Runtime.exec().waitFor() not actually waiting for

Question

I've got some code that uses Runtime.exec() to run an external .jar (built as an IzPack installer).

If I run this external.jar from the command line like so:

java -jar external.jar

Then the command prompt does not return control until the application is finished. However, if I run external.jar from within some java class, using:

Process p = Runtime.getRuntime().exec("java -jar external.jar");
int exitCode = p.waitFor();
System.out.println("Process p returned: " + exitCode);

Then p returns almost instantly with a success code of 0, despite external.jar having not yet completed execution (i've also tried this via the ProcessBuilder route of external file execution).

Why does it wait to return from the command line, but not when executed from within another java program?

I've also set up 3 jars, A, B and C where A calls B which calls C (using Runtime.exec()), where C Thread.sleeps for 10 seconds, as a simple test, and as expected, A doesn't return until 10 seconds after it runs.

I figure this is probably some kind of a threading issue with external.jar where execution is being handed over from one thing to another, but given that it works directly from the command line i kind of expected to see the same behaviour (perhaps naively) when called from within another java program.

I've tested this on Windows and Ubuntu with Java 6.

Thanks!

Answer 1

another possible way to achieve this might be to capture the output of the process and wait for it to finish.

For example:

Process tr = Runtime.getRuntime().exec( new String[]{"wkhtmltopdf",mainPage,mainPagePDF});
BufferedReader stdOut=new BufferedReader(new InputStreamReader(tr.getInputStream()));
String s;
while((s=stdOut.readLine())!=null){
       //nothing or print
}

Normally the output stream is tr.getInputStream() but depending on the program you are executing the process output stream migh be:

• tr.getInputStream()
• tr.getErrorStream()
• tr.getOutputStream()

By doing this while loop you force your program to wait the process to finish.