Java- Execute Shell Command Not Working

Question

I'm making a simple Java socket program that sends text from one computer to another.

Code:

    Scanner scan =  new Scanner(System.in);
    System.out.println("Starting Server...");
    ServerSocket server = new ServerSocket(7777);

The program binds the socket to port 7777 on line 3 on the code example above. However, this program sometimes returns a BindException. To fix that, I added this line of code before the bind occurs:

Runtime.getRuntime().exec("lsof -t -i:7777 | xargs kill");

So, in all:

    Scanner scan =  new Scanner(System.in);
    System.out.println("Starting Server...");
    Runtime.getRuntime().exec("lsof -t -i:7777 | xargs kill");
    ServerSocket server = new ServerSocket(7777);

This should run the shell command to kill any processes running on port 7777. But, it doesn't. If type the same command into Terminal.app, It works, and if I use the same syntax as line 4 of the above example and use a different command, like "say hello", that command works, but not the kill one.

So,

Why does that command not execute?

Thanks.

Why the downvote? Anything I can change?

xǝlɐ
– xǝlɐ

2014-07-12 18:07:52 +00:00
Commented Jul 12, 2014 at 18:07 — xǝlɐ
– xǝlɐ, Commented Jul 12, 2014 at 18:07

Sylvain Leroux · Accepted Answer · 2014-07-15 17:53:27Z

8

Runtime.exec will not launch the command through a shell, whereas this is required as you use a pipe. Try that instead:

Runtime.getRuntime().exec(
              new String[]{"sh","-c","lsof -t -i:7777 | xargs kill"},
              null, null);

I you need to wait for completion before continuing execution, use Process.waitFor:

Process p = Runtime.getRuntime().exec(
              new String[]{"sh","-c","lsof -t -i:7777 | xargs kill"},
              null, null);
int exitCode = p.waitFor()

edited Jul 15, 2014 at 17:53

answered Jul 12, 2014 at 18:16

Sylvain Leroux

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3 Comments

xǝlɐ Over a year ago

Thanks. That works great!! I also needed to add a Thread.sleep(156) below the command execution to give it time to execute. I found, on my computer, that 156ms was the minimum amount of time I could use.

Sylvain Leroux Over a year ago

@user3731821 I've edited my answer to point out Process.waitFor. Definitively more robust than empirical timing ;)

xǝlɐ Over a year ago

For some reason, that doesn't wait long enough on my computer. I bind the socket directly after I kill the processes on the port, Should I be doing something else first?

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