Java同步外部文件读写

Question

I would like to create a pipe for read/write data between 2 JVM process (not the same process). Im using a FIFO pipe in Unix to share the data.

I did a Reader:

/** Read a named pipe file */
public class PipeReader {

private String path;

public PipeReader (String path) {
    this.path = path;
}

public String readpipe () throws IOException { 
    String res = null;
    RandomAccessFile pipe = null;

    try {
        // Connect to the named pipe
        pipe = new RandomAccessFile (path, "r");

        // Read response from pipe
        while (true) {
           res = pipe.readLine();
           System.out.println("Read message:" + res);   
        }    
    } catch (Exception e) { 
        e.printStackTrace();
    } finally {
        pipe.close();
    }
    return res;
}

And a Writer using FileLock:

public class PipeWriter {

private String path;

public PipeWriter (String path) {
    this.path = path;
}

public String writepipe (String mm) throws IOException { 
    String res = null;
    RandomAccessFile pipe = null;
    try {
        // Connect to the named pipe
        pipe = new RandomAccessFile (path, "rw");
        FileChannel channel = pipe.getChannel();
        int i = 0;
        while (i<5) {   
            // Write request to the pipe
            FileLock lock = channel.lock();
            pipe.write(mm.getBytes());
            lock.release();
            System.out.println("PipeWriten" + mm);
            Thread.sleep(3000);
            i++;
        }



        // do something with res
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        // Close the pipe
        pipe.close();
    }


    return res;
}

The first time pipe.readLine() block this thread and wait until PipeWriter write a line in the pipe and release the lock, when the writer finishes the reader read it. This is the behavior until PipeWriter finishes (after the 5 writings). After that, pipe.readLine() don't block the thread and continues looping the reading so I get "Read message: null" a lot of times. How could I fix it? I think I am missing something in the Reader. Are there any method to synchronize a file shared by 2 process, not using FileLock? (something like semaphores but for processes, not threads).

Thanks in advance.

Answer 1

A pipe's not like a file (which is good, since you can't wait for a file) it's a connection between the reader and the writer. It's more like a telephone. When the other side hangs up, you have to hang up too. Then you can wait for the next call.

When read gets zero, that means the connection is now closed, and you should close the pipe. If you want to wait for a new connection, re-open the pipe.

Answer 2

This is what I get: In the beginning, Reader is blocked waiting in pipe.readLine(). When Writer write a line and release the locker it read it. But then the Writer finishes, the Reader don't wait in readLine() anymore, and read null all the time.