Programming Tutorials

Executing other system commands and programs from a Java Program

By: Emiley J Printer Friendly Format    

In safe environments, you can use Java to execute other heavyweight processes (that is, programs) on your multitasking operating system. Several forms of the exec( ) method allow you to name the program you want to run as well as its input parameters. The exec( ) method returns a Process object, which can then be used to control how your Java program interacts with this new running process. Because Java can run on a variety of platforms and under a variety of operating systems, exec( ) is inherently environment-dependent.

The following example uses exec( ) to launch notepad, Windows' simple text editor. Obviously, this example must be run under the Windows operating system.

// Demonstrate exec().
class ExecDemo {
public static void main(String args[]) {
Runtime r = Runtime.getRuntime();
Process p = null;
try {
p = r.exec("notepad");
} catch (Exception e) {
System.out.println("Error executing notepad.");
}
}
}

There are several alternate forms of exec( ), but the one shown in the example is the most common. The Process object returned by exec( ) can be manipulated by Process' methods after the new program starts running. You can kill the subprocess with the destroy( ) method. The waitFor( ) method causes your program to wait until the subprocess finishes. The exitValue( ) method returns the value returned by the subprocess when it is finished. This is typically 0 if no problems occur. Here is the preceding exec( ) example modified to wait for the running process to exit:

// Wait until notepad is terminated.
class ExecDemoFini {
public static void main(String args[]) {
Runtime r = Runtime.getRuntime();
Process p = null;
try {
p = r.exec("notepad");
p.waitFor();
} catch (Exception e) {
System.out.println("Error executing notepad.");
}
System.out.println("Notepad returned " + p.exitValue());
}
}

While a subprocess is running, you can write to and read from its standard input and output. The getOutputStream( ) and getInputStream( ) methods return the handles to standard in and out of the subprocess.