Concurrent read/write of named pipe in Java (on windows)

Question

I'm trying to provide communication between a C# app and a Java app on windows using named pipes with the method described by v01ver in this question: How to open a Windows named pipe from Java?

I'm running into a problem on the Java side because I have a reader thread constantly waiting for input on the pipe and when I try to write to the pipe from my main thread it gets stuck forever.

final RandomAccessFile pipe;
try {
   pipe = new RandomAccessFile("\\\\.\\pipe\\mypipe", "rw");
}
catch (FileNotFoundException ex) {
   ex.printStackTrace();
   return;
}

Thread readerThread = new Thread(new Runnable() {
   @Override
   public void run() {
      String line = null;
      try {
         while (null != (line = pipe.readLine())) {
            System.out.println(line);
         }
      }
      catch (IOException ex) {
         ex.printStackTrace();
      }
   }
});
readerThread.start();

try { Thread.sleep(500); } catch (InterruptedException e) {}

try {
   System.out.println("Writing a message...");
   pipe.write("Hello there.\n".getBytes());
   System.out.println("Finished.");
}
catch (IOException ex) {
   ex.printStackTrace();
}

The output is:

Writing a message...

and then it waits forever.

How can I write to a named pipe while waiting for input in another thread?

Answer 1

This is expected behaviour of pipes. It is supposed to hang untill other process connects to the pipe and reads it.

Answer 2

I have a same problem -- communication between a C#/Python app and a Java app on windows using named pipes:

We have example of Client Code written on Java, but in line String echoResponse = pipe.readLine(); tread waits forever.

try {
    // Connect to the pipe
    RandomAccessFile pipe = new RandomAccessFile("\\\\.\\pipe\\testpipe", "rw");
    String echoText = "Hello word\n";
    // write to pipe
    pipe.write ( echoText.getBytes() );
    // read response
    String echoResponse = pipe.readLine();
    System.out.println("Response: " + echoResponse );
    pipe.close();

    } catch (Exception e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
    }

Solution of problem: I have a ServerPipe code written on Python from here Example Code - Named Pipes: and run its on Python 2.6.6

from ctypes import *

PIPE_ACCESS_DUPLEX = 0x3
PIPE_TYPE_MESSAGE = 0x4
PIPE_READMODE_MESSAGE = 0x2
PIPE_WAIT = 0
PIPE_UNLIMITED_INSTANCES = 255
BUFSIZE = 4096
NMPWAIT_USE_DEFAULT_WAIT = 0
INVALID_HANDLE_VALUE = -1
ERROR_PIPE_CONNECTED = 535

MESSAGE = "Default answer from server\0"
szPipename = "\\\\.\\pipe\\mynamedpipe"


def ReadWrite_ClientPipe_Thread(hPipe):
    chBuf = create_string_buffer(BUFSIZE)
    cbRead = c_ulong(0)
    while 1:
        fSuccess = windll.kernel32.ReadFile(hPipe, chBuf, BUFSIZE,
byref(cbRead), None)
        if ((fSuccess ==1) or (cbRead.value != 0)):
            print chBuf.value
            cbWritten = c_ulong(0)
            fSuccess = windll.kernel32.WriteFile(hPipe,
                                                 c_char_p(MESSAGE),
                                                 len(MESSAGE),
                                                 byref(cbWritten),
                                                 None
                                                )
        else:
            break
        if ( (not fSuccess) or (len(MESSAGE) != cbWritten.value)):
            print "Could not reply to the client's request from the
pipe"
            break
        else:
            print "Number of bytes written:", cbWritten.value

    windll.kernel32.FlushFileBuffers(hPipe)
    windll.kernel32.DisconnectNamedPipe(hPipe)
    windll.kernel32.CloseHandle(hPipe)
    return 0

def main():
    THREADFUNC = CFUNCTYPE(c_int, c_int)
    thread_func = THREADFUNC(ReadWrite_ClientPipe_Thread)
    while 1:
        hPipe = windll.kernel32.CreateNamedPipeA(szPipename,
                                                 PIPE_ACCESS_DUPLEX,
                                                 PIPE_TYPE_MESSAGE |
                                                 PIPE_READMODE_MESSAGE
|
                                                 PIPE_WAIT,

PIPE_UNLIMITED_INSTANCES,
                                                 BUFSIZE, BUFSIZE,

NMPWAIT_USE_DEFAULT_WAIT,
                                                 None
                                                )
        if (hPipe == INVALID_HANDLE_VALUE):
            print "Error in creating Named Pipe"
            return 0

        fConnected = windll.kernel32.ConnectNamedPipe(hPipe, None)
        if ((fConnected == 0) and (windll.kernel32.GetLastError() ==
ERROR_PIPE_CONNECTED)):
            fConnected = 1
        if (fConnected == 1):
            dwThreadId = c_ulong(0)
            hThread = windll.kernel32.CreateThread(None, 0,
thread_func, hPipe, 0, byref(dwThreadId))
            if (hThread == -1):
                print "Create Thread failed"
                return 0
            else:
                windll.kernel32.CloseHandle(hThread)
        else:
            print "Could not connect to the Named Pipe"
            windll.kernel32.CloseHandle(hPipe)
    return 0


if __name__ == "__main__":
    main()

After server have start you can use slightly modified version of the Java Client code:

try {
    // Connect to the pipe
    RandomAccessFile pipe = new RandomAccessFile("\\\\.\\pipe\\mynamedpipe", "rw");
    String echoText = "Hello world\n";
    // write to pipe
    pipe.write(echoText.getBytes());

    //String aChar;
    StringBuffer fullString = new StringBuffer();

    while(true){
        int charCode = pipe.read();
        if(charCode == 0) break;
        //aChar = new Character((char)charCode).toString();
        fullString.append((char)charCode);
    }

    System.out.println("Response: " + fullString);
    pipe.close();
}
catch (Exception e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

It works well in NetBeans 6.9.1.

Answer 3

I suppose that RandomAccessFile is not the right API here. Try a FileInputStream + FileOutputStream on the Java side. But that is only a guess, as I last used the Windows API in times when named pipes didn't yet exist.

Answer 4

Don't worry, using RandomAccessFile to access a named pipe is correct. A named pipe is a file system object. Under Linux/Unix it is also called "fifo". Those objects are readable just like a file. (and not the same as pipes used between processes which are abstracted by Java Pipe class).

However I see two problems with your program. I cannot test it currently as I would need your test server (feel free to publish). Your reader thread waits for answers from the other side (i.e. the server). It uses readLine(), I would use a different method (for debugging reading char by char might be the best).

With Java (without JNI) you cannot actually create a named pipe (server side). Opening a named pipe with the generic method used by RandomAccessFile you will get a byte-type stream which can be one-way or duplex.

BTW: JTDS (the free JDBC driver for SQL Server) can optionally use a named pipe to access SQL server, even over the network. And it is using exactly the RandomAccessFile method.

BTW2: there is a makepipe.exe test server on older MS SQL Server installation media, however I did not find a trusted source to get that file.

Answer 5

I'm not familiar with JAVA, and my C# is pretty elementary too. However I'm had a similar problem with a multithreaded C++ client that I fixed by opening the pipe for overlapped IO. Until I did this, Windows serialized reads and writes, effectively causing an unsatisfied (blocking) ReadFile to prevent completion of a subsequent WriteFile until the read was done.

See CreateFile function
FILE_FLAG_OVERLAPPED