Sockets Discover port availability using Java

Question

How do I programmatically determine the availability of a port in a given machine using Java   i e given a port number  determine whether it is already being used or not

User · Answer

It appears that as of Java 7  David Santamaria s answer doesn t work reliably any more  It looks like you can still reliably use a Socket to test the connection  however   private static boolean available int port        System out println  --------------Testing port     port       Socket s   null      try           s   new Socket  localhost   port               If the code makes it this far without an exception it means            something is using the port and has responded          System out println  --------------Port     port     is not available            return false        catch  IOException e            System out println  --------------Port     port     is available            return true        finally           if  s    null               try                   s close                  catch  IOException e                    throw new RuntimeException  You should handle this error     e

User · Answer

This is the implementation coming from the Apache camel project          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  lt  MIN PORT NUMBER    port  gt  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      They are checking the DatagramSocket as well to check if the port is avaliable in UDP and TCP   Hope this helps

User · Answer

In my case it helped to try and connect to the port - if service is already present  it would respond       try           log debug      Checking if port open by trying to connect as a client   portNumber           Socket sock   new Socket  localhost   portNumber                     sock close            log debug      Someone responding on port - seems not open   portNumber           return false        catch  Exception e                     if  e getMessage   contains  refused                  return true                log error  Troubles checking if port is open   e           throw new RuntimeException e

User · Answer

If you re not too concerned with performance  you could always try listening on a port using the ServerSocket class  If it throws an exception odds are it s being used   public static boolean isAvailable int portNr      boolean portFree    try  var ignored   new ServerSocket portNr           portFree   true      catch  IOException e          portFree   false        return portFree      EDIT  If all you re trying to do is select a free port then new ServerSocket 0  will find one for you

User · Answer

A cleanup of the answer pointed out by David Santamaria          Check to see if a port is available         param port               the port to check for availability      public static boolean isPortAvailable int port        try  var ss   new ServerSocket port   var ds   new DatagramSocket port             return true        catch  IOException e            return false            This is still subject to a race condition pointed out by user207421 in the comments to David Santamaria s answer  something could grab the port after this method closes the ServerSocket and DatagramSocket and returns

User · Answer

It appears that as of Java 7  David Santamaria s answer doesn t work reliably any more  It looks like you can still reliably use a Socket to test the connection  however   private static boolean available int port        System out println  --------------Testing port     port       Socket s   null      try           s   new Socket  localhost   port               If the code makes it this far without an exception it means            something is using the port and has responded          System out println  --------------Port     port     is not available            return false        catch  IOException e            System out println  --------------Port     port     is available            return true        finally           if  s    null               try                   s close                  catch  IOException e                    throw new RuntimeException  You should handle this error     e

User · Answer

The following solution is inspired by the SocketUtils implementation of Spring-core  Apache license    Compared to other solutions using Socket      it is pretty fast  testing 1000 TCP ports in less than a second    public static boolean isTcpPortAvailable int port        try  ServerSocket serverSocket   new ServerSocket                 setReuseAddress false  is required only on OSX              otherwise the code will not work correctly on that platform                   serverSocket setReuseAddress false           serverSocket bind new InetSocketAddress InetAddress getByName  localhost    port   1           return true        catch  Exception ex            return false

User · Answer

In my case I had to use DatagramSocket class   boolean isPortOccupied int port        DatagramSocket sock   null      try           sock   new DatagramSocket port           sock close            return false        catch  BindException ignored            return true        catch  SocketException ex            System out println ex           return true            Don t forget to import first  import java net DatagramSocket  import java net BindException  import java net SocketException

User · Answer

The try catch socket based solutions   might not yield accurate results  the socket address is  localhost  and in some cases the port could be  occupied  not by the loopback interface and at least on Windows I ve seen this test fails i e  the prot falsely declared as available    There is a cool library named SIGAR   the following code can hook you up    Sigar sigar   new Sigar    int flags   NetFlags CONN TCP   NetFlags CONN SERVER   NetFlags CONN CLIENT              NetConnection   netConnectionList   sigar getNetConnectionList flags   for  NetConnection netConnection   netConnectionList       if   netConnection getLocalPort      port           return false    return true

User · Answer

This is the implementation coming from the Apache camel project          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  lt  MIN PORT NUMBER    port  gt  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      They are checking the DatagramSocket as well to check if the port is avaliable in UDP and TCP   Hope this helps

User · Answer

I have Tried something Like this and it worked really fine with me              Socket Skt              String host    localhost               int i   8983     port no                    try                       System out println  Looking for    i                       Skt   new Socket host  i                       System out println  There is a Server on port                         i     of     host                                       catch  UnknownHostException e                        System out println  Exception occured   e                                        catch  IOException e                         System out println  port is not used

