How to find an available port

Question

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

User · Answer

If you are using the Spring Framework  the most straightforward way to do this is  private Integer laancNotifyPort   SocketUtils findAvailableTcpPort     You can also set an acceptable range  and it will search in this range  private Integer laancNotifyPort   SocketUtils findAvailableTcpPort 9090  10090    This is a convenience method that abstracts away the complexity but internally is similar to a lot of the other answers on this thread

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If you want to create your own server using a ServerSocket  you can just have it pick a free port for you     ServerSocket serverSocket   new ServerSocket 0     int port   serverSocket getLocalPort      Other server implementations typically have similar support  Jetty for example picks a free port unless you explicitly set it     Server server   new Server      ServerConnector connector   new ServerConnector server        don t call  connector setPort port     server addConnector connector     server start      int port   connector getLocalPort

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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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If you use Spring you may try http   docs spring io spring docs 4 0 5 RELEASE javadoc-api org springframework util SocketUtils html findAvailableTcpPort--

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If you are using Spring  the answer provided by Michail Nikolaev is the simplest and cleanest one  and IMO should be upvoted  Just for convenience purposes  I ll add an inline example using the Springframwework SocketUtils findAvailableTcpPort   method   int randomAvailablePort   SocketUtils findAvailableTcpPort     It s as easy as that  just one line     Of course  the Utils class offer many other interesting methods  I suggest having a look at the docs

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This works for me on Java 6      ServerSocket serverSocket   new ServerSocket 0       System out println  listening on port     serverSocket getLocalPort

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See ServerSocket   Creates a server socket  bound to the specified port  A port of 0 creates a socket on any free port

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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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If your server starts up  then that socket was not used    EDIT  Something like    ServerSocket s   null    try        s   new ServerSocket  0       catch  IOException ioe       for  int i   START  i  lt  END   i     try           s   new ServerSocket  i          catch  IOException ioe          At this point if s is null we are helpless if  s    null         throw new IOException         Strings format  Unable to open server in port range  d- d   START END

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If you need in range use   public int nextFreePort int from  int to        int port   randPort from  to       while  true            if  isLocalPortFree port                 return port            else               port   ThreadLocalRandom current   nextInt from  to                      private boolean isLocalPortFree int port        try           new ServerSocket port  close            return true        catch  IOException e            return false

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The Eclipse SDK contains a class SocketUtil  that does what you want  You may have a look into the git source code

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If you pass 0 as the port number to the constructor of ServerSocket  It will allocate a port for you

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According to Wikipedia  you should use ports 49152 to 65535 if you don t need a  well known  port   AFAIK the only way to determine wheter a port is in use is to try to open it

User · Answer

Using  ServerSocket  class we can identify whether given port is in use or free  ServerSocket provides a constructor that take an integer  which is port number  as  argument and initialise server socket on the port  If ServerSocket throws any IO Exception  then we can assume this port is already in use   Following snippet is used to get all available ports   for  int port   1  port  lt  65535  port               try                     ServerSocket socket   new ServerSocket port                     socket close                      availablePorts add port              catch  IOException e                    Reference link

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I have recently released a tiny library for doing just that with tests in mind  Maven dependency is    lt dependency gt           lt groupId gt me alexpanov lt  groupId gt           lt artifactId gt free-port-finder lt  artifactId gt           lt version gt 1 0 lt  version gt   lt  dependency gt    Once installed  free port numbers can be obtained via   int port   FreePortFinder findFreeLocalPort

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It may not help you much  but on my  Ubuntu  machine I have a file  etc services in which at least the ports used reserved by some of the apps are given  These are the standard ports for those apps    No guarantees that these are running  just the default ports these apps use  so you should not use them if possible    There are slightly more than 500 ports defined  about half UDP and half TCP   The files are made using information by IANA  see IANA Assigned port numbers

[java] How to find an available port?

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