Execute a Java program from our Java program

Question

How to execute another Java program from our Java program? I used Runtime.getRuntime().exec("_") but it throws a runtime IOException. I don't know whether I have the problem with specifying the path or something else. Can anyone please help me with the code. Thanks in advance.

Regards, Arun

Answer 1

How about just calling the main from your java program?

Test.main(null);

This worked fine for me

Answer 2

Is there any reason you can't just call it directly in your Java code?

If there is a reason I've not tried it for executing a Java Program but you could try Jakarta Commons Exec works well for executing most programs.

Answer 3

You must pass the path of your executable at the exec method. Are you really trying to execute the "-" process?

Also, have a look at this for some useful tips.

Answer 4

I can't remember the exact code that I used to get this to work, but you have to pass "java.exe" (or the equivalent) as the executable, and then the class or jar to run as the parameter, with the correct working directory. So it's not as simple as just calling one method.

Answer 5

You're trying to execute "C:/". You'll want to execute something like:

"javaw.exe d:\\somejavaprogram\\program.jar"

Notice the path separators.

I'm assuming this is for an ad-hoc project, rather than something large. However, for best practice running external programs from code:

• Don't hardcode the executable location, unless you're certain it will never change
• Look up directories like %windir% using System.getenv
• Don't assume programs like javaw.exe are in the search path: check them first, or allow the user to specify a location
• Make sure you're taking spaces into account: "cmd /c start " + myProg will not work if myProg is "my program.jar".

Answer 6

I had to do this recently.
Here is how I did it, picking up only the relevant parts:

private static final String[] straJavaArgs =
{
   "?i/j2re/bin/java",
   "-ms64m",
   "-mx64m",
   "-Djava.ext.dirs=?i/lib;?i/jar/lib;?i/jar"
};

// ...

  // AppDesc appToRun;
  List<String> params = new ArrayList<String>();
  // Java exe and parameters
  params.addAll(ExpandStrings(straJavaArgs));
  // Common VM arguments
  params.addAll(Arrays.asList(AppDesc.GetCommonVMArgs()));
  // Specific VM arguments
  params.addAll(ExpandStrings(appToRun.GetVMArgs()));
  // The program to run
  params.add(appToRun.GetClass());
  // Its arguments
  params.addAll(ExpandStrings(appToRun.GetProgramArgs()));
  // The common arguments
  params.addAll(ExpandStrings(AppDesc.GetCommonProgramArgs()));

  ProcessBuilder processBuilder = new ProcessBuilder(params);
  process = processBuilder.start();
  return CaptureProcessOutput(); // Uses a StreamGobbler class

protected ArrayList<String> ExpandStrings(String[] stra)
{
  ArrayList<String> alResult = new ArrayList<String>();
  for (int i = 0; i < stra.length; i++)
  {
     // Super flexible, eh? Ad hoc for the current task, at least...
     alResult.add(stra[i]
           .replaceAll("\\?i", strInstallDir)
           .replaceAll("\\?c", strConfigDir)
     );
  }
  return alResult;
}

public enum AppDesc
{
// Enumerate the applications to run, with their parameters
}

Incomplete, if you need more details, just ask.

Answer 7

java.io.IOException: CreateProcess: c:/ error=5
        at java.lang.Win32Process.create(Native Method)
        at java.lang.Win32Process.<init>(Win32Process.java:63)
        at java.lang.Runtime.execInternal(Native Method)

If I recall correctly, error code 5 means access denied. This could be because your path is incorrect (trying to execute "c:/") or you are bumping against your OS security (in which case, look at the permissions).

If you are having trouble locating the Java executable, you can usually find it using system properties:

public class LaunchJre {

    private static boolean isWindows() {
     String os = System.getProperty("os.name");
     if (os == null) {
      throw new IllegalStateException("os.name");
     }
     os = os.toLowerCase();
     return os.startsWith("windows");
    }

    public static File getJreExecutable() throws FileNotFoundException {
     String jreDirectory = System.getProperty("java.home");
     if (jreDirectory == null) {
      throw new IllegalStateException("java.home");
     }
     File exe;
     if (isWindows()) {
      exe = new File(jreDirectory, "bin/java.exe");
     } else {
      exe = new File(jreDirectory, "bin/java");
     }
     if (!exe.isFile()) {
      throw new FileNotFoundException(exe.toString());
     }
     return exe;
    }

    public static int launch(List<String> cmdarray) throws IOException,
      InterruptedException {
     byte[] buffer = new byte[1024];

     ProcessBuilder processBuilder = new ProcessBuilder(cmdarray);
     processBuilder.redirectErrorStream(true);
     Process process = processBuilder.start();
     InputStream in = process.getInputStream();
     while (true) {
      int r = in.read(buffer);
      if (r <= 0) {
       break;
      }
      System.out.write(buffer, 0, r);
     }
     return process.waitFor();
    }

    public static void main(String[] args) {
     try {
      Runtime.getRuntime().exec("c:/");

      List<String> cmdarray = new ArrayList<String>();
      cmdarray.add(getJreExecutable().toString());
      cmdarray.add("-version");
      int retValue = launch(cmdarray);
      if (retValue != 0) {
       System.err.println("Error code " + retValue);
      }
      System.out.println("OK");
     } catch (IOException e) {
      e.printStackTrace();
     } catch (InterruptedException e) {
      e.printStackTrace();
     }
    }

}

(Tested Windows XP, Sun JRE 1.6; Ubuntu 8.04, OpenJDK JRE 1.6)

This is the equivalent of running:

java -version

You may also want to look at the "java.library.path" system property (and "path.separator") when trying to locate the executable.

Answer 8

You can either launch another JVM (as described in detail in other answers). But that is not a solution i would prefer.

Reasons are:

• calling a native program from java is "dirty" (and sometimes crashes your own VM)
• you need to know the path to the external JVM (modern JVMs don't set JAVA_HOME anymore)
• you have no control on the other program

Main reason to do it anyway is, that the other application has no control over your part of the program either. And more importantly there's no trouble with unresponsive system threads like the AWT-Thread if the other application doesn't know its threading 101.

But! You can achieve more control and similar behaviour by using an elementary plugin technique. I.e. just call "a known interface method" the other application has to implement. (in this case the "main" method).

Only it's not quite as easy as it sounds to pull this off.

• you have to dynamically include required jars at runtime (or include them in the classpath for your application)
• you have to put the plugin in a sandbox that prevents compromising critical classes to the other application

And this calls for a customized classloader. But be warned - there are some well hidden pitfalls in implementing that. On the other hand it's a great exercise.

So, take your pick: either quick and dirty or hard but rewarding.

Answer 9

I had a similiar problem. I needed to run a section of Java code in a seperate VM as it invoked native code via JNI that occasionally blew up taking out the entire VM.

I cheated a little though. I initially used Runtime to invoke a simple batch command file and put the work-in-progress java command in there. This enabled me to tweak it as needed and to run the command in a DOS prompt for easy testing. Once it was finished I simply copied the result into the Runtime invocation.

Answer 10

public class Test {    
  public static void main(String[] args) throws Exception {    
    Process p = Runtime.getRuntime().exec("\"c:/program files/windows/notepad.exe\"");
    p.waitFor();
  }

}

The above works quite well, instead of passing \"c:/program files/windows/notepad.exe\" as the arguments for the executable, use the path to your program, I'm not sure if this solution is JVM version dependent, or if it can use relative paths.

Answer 11

The code provided by jex above is dangerous. Read the following article that explains it in greater details: http://www.javaworld.com/javaworld/jw-12-2000/jw-1229-traps.html It also explains how to call external native program from your Java code properly.