User · Answer

A cleanup of the answer pointed out by David Santamaria          Check to see if a port is available         param port               the port to check for availability      public static boolean isPortAvailable int port        try  var ss   new ServerSocket port   var ds   new DatagramSocket port             return true        catch  IOException e            return false            This is still subject to a race condition pointed out by user207421 in the comments to David Santamaria s answer  something could grab the port after this method closes the ServerSocket and DatagramSocket and returns

User · Answer

This is the implementation coming from the Apache camel project          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  lt  MIN PORT NUMBER    port  gt  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      They are checking the DatagramSocket as well to check if the port is avaliable in UDP and TCP   Hope this helps

User · Answer

The following solution is inspired by the SocketUtils implementation of Spring-core  Apache license    Compared to other solutions using Socket      it is pretty fast  testing 1000 TCP ports in less than a second    public static boolean isTcpPortAvailable int port        try  ServerSocket serverSocket   new ServerSocket                 setReuseAddress false  is required only on OSX              otherwise the code will not work correctly on that platform                   serverSocket setReuseAddress false           serverSocket bind new InetSocketAddress InetAddress getByName  localhost    port   1           return true        catch  Exception ex            return false

User · Answer

For Java 7 you can use try-with-resource for more compact code   private static boolean available int port        try  Socket ignored   new Socket  localhost   port             return false        catch  IOException ignored            return true

User · Answer

If you re not too concerned with performance  you could always try listening on a port using the ServerSocket class  If it throws an exception odds are it s being used   public static boolean isAvailable int portNr      boolean portFree    try  var ignored   new ServerSocket portNr           portFree   true      catch  IOException e          portFree   false        return portFree      EDIT  If all you re trying to do is select a free port then new ServerSocket 0  will find one for you

User · Answer

If you re not too concerned with performance  you could always try listening on a port using the ServerSocket class  If it throws an exception odds are it s being used   public static boolean isAvailable int portNr      boolean portFree    try  var ignored   new ServerSocket portNr           portFree   true      catch  IOException e          portFree   false        return portFree      EDIT  If all you re trying to do is select a free port then new ServerSocket 0  will find one for you

User · Answer

In my case it helped to try and connect to the port - if service is already present  it would respond       try           log debug      Checking if port open by trying to connect as a client   portNumber           Socket sock   new Socket  localhost   portNumber                     sock close            log debug      Someone responding on port - seems not open   portNumber           return false        catch  Exception e                     if  e getMessage   contains  refused                  return true                log error  Troubles checking if port is open   e           throw new RuntimeException e

User · Answer

I have Tried something Like this and it worked really fine with me              Socket Skt              String host    localhost               int i   8983     port no                    try                       System out println  Looking for    i                       Skt   new Socket host  i                       System out println  There is a Server on port                         i     of     host                                       catch  UnknownHostException e                        System out println  Exception occured   e                                        catch  IOException e                         System out println  port is not used

User · Answer

For Java 7 you can use try-with-resource for more compact code   private static boolean available int port        try  Socket ignored   new Socket  localhost   port             return false        catch  IOException ignored            return true

User · Answer

If you re not too concerned with performance  you could always try listening on a port using the ServerSocket class  If it throws an exception odds are it s being used   public static boolean isAvailable int portNr      boolean portFree    try  var ignored   new ServerSocket portNr           portFree   true      catch  IOException e          portFree   false        return portFree      EDIT  If all you re trying to do is select a free port then new ServerSocket 0  will find one for you

User · Answer

In my case I had to use DatagramSocket class   boolean isPortOccupied int port        DatagramSocket sock   null      try           sock   new DatagramSocket port           sock close            return false        catch  BindException ignored            return true        catch  SocketException ex            System out println ex           return true            Don t forget to import first  import java net DatagramSocket  import java net BindException  import java net SocketException

User · Answer

The try catch socket based solutions   might not yield accurate results  the socket address is  localhost  and in some cases the port could be  occupied  not by the loopback interface and at least on Windows I ve seen this test fails i e  the prot falsely declared as available    There is a cool library named SIGAR   the following code can hook you up    Sigar sigar   new Sigar    int flags   NetFlags CONN TCP   NetFlags CONN SERVER   NetFlags CONN CLIENT              NetConnection   netConnectionList   sigar getNetConnectionList flags   for  NetConnection netConnection   netConnectionList       if   netConnection getLocalPort      port           return false    return true

User · Answer

This is the implementation coming from the Apache camel project          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  lt  MIN PORT NUMBER    port  gt  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      They are checking the DatagramSocket as well to check if the port is avaliable in UDP and TCP   Hope this helps

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