recorder not running - java error

I have downloaded Gatling and run it at ubuntu machine.
however at terminal, error displayed after clicked ‘Start’.

$ ./recorder.sh
GATLING_HOME is set to /home/local/tester/Downloads/gatling-charts-highcharts-bundle-3.7.4
Exception in thread “AWT-EventQueue-0” java.security.PrivilegedActionException: java.security.PrivilegedActionException: java.net.BindException: Address already in use
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
Caused by: java.security.PrivilegedActionException: java.net.BindException: Address already in use
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745)
at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743)
… 9 more
Caused by: java.net.BindException: Address already in use
at java.base/sun.nio.ch.Net.bind0(Native Method)
at java.base/sun.nio.ch.Net.bind(Net.java:455)
at java.base/sun.nio.ch.Net.bind(Net.java:447)
at java.base/sun.nio.ch.ServerSocketChannelImpl.bind(ServerSocketChannelImpl.java:227)
at io.netty.channel.socket.nio.NioServerSocketChannel.doBind(NioServerSocketChannel.java:134)
at io.netty.channel.AbstractChannel$AbstractUnsafe.bind(AbstractChannel.java:562)
at io.netty.channel.DefaultChannelPipeline$HeadContext.bind(DefaultChannelPipeline.java:1334)
at io.netty.channel.AbstractChannelHandlerContext.invokeBind(AbstractChannelHandlerContext.java:506)
at io.netty.channel.AbstractChannelHandlerContext.bind(AbstractChannelHandlerContext.java:491)
at io.netty.channel.DefaultChannelPipeline.bind(DefaultChannelPipeline.java:973)
at io.netty.channel.AbstractChannel.bind(AbstractChannel.java:260)
at io.netty.bootstrap.AbstractBootstrap$2.run(AbstractBootstrap.java:356)
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:164)
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:469)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:503)
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986)
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.base/java.lang.Thread.run(Thread.java:829)

I changed to port 8001 and it works. any reason?

It’s pretty explicit: “Address already in use”
You already have another process binded on port 8000.

sudo ss -tulpn | grep LISTEN