The exec( ) method in the
Runtime class lets you run an external program.
The command line you give will be broken into
strings by a simple
StringTokenizer (Section 3.3) and
passed on to the operating system’s “execute a
program” system call. As a simple example, here is a simple
program that uses exec( ) to run
kwrite
, a windowed
text editor
program.[59] On
MS-Windows, you’d have
to change the name to notepad or
wordpad, possibly including the full pathname,
e.g., c:\WINDOWS\NOTEPAD.EXE (double
backslashes because the backslash is special in Java strings).
// file ExecDemoSimple.java
public class ExecDemoSimple {
public static void main(String av[]) throws java.io.IOException {
// Run the "notepad" program or a similar editor
Process p = Runtime.getRuntime( ).exec("kwrite");
}
}When you compile and run it, the appropriate editor window appears:
$ jr ExecDemoSimple + jikes +E -d . ExecDemoSimple.java + java ExecDemoSimple # causes a KWrite window to appear. $
Example 26-1 runs the MS-Windows or Unix version of Netscape, assuming Netscape was installed in the default directory. It passes as an argument the name of a help file, offering a kind of primitive “help” mechanism, as displayed in Figure 26-1.
Example 26-1. ExecDemoNS.java
import com.darwinsys.util.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.net.*;
import java.awt.*;
/**
* ExecDemoNS shows how to execute a 32-bit Windows program from within Java.
*/
public class ExecDemoNS extends JFrame {
/** The name of the help file. */
protected final static String HELPFILE = "./help/index.html";
/** The path to the Netscape binary */
protected static String netscape;
/** A process object tracks one external running process */
Process p;
/** main - instantiate and run */
public static void main(String av[]) throws Exception {
new ExecDemoNS( ).setVisible(true);
}
/** Constructor - set up strings and things. */
public ExecDemoNS( ) {
super("ExecDemo: Netscape");
String osname = System.getProperty("os.name");
if (osname == null)
throw new IllegalArgumentException("no os.name");
netscape = // Windows or Unix only for now, sorry Mac fans
(osname.toLowerCase( ).indexOf("windows")!=-1) ?
"c:/program files/netscape/communicator/program/netscape.exe" :
"/usr/local/netscape/netscape";
Container cp = getContentPane( );
cp.setLayout(new FlowLayout( ));
JButton b;
cp.add(b=new JButton("Exec"));
b.addActionListener(new ActionListener( ) {
public void actionPerformed(ActionEvent evt) {
doHelp( );
}
});
cp.add(b=new JButton("Wait"));
b.addActionListener(new ActionListener( ) {
public void actionPerformed(ActionEvent evt) {
doWait( );
}
});
cp.add(b=new JButton("Exit"));
b.addActionListener(new ActionListener( ) {
public void actionPerformed(ActionEvent evt) {
System.exit(0);
}
});
pack( );
}
/** Start the help, in its own Thread. */
public void doHelp( ) {
new Thread( ) {
public void run( ) {
try {
// Get the URL for the Help File
URL helpURL = this.getClass().getClassLoader( ).
getResource(HELPFILE);
// Start Netscape from the Java Application. A Java
// Applet would not be allowed to, nor need to :-)
p = Runtime.getRuntime( ).exec(netscape + " " + helpURL);
Debug.println("trace", "In main after exec");
} catch (Exception ex) {
JOptionPane.showMessageDialog(ExecDemoNS.this,
"Error" + ex, "Error",
JOptionPane.ERROR_MESSAGE);
}
}
}.start( );
}
public void doWait( ) {
try {
p.waitFor( ); // wait for process to complete
Debug.println("trace", "Process is done");
} catch (Exception ex) {
JOptionPane.showMessageDialog(this,
"Error" + ex, "Error",
JOptionPane.ERROR_MESSAGE);
}
}
}
[59] kwrite is Unix-specific; it’s a part of the K Desktop Environment (KDE). See http://www.kde.org.