields used as scan parameters

InetAddress address;
int port;


// Handle the button's action event
public void actionPerformed(ActionEvent ev) {
out("check parameters...");
if(validParameters()) {
out("scannering ...");
try {
while(true){
ServerSocket s = new ServerSocket(port, 5);
// No exception: SUCCESS
out("scan:\n");
out(" "+s+"\n");
s.close();
}
}
catch(Exception ex) {
// Could not connect
if(ex instanceof SecurityException) out(ex.getMessage());
else out("Port "+port+" is closed ");
}
}
}

// Validate scan parameters
private boolean validParameters() {
try {

String portString = portTextField.getText();
// Convert the portString to an int
port = Integer.decode(portString).intValue();
// Make sure that it's in range
if(port > 65535) throw new ScannerException("Invalid port.");
}catch(Exception e) {
// Handle any validation-related exceptions
out(e.getMessage());
return false;
}
return true;
}

// Convenience method for writing results
private void out(String msg) {
resultsTextArea.setText(msg);
}
}

// Used to identify invalid scan parameters
private class ScannerException extends Exception {

public ScannerException(String msg) {
super(msg);
}
}
}

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but when I run the applet,the CPU always works 100%,how to
reduce the resource needs?

after ServerSocket s = new ServerSocket(port, 5); i don't seem to see where you call: s.accept();
my guess is that will not only solve your problem, but also make your program correct.

really?I'll try

well,after I add the s.accept();
the program just output"scannering....."
my purpose is just to scan the local port
so I don't need to use s.accept();
"bind","listen" is enough.
I was told that because of the "while" sentence,my CPU run 100%,is there
any way to reduce resource requirement?I was told C could do that ,but he was not sure whether java could or not.it seens to make the program pause some time,then run,not always run,I am not sure.

to me, "scan" sounds like what a client would do, you know, like hackers scan the internet for servers that are accepting connections. so, if that's what you meant by "scan," then i think you should use Socket instead of ServerSocket. because it's hard for me to see the usefulness of a ServerSocket when it's not used to accept connections.
but then again, since you're working with a local port, you have to have some process accepting connections at that port.
also, i don't quite understand what you meant (my networking knowledge is limited to a quarter of java): ""bind","listen" is enough". isn't it the same as accepting?
basically, clients connect, and servers accept. but you're doing neither (or so it seems).

well,I just need to listen to my local port,so I don't need server and client,case I just listen to my local port,I don't need to connect to other computers.
ServerSocket s=new ServerSocket(port,5) could bind and listen to the local
port,so this is enough for my job.
s.accepts() Listens for a connection to be made to this socket and accepts it.But since I don't need to connect to others ,this is no use.
this is just what I thought,maybe I am wrong(case I am also lack of networking knowledges of java).why don't you test it and tell me the result?I am also puzzled why I add the s.accept()there is no result,maybe you could show me the reason.
well,my e-mail is hou007xz@hotmail.com,tomorrow I'll go traveling for 3 days,maybe we could communicate when I go back.Anyway,thank you very much .

the pupose for this program is to listen to your local port all the time and show the situation of your local port.