How to Execute External Programs in Java: A Guide to the Process Interface?

Starting Processes in Java

Similar to the .Net framework where processes are launched using System.Diagnostics.Process.Start("processname"), Java provides a convenient way to execute external programs.

Java Process Interface

In Java, the Runtime class offers the exec() method to start a process. The syntax is:

Process p = Runtime.getRuntime().exec("processname");

where "processname" represents the executable or command to be launched.

Cross-Platform Compatibility

To achieve cross-platform compatibility, you can use the System class to determine the current operating system and adjust the process launch accordingly. For example, on Windows systems, you can use the "%windir%system32" path before the executable name.

Process Input and Output

The Process class provides input and output streams to communicate with the running process. You can use the getInputStream() and getOutputStream() methods to access these streams.

Example Walkthrough

The provided Java code demonstrates how to launch the command tree.com /A on a Windows system:

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.nio.file.Paths;

public class CmdExec {

  public static void main(String[] args) {
    try {
      Process p = Runtime.getRuntime().exec(
          Paths.get(System.getenv("windir"), "system32", "tree.com /A").toString()
      );

      try (BufferedReader input = new BufferedReader(new InputStreamReader(p.getInputStream()))) {
        String line;

        while ((line = input.readLine()) != null) {
          System.out.println(line);
        }
      }

    } catch (Exception err) {
      err.printStackTrace();
    }
  }
}

This code launches the tree command with the /A option, which displays a directory listing in tree format. The output from the command is then read and printed to the console.

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