Java - 检查端口是否正在使用而不连接它

Question

My situation:

I have two threads running at the same time, one thread wants to wait until a port is in use by the other thread, which will run through a certain number of steps before it reaches the point where it uses the port.

Another restraint I have to commit to is that the thread going through the steps before it reaches opening the port has no easy communication towards the other thread, as it is in JNI native c code using an API (Hopefully not applicable to the problem, its running through a big method)

Question:

Is there a way for java to be able to wait until the port is in use? I have looked at stackoverflow's solutions but they have a few problems:

1. Creating a new socket will cause the port to be in use, my other thread will not be able to access this socket.
2. Using a netstat parser removes portability from the project.

If I have overlooked a stackoverflow answer please scold me for my inability to search or something, anything would be helpful.

Answer 1

Is there a way for java to be able to wait until the port is in use?

Not in pure Java.

But assuming that the two threads are running in the same JVM, they can communicate by other means. So you could use a shared flag (eg a volatile boolean variable or an AtomicBoolean ) or a CountDownLatch or a Semaphore . You can use any of these to implement a simple where one thread (in effect) tells the other one that it is using the port.

(In fact, if you want one thread to wait for the other, a latch is the best choice.)

---

This is not the kind of thing that would be easy to do in JNI. AFAIK, the netstat program does not use normal syscalls to do this. I think it uses either the "/proc" tree or (in old implementations) direct access to kernel memory to get the information it needs. You could replicate this in a native library, but I doubt that you will find an existing library to do this. And of course, a Windows implementation would be completely different ...

Answer 2

Have you considered or explored Apache camel ? Here is the sample code from apache camel to check the port availability. Code is quite simple and clean:

/**
 * Checks to see if a specific port is available.
 *
 * @param port the port to check for availability
 */
public static boolean available(int port) {
    if (port < MIN_PORT_NUMBER || port > MAX_PORT_NUMBER) {
        throw new IllegalArgumentException("Invalid start port: " + port);
    }

    ServerSocket ss = null;
    DatagramSocket ds = null;
    try {
        ss = new ServerSocket(port);
        ss.setReuseAddress(true);
        ds = new DatagramSocket(port);
        ds.setReuseAddress(true);
        return true;
    } catch (IOException e) {
    } finally {
        if (ds != null) {
            ds.close();
        }

        if (ss != null) {
            try {
                ss.close();
            } catch (IOException e) {
                /* should not be thrown */
            }
        }
    }

    return false;
}

Code source: http://svn.apache.org/viewvc/camel/trunk/components/camel-test/src/main/java/org/apache/camel/test/AvailablePortFinder.java?view=markup#l130

Answer 3

Just try to connect to it, and sleep/retry a few times until it succeeds. There is no magic predictor of success.