CODEWITHSUNDEEP
javaprocess
Jeff Bullard
Jeff Bullard

Reputation: 41

Java hangs when trying to close a ProcessBuilder OutputStream

I have the following Java code to start a ProcessBuilder, open an OutputStream, have the process write a string to an OutputStream, and then close the OutputStream. The whole thing hangs indefinitely when I try to close the OutputStream. This only happens on Windows, never on Mac or Linux.

Some of the related questions seem to be close to the same problem I'm having, but I haven't been able to figure out how to apply the answers to my problem, as I am a relative newbie with Java. Here is the code. You can see I have put in a lot of println statements to try to isolate the problem.

    System.out.println("GenMic trying to get the input file now");
    System.out.flush();
    OutputStream out = child.getOutputStream();
    try {
        System.out.println("GenMic getting ready to write the input file to out");
        System.out.flush();
        out.write(intext.getBytes());  // intext is a string previously created
        System.out.println("GenMic finished writing to out");
        System.out.flush();
        out.close();
        System.out.println("GenMic closed OutputStream");
        System.out.flush();
    } catch (IOException iox) {
        System.out.println("GenMic caught IOException 2");
        System.out.flush();
        String detailedMessage = iox.getMessage();
        System.out.println("Exception: " + detailedMessage);
        System.out.flush();
        throw new RuntimeException(iox);
    }

And here is the output when this chunk is executed:

GenMic trying to get the input file now

GenMic getting ready to write the input file to out

GenMic finished writing to out

Upvotes: 4

Views: 4760

Answers (3)

Alexander Pogrebnyak
Alexander Pogrebnyak

Reputation: 45616

You need to make sure that the streams returned by getInputStream() and getOutputStream() are drained on individual threads and these threads are different from the one on which you close the stream returned by getOutputStream().

Basically it is a requirement to have at least 3 threads per sub-process if you want to manipulate and examine its stdin, stdout and stderr. One of the threads, depending on your circumstances, may be your current execution thread ( the one on which you create ProcessBuilder ).

Upvotes: 4

Nathan Hughes
Nathan Hughes

Reputation: 96454

When that happened to me it was because I hadn't read everything from the stream being written to by the process.

The API docs for the java.lang.Process class say:

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 (getOutputStream(), getInputStream(), 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.

I would try calling getInputStream() on the Process instance and writing a loop to read one byte at a time until it reaches EOF. And I'd do the same thing with getErrorStream() just in case the process is writing to stderr.

Upvotes: 3

nos
nos

Reputation: 229342

Do you have anything reading from stdout/stderr of the process ?

It's quite likely the process tries to output something, but gets blocked since noone is reading the output. Meaning your out.flush() or out.close() blocks as the process can't get around to process the input since its blocked doing output.

Upvotes: 1

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