How to find an available port?

I want to start a server which listen to a port. I can specify port explicitly and it works. But I would like to find a port in an automatic way. In this respect I have two questions.

In which range of port numbers should I search for? (I used ports 12345, 12346, and 12347 and it was fine).

How can I find out if a given port is not occupied by another software?

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If you don't mind the port used, specify a port of 0 to the ServerSocket constructor and it will listen on any free port.

ServerSocket s = new ServerSocket(0);
System.out.println("listening on port: " + s.getLocalPort());

If you want to use a specific set of ports, then the easiest way is probably to iterate through them until one works. Something like this:

public ServerSocket create(int[] ports) throws IOException {
    for (int port : ports) {
        try {
            return new ServerSocket(port);
        } catch (IOException ex) {
            continue; // try next port
        }
    }

    // if the program gets here, no port in the range was found
    throw new IOException("no free port found");
}

Could be used like so:

try {
    ServerSocket s = create(new int[] { 3843, 4584, 4843 });
    System.out.println("listening on port: " + s.getLocalPort());
} catch (IOException ex) {
    System.err.println("no available ports");
}

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如果您将0作为端口号传递给ServerSocket的构造函数，它将为您分配一个端口。

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Starting from Java 1.7 you can use try-with-resources like this:

  private Integer findRandomOpenPortOnAllLocalInterfaces() throws IOException {
    try (
        ServerSocket socket = new ServerSocket(0);
    ) {
      return socket.getLocalPort();

    }
  }

If you need to find an open port on a specific interface check ServerSocket documentation for alternative constructors.

Warning: Any code using the port number returned by this method is subject to a race condition - a different process / thread may bind to the same port immediately after we close the ServerSocket instance.

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