Exec calls do not appear to be returning to the program

Question

After our NT server was rebooted, all of the sudden the Java program I maintain failed to work properly. The program creates a thread and within that thread multiple exec calls are made. It appears that the program is hanging in the portion of the code that is handling the InputStream and ErrorStream of the processes created by exec():
Process process = null;
String processArgs[] = { .... };
try
java.util.List processes = new java.util.Vector();
java.io.InputStream streams =
new java.io.InputStream[jobList.length*2];
int streamCount = 0;
for (int streamLoop = 0; streamLoop < jobList.length; streamLoop++)
process = Runtime.getRuntime().exec(processArgs);
processes.add(process);
streams[streamCount++] = process.getInputStream();
streams[streamCount++] = process.getErrorStream();
int size = 0;
int remainingCount = streamCount;
byte b[] = new byte[4098];
do
for (int loop = 0; loop < streamCount; loop++)
size = streams[loop].read(b, 0, b.length);
if (size == -1)
remainingCount--;
streams[loop] = null;
} while (remainingCount > 0);
java.util.Iterator processList = processes.iterator();
while (processList != null && processList.hasNext())
process = (Process) processList.next();
process.waitfor();
catch (Exception e)
This chunk of code accepts an array of files that need to be processed simultaneously. The code should exit once all of the child processes have exited.
After the reboot of NT, this code just hangs. I look at the log of the child processes (the child process is a vendor program) and it appears to successfully exit. Running the child processes from the command line or from a DOS batch script doesn't cause anything to hang.
To debug this, I placed the code chunk:
else
System.out.println(new String(b, 0, size));
below the condition for: if (size == -1). Magically, the program no longer hung. Subsequent debugging points to the program hanging in the portion of the code that reads the Input and Error streams from the process.
Since I'm a C/Perl programmer, I'm not quite understanding why I even need to deal with the child processes output when I don't even care what's in it. And I'm completely baffled as to why this program would hang at all... especially when this problem has only cropped up since the server was rebooted. I believe it has something to do with the child processes putting out different length output streams and perhaps tweaking with the read buffer size would help, but that really doesn't explain the underlying problem.
Does anyone have a clue what may be going on given my explaination of the problem?
TIA,
Curt

Answer 1

do {
    for (int loop = 0; loop < streamCount; loop++) {
        size = streams[loop].read(b, 0, b.length);
        if (size == -1) {
            remainingCount--;
            streams[loop] = null;
} while (remainingCount > 0);Like they say in the API documentation of read(byte[], int, int), "this method blocks until input data is available, end of file is detected, or an exception is thrown." This is useful in some situations but not here. You can check how many bytes are available with the method "available()". So I suggest modifying        size = streams[loop].read(b, 0, b.length);to        int available = Math.min(streams[loop].available(), b.length);
        size = streams[loop].read(b, 0, available);Also, you need to check if streams[loop] is null before trying to read since when you start reaching the ends of the streams some references in that array will be null and you'll be getting NullPointerExceptions.. so the fragment should be        if (streams[loop] != null) {
            int available = Math.min(streams[loop].available(), b.length);
            size = streams[loop].read(b, 0, available);
        }I think you should also remember to close the streams, so change the rest to        if (size == -1) {
            remainingCount--;
            streams[loop].close();
            streams[loop] = null;
        }> Since I'm a C/Perl programmer, I'm not quite understanding
why I even need to deal with the child processes output when
I don't even care what's in it.My C experience is pretty much null and void but don't you have to do that in C too?
I don't know, maybe that's yet another quirk of the language...