Sunday, October 07, 2012

Java: Find an Available Port Number

In some cases, such as in unit tests, you might need to start up a server or an rmiregistry. What port number do you use? You cannot hardcode the port number because when your unit test runs on a continuous build server or on a colleague's machine, it might already be in use. Instead, you need a way to find an available port on the current machine.

According to IANA (Internet Assigned Numbers Authority), the ports that we are free to use lie in the range 1024-49151:

Port numbers are assigned in various ways, based on three ranges: System Ports (0-1023), User Ports (1024-49151), and the Dynamic and/or Private Ports (49152-65535)
The following utility class can help find an available port on your local machine:
import java.io.IOException;
import java.net.DatagramSocket;
import java.net.ServerSocket;
 
/**
 * Finds an available port on localhost.
 */
public class PortFinder {
 
  // the ports below 1024 are system ports
  private static final int MIN_PORT_NUMBER = 1024;
 
  // the ports above 49151 are dynamic and/or private
  private static final int MAX_PORT_NUMBER = 49151;
 
  /**
   * Finds a free port between 
   * {@link #MIN_PORT_NUMBER} and {@link #MAX_PORT_NUMBER}.
   *
   * @return a free port
   * @throw RuntimeException if a port could not be found
   */
  public static int findFreePort() {
    for (int i = MIN_PORT_NUMBER; i <= MAX_PORT_NUMBER; i++) {
      if (available(i)) {
        return i;
      }
    }
    throw new RuntimeException("Could not find an available port between " + 
                               MIN_PORT_NUMBER + " and " + MAX_PORT_NUMBER);
  }
 
  /**
   * Returns true if the specified port is available on this host.
   *
   * @param port the port to check
   * @return true if the port is available, false otherwise
   */
  private static boolean available(final int port) {
    ServerSocket serverSocket = null;
    DatagramSocket dataSocket = null;
    try {
      serverSocket = new ServerSocket(port);
      serverSocket.setReuseAddress(true);
      dataSocket = new DatagramSocket(port);
      dataSocket.setReuseAddress(true);
      return true;
    } catch (final IOException e) {
      return false;
    } finally {
      if (dataSocket != null) {
        dataSocket.close();
      }
      if (serverSocket != null) {
        try {
          serverSocket.close();
        } catch (final IOException e) {
          // can never happen
        }
      }
    }
  }
}

3 comments:

  1. am I free to use this as is in my uni assignment?

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  2. I am not sure how it was back in 2012 (when you wrote this article), but at least today it is possible to just use new ServerSocket(0) or DatagramSocket(0). After the system created the socket, you could just ask it with getLocalPort() which port it got assigned.
    The solution posted here smells like race condition.
    I am commenting here only because I had the same question a few days ago. Anybody who find this post should better learn how to do it right.

    ReplyDelete