Cannot assign requested address using ServerSocket.socketBind

When I'm trying to set up a socket server, I've got an error message:

Exception in thread "main" java.net.BindException: Cannot assign requested address: JVM_Bind
    at java.net.PlainSocketImpl.socketBind(Native Method)
    at java.net.PlainSocketImpl.bind(PlainSocketImpl.java:383)
    at java.net.ServerSocket.bind(ServerSocket.java:328)
    at java.net.ServerSocket.<init>(ServerSocket.java:194)
    at java.net.ServerSocket.<init>(ServerSocket.java:106)
    at socketyserver.SocketyServer.main(SocketyServer.java:12)
Java Result: 1

Whole code is simplest as it can be:

public static void main(String[] args) throws UnknownHostException, IOException
{
    ServerSocket serverSocket;
    serverSocket = new ServerSocket(9999);
}

I'm 100% sure that my ports are forwarded, Windows Firewall is off. Nothing blocks port 9999. What else can go wrong?

It may be related to a misconfiguration in your /etc/hosts. In my case, it was like this: 192.168.1.11 localhost instead of 127.0.0.1 localhost

As other people have pointed out, it is most likely related to another process using port 9999. On Windows, run the command:

netstat -a -n | grep "LIST"

And it should list anything there that's hogging the port. Of course you'll then have to go and manually kill those programs in Task Manager. If this still doesn't work, replace the line:

serverSocket = new ServerSocket(9999);

With:

InetAddress locIP = InetAddress.getByName("192.168.1.20");
serverSocket = new ServerSocket(9999, 0, locIP);

Of course replace 192.168.1.20 with your actual IP address, or use 127.0.0.1.

Just for others who may look at this answer in the hope of solving a similar problem, I got a similar message because my ip address changed.

java.net.BindException: Cannot assign requested address: bind
    at sun.nio.ch.Net.bind(Native Method)
    at sun.nio.ch.ServerSocketChannelImpl.bind(ServerSocketChannelImpl.java:126)
    at sun.nio.ch.ServerSocketAdaptor.bind(ServerSocketAdaptor.java:59)
    at org.eclipse.jetty.server.nio.SelectChannelConnector.open(SelectChannelConnector.java:182)
    at org.eclipse.jetty.server.AbstractConnector.doStart(AbstractConnector.java:311)
    at org.eclipse.jetty.server.nio.SelectChannelConnector.doStart(SelectChannelConnector.java:260)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:59)
    at org.eclipse.jetty.server.Server.doStart(Server.java:273)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:59)

The error says Cannot assign requested address. This means that you need to use the correct address for one of your network interfaces or 0.0.0.0 to accept connections from all interfaces.

The other solutions about ports only work after sometimes-failing black magic (like working after some computer restarts but not others) because the port is completely irrelevant.

Java documentation for java.net.BindExcpetion,

Signals that an error occurred while attempting to bind a socket to a local address and port. Typically, the port is in use, or the requested local address could not be assigned.

Cause:

The error is due to the second condition mentioned above. When you start a server(Tomcat,Jetty etc) it listens to a port and bind a socket to an address and port. In Windows and Linux the hostname is resolved to IP address from /etc/hosts This host to IP address mapping file can be found at C:\Windows\System32\Drivers\etc\hosts. If this mapping is changed and the host name cannot be resolved to the IP address you get the error message.

Solution:

Edit the hosts file and correct the mapping for hostname and IP using admin privileges.

eg:

#127.0.0.1 localhost
192.168.52.1 localhost

Read more: java.net.BindException : cannot assign requested address.