Interact with Powershell process called from Java application

Question

I'm trying to run a Java application which creates a new powershell process on startup and then later on interacts with it multiple times. Calling powershell.exe and have it execute a single command and return the output works fine for me. The problem arises if I don't want the powershell process to immediately finish/exit but to stay open so I can write to its outputStream and receive results back from the inputStream.

            String input = "dir";

            String[] commandList = {"powershell.exe", "-Command", "dir"}; 

            ProcessBuilder pb = new ProcessBuilder(commandList); 

            Process p = pb.start(); 

            if(input != null) { 
                PrintWriter writer = new PrintWriter(new OutputStreamWriter(new BufferedOutputStream(p.getOutputStream())), true); 
                writer.println(input); 
                writer.flush(); 
                writer.close(); 

            } 

            //p.getOutputStream().close();

            Gobbler outGobbler = new Gobbler(p.getInputStream()); 
            Gobbler errGobbler = new Gobbler(p.getErrorStream()); 
            Thread outThread = new Thread(outGobbler); 
            Thread errThread = new Thread(errGobbler); 
            outThread.start(); 
            errThread.start(); 

            System.out.println("Waiting for the Gobbler threads to join...");

            outThread.join(); 
            errThread.join(); 

            System.out.println("Waiting for the process to exit...");                                                                           

                int exitVal = p.waitFor(); 
                System.out.println("\n****************************"); 
                System.out.println("Command: " + "cmd.exe /c dir"); 
                System.out.println("Exit Value = " + exitVal); 
                List<String> output = outGobbler.getOuput(); 
                input = ""; 
                for(String o: output) { 
                        input += o; 
                } 

            System.out.println("Final Output:"); 
            System.out.println(input);

This code returns the result of the "dir" command from a powershell - fine. But as you can see, I'm trying to run a second "dir" command using

                PrintWriter writer = new PrintWriter(new OutputStreamWriter(new BufferedOutputStream(p.getOutputStream())), true); 
                writer.println(input); 
                writer.flush();

This has no effect whatsoever - no second dir output is shown when I run my code. I've also experimented with a powershell.exe option to open the powershell but not close it immediately:

String[] commandList = {"powershell.exe", "-NoExit", "-Command", "dir"};

But then my code hangs, meaning the Gobbler's who consume the process's inputStream don't read anything - strangely enough: they don't even read the first line - there must be at least some output....

I've also tried to close the process's outputStream after writing the second "dir" command to it - didn't change anything.

Any help is highly appreciated. Thanks Kurt

Andy Arismendi · Accepted Answer · 2012-01-05 09:59:15Z

0

This sounds about right for the nature of a process spun up by another process. I think you're experiencing pretty standard behavior.

This is the key: p.waitFor()

From Java docs:

causes the current thread to wait, if necessary, until the process represented by this Process object has terminated.

You won't be able to receive the PowerShell output stream until it has terminated. When you run with -NoExit it never exits which is why you are experiencing the hang.

If you run ProcExp from Sysinternals you'll be able to see your Java process spin up an child PowerShell process.

So I don't think you'll be able to interact with it like it's a live object in memory.

answered Jan 5, 2012 at 9:59

Andy Arismendi

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6 Comments

Kurt Over a year ago

Thanks Andy. Then there must be a difference between calling powershell.exe and cmd.exe. Because when I do change the command to this: String[] commandList = {"cmd.exe", "/k", "dir"}; I can actually write another "dir" command to the cmd.exe and get its feedback. So in this scenario I keep the cmd.exe running (/k switch) and can send subsequent commands to it. No way that I can do the same with powershell.exe?

Andy Arismendi Over a year ago

@Kurt When you have cmd.exe run a second command are you running it with pb.start() again? If so I think you might be spinning up another cmd.exe. To make sure watch the process tree with ProcExp to get a better look at whats happening behind the scenes.

Kurt Over a year ago

no, I use exactly the same code as above - without a second call to pb.start(). When I run the code I can see one cmd.exe and one conhost.exe opening - not two. In the output you can also see that my second "dir" command is input into the open cmd.exe:

Kurt Over a year ago

Here is the abbreviated output: Directory of C:\Users\k\Documents\Eclipse Workspace\PipeRedirection <...contents of directory...> C:\Users\k\Documents\Eclipse Workspace\PipeRedirection>dir Directory of C:\Users\k\Documents\Eclipse Workspace\PipeRedirection <...contents of directory...>

Andy Arismendi Over a year ago

One last suggestion, try calling powershell indirectly through cmd.exe: cmd.exe /k powershell.exe -command dir.

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