Java hangs when trying to close a ProcessBuilder OutputStream

Question

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 who

Answer 1

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 ).

Answer 2

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.

Answer 3

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.