Here you can find the source of getFreePort()
public static int getFreePort()
//package com.java2s; /**// ww w. ja v a2 s . c o m * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ import java.io.IOException; import java.net.DatagramSocket; import java.net.ServerSocket; public class Main { public static final int MAX_PORT_NUMBER = 65535; /** * Uses Minas AvailablePortFinder to find a port, starting at 14000. * * @return a free port. */ public static int getFreePort() { int startPort = 14000; return getFreePort(startPort); } /** * Uses Minas AvailablePortFinder to find a port, starting at startPort. * * @return a free port. */ public static int getFreePort(int pStartPort) { int startPort = pStartPort; while (!available(startPort)) { startPort++; } return startPort; } /** * 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 < 1000 || 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; } }