I’m trying to expose a separate REST API in my Spring Boot application, bound to a different port from the default server and management ports. To achieve this, I created a new TomcatServletWebServerFactory and bound the new port to the container. I also set up a new GenericWebApplicationContext and linked my REST controller to this context.

Additionally, I transferred the default parent context to the new context, so that the beans and configurations are shared between them.

However, when I attempt to launch the new child context, I encounter a PortInUseException. Despite this error, the application itself continues running, but the new port doesn’t open as expected.

Here’s an outline of my approach:

• Created a new TomcatServletWebServerFactory.
• Set a custom port for the new container.
• Created a new GenericWebApplicationContext.
• Transferred the parent context for bean access.
• Connected the REST controller to the new context.

Issue: The new context throws a PortInUseException, and the separate port fails to start, even though the main service continues functioning normally. How can I resolve this issue and get the new port working?

Any guidance or insights would be greatly appreciated. Thank you!

Tomcat Configuration Class:

@Slf4j
@Configuration
public class TopologyServerConfig {

    private final ApplicationContext defaultCtx;

    @Value("${server.topology.port:8099}")
    private int topologyServerPort;

    @Value("${spring.lifecycle.timeout-per-shutdown-phase:3000}")
    private long timeOutPerShutdownPhase;

    public TopologyServerConfig(ApplicationContext defaultCtx) {
        this.defaultCtx = defaultCtx;
    }

    @Bean
    public ServletWebServerFactory topologyServletContainer() {
        TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
        factory.setPort(topologyServerPort);  // Set port to 8099
        return factory;
    }

    @EventListener(ApplicationReadyEvent.class)
    public void startTopologyCtxAfterDefaultCtx() {
        log.info("Default context is fully started. Now starting topology context on port {}...", topologyServerPort);

        // Create a new context for the topology server
        GenericWebApplicationContext topologyCtx = new GenericWebApplicationContext();
        topologyCtx.setParent(defaultCtx);
        topologyCtx.registerBean(StreamTopologyDisplayController.class);
        topologyCtx.registerBean(ServletWebServerFactory.class, this::topologyServletContainer);
        topologyCtx.registerShutdownHook();

        try {
            ServletWebServerFactory topologyFactory = topologyServletContainer();
            WebServer topologyServer = topologyFactory.getWebServer(servletContext -> {
                topologyCtx.setServletContext(servletContext);
                topologyCtx.refresh();
            });

            topologyServer.start(); // <- exception happening form this line
            log.info("Topology web server started at port {}", topologyFactory.getWebServer().getPort());
        } catch (Exception e) {
            log.error("Failed to start topology web server on port " + topologyServerPort, e);
            System.exit(1);
        }
    }
}

REST Controller class:

@Slf4j
@RestController
@RequestMapping("/topology")
public class StreamTopologyDisplayController {

    @GetMapping
    public String getStreamTopology() {
        log.info("hit the getStreamTopology");
        if (Constant.STREAM_TOPOLOGY_DESCRIPTION.isEmpty()) {
            return "Not Found";
        }

        return Constant.STREAM_TOPOLOGY_DESCRIPTION;
    }
}

Exception:

org.springframework.boot.web.server.PortInUseException: Port 8097 is already in use

Issue: The new context throws a PortInUseException, and the separate port fails to start, even though the main service continues functioning normally. How can I resolve this issue and get the new port working?

Any guidance or insights would be greatly appreciated. Thank you!