Here you can find the source of getFreePort()
public static int getFreePort() throws Exception
//package com.java2s; /*/* w w w . j a v a 2s . com*/ * 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.net.InetAddress; import java.net.InetSocketAddress; import java.net.ServerSocket; public class Main { public static int getFreePort() throws Exception { try (ServerSocket s = new ServerSocket()) { s.setReuseAddress(true); s.bind(new InetSocketAddress((InetAddress) null, 0)); return s.getLocalPort(); } } }