CODEWITHSUNDEEP
javaprocessbuilderzombie-process
chimaira
chimaira

Reputation: 181

Linux zombies processes left unterminated with Java ProcessBuilder

I have a java Spring REST API with a controller that runs a linux command with the ProcessBuilder class

. The command is a generated 'find' command

The problem is that I found a lot of unterminated processes in the hosting server after fiew days of use. I don't know why they still there and not ended or destroyed . (I checked with a ps -ef command)

Here is my runCmd function:


public static final BufferedReader runCmd(String cmd) throws IOException, InterruptedException {
        ProcessBuilder processBuilder = new ProcessBuilder();
        processBuilder.command("bash", "-c",  cmd);

        processBuilder.redirectErrorStream(true);
        Process process = processBuilder.start();

        BufferedReader output = new BufferedReader(new InputStreamReader(process.getInputStream()));

        int ret = process.waitFor();
        return output;
}

Is there a way to make sur that there is no more process left behind ?

UPDATE

The problem comes only from commands with a very large output stream (std output) Thanks for the hint @DuncG

As this output is important, I can't ignore it. I have to find a way to consume it.

Any Idea on how to do it with Runnable Threads ?

Thanks

Upvotes: 2

Views: 887

Answers (1)

DuncG
DuncG

Reputation: 15186

Are your commands generating a lot of output? The cause of the zombies may be simply that cmd has written a lot of output the STDOUT and the stream is blocking in the BufferedReader.

You can test if this the case by adding redirect to null - just append " > /dev/null" the end of cmd. This discards the sub-process output and means the BufferedReader is not full of unread data / blocking the sub-process.

processBuilder.command("bash", "-c",  cmd + " > /dev/null");

If that fixes the zombie problem you can revert the redirect and make ProcessBuilder redirect output to files (before calling start()), or you'll need to add a thread to consume the IO as it is generated.

Path tmpdir = Path.of(System.getProperty("java.io.tmpdir"));
Path out = tmpdir.resolve("stdout.log");
processBuilder.redirectErrorStream(true);
processBuilder.redirectOutput(out.toFile());

At the end you should return the out file for caller to check, or could return Files.newBufferedReader(out).

If you don't use the redirect to file as above, this will store using thread to capture the output into memory buffer. Note you'd need to duplicate for STDERR too if not redirecting ERR->OUT:

Process p = pb.start();
ByteArrayOutputStream stdout = new ByteArrayOutputStream(8192);
new Thread(() -> copy(p.getInputStream(), stdout), "STDOUT").start();
int rc = p.waitFor();
byte[] sour = stdout.toByteArray()

Using method:

private static void copy(InputStream in, OutputStream buf)
{
    try(var autoClose = in; var autoClose2 = buf)
    {
        in.transferTo(buf);
    }
    catch(IOException io)
    {
        throw new UncheckedIOException(io);
    }
}

Upvotes: 4

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