Execute An External Program From Java
17 April 2007A Java programmer may need to access the environment under which the application is running. It is supported by “ getRuntime ” method in Runtime Class. Programmer need not create an instance of Runtime class, Instead the Java application has a single instance of class Runtime environment in which the application is running.
Runtime class has several methods that are useful to execute an external program, getting Information about processors, Memory Information etc.
Following program executes an external program JavaTipsEbook.exe which is located at Local Drive.
import java.io.*; public class CommandExection { public CommandExection(String commandline) { try { String line; Process p = Runtime.getRuntime().exec(commandline); BufferedReader input = new BufferedReader (new InputStreamReader(p.getInputStream())); while ((line = input.readLine()) != null) { System.out.println(line); } input.close(); } catch (Exception err) { err.printStackTrace(); } } public static void main(String argv[]) { new CommandExection("c:JavaTipsEbook.exe"); } }
Code Explanation: Here “ exec ” method of Runtime class is used, Which takes a string as an argument and instructs underlying environment to run the command. User can run any command like “ mkdir ”,“ cmd ” etc. It returns an object of Process Class which is processed using BufferedReader to show the result returned by underlying run time environment .
Runtime Class has several other methods as well. Description of some important methods are as below.
public void exit(int status): Terminates the currently running Java Virtual Machine by initiating its shutdown sequence.
public void halt(int status): Forcibly terminates the currently running Java Virtual Machine. This method never returns normally.
public int availableProcessors(): Returns the number of processors available to the Java Virtual Machine.
public long freeMemory(): Returns the amount of free memory in the Java Virtual Machine.
public long totalMemory(): Returns the total amount of memory in the Java Virtual Machine.
public long maxMemory(): Returns the maximum amount of memory that the Java Virtual Machine will attempt to use.
There are a lot of other useful methods too. For that refer Java API For Runtime Class.
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Note the documentation from Process (from Java 1.4 JavaDocs)
If you execute a sub-process that generates stderr or stdout data, it could block or deadlock if you don’t read it…
The created subprocess does not have its own terminal or console. All its standard io (i.e. stdin, stdout, stderr) operations will be redirected to the parent process through three streams (Process.getOutputStream(), Process.getInputStream(), Process.getErrorStream()). The parent process uses these streams to feed input to and get output from the subprocess. Because some native platforms only provide limited buffer size for standard input and output streams, failure to promptly write the input stream or read the output stream of the subprocess may cause the subprocess to block, and even deadlock.