SSE - ClientAbortException doesn't reach @ExceptionHandler

[icguy]

I have a proof of concept Spring Boot application with the following controller returning server sent events:

@RestController
class FooController {
    private val logger = LoggerFactory.getLogger(this::class.java)

    @GetMapping("/")
    fun foo(request: HttpServletRequest): SseEmitter {
        val sse = SseEmitter(-1)
        thread {
            var i = 0
            while (true) {
                Thread.sleep(1000)
                try {
                    sse.send(SseEmitter.event().data(++i).build())
                } catch (ex: Exception) {
                    logger.info("Exception caught", ex)
                    break
                }
            }
        }
        return sse
    }

    @ExceptionHandler(Exception::class)
    fun handleException(ex: Exception): ResponseEntity<*> {
        logger.error("Error in exception handler", ex)
        return ResponseEntity.status(500).body(null)
    }
}

If I run it from the command line using gradlew.bat bootRun these exceptions are thrown when a client disconnects :

2025-07-01T09:26:03.527+02:00  INFO 24452 --- [demo] [       Thread-1] com.example.demo.FooController           : Exception caught

org.springframework.web.context.request.async.AsyncRequestNotUsableException: ServletOutputStream failed to flush: java.io.IOException: An established connection was aborted by the software in your host machine
        at org.springframework.web.context.request.async.StandardServletAsyncWebRequest$LifecycleHttpServletResponse.handleIOException(StandardServletAsyncWebRequest.java:346) ~[spring-web-6.2.8.jar:6.2.8]
        at org.springframework.web.context.request.async.StandardServletAsyncWebRequest$LifecycleServletOutputStream.flush(StandardServletAsyncWebRequest.java:418) ~[spring-web-6.2.8.jar:6.2.8]
        at java.base/java.io.FilterOutputStream.flush(FilterOutputStream.java:153) ~[na:na]
        at com.fasterxml.jackson.core.json.UTF8JsonGenerator.flush(UTF8JsonGenerator.java:1205) ~[jackson-core-2.18.4.1.jar:2.18.4.1]
        at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:1063) ~[jackson-databind-2.18.4.jar:2.18.4]
        at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:485) ~[spring-web-6.2.8.jar:6.2.8]
        at org.springframework.http.converter.AbstractGenericHttpMessageConverter.writeInternal(AbstractGenericHttpMessageConverter.java:135) ~[spring-web-6.2.8.jar:6.2.8]
        at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:235) ~[spring-web-6.2.8.jar:6.2.8]
        at org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitterReturnValueHandler$DefaultSseEmitterHandler.sendInternal(ResponseBodyEmitterReturnValueHandler.java:310) ~[spring-webmvc-6.2.8.jar:6.2.8]
        at org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitterReturnValueHandler$DefaultSseEmitterHandler.send(ResponseBodyEmitterReturnValueHandler.java:297) ~[spring-webmvc-6.2.8.jar:6.2.8]
        at org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitter.sendInternal(ResponseBodyEmitter.java:226) ~[spring-webmvc-6.2.8.jar:6.2.8]
        at org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitter.send(ResponseBodyEmitter.java:217) ~[spring-webmvc-6.2.8.jar:6.2.8]
        at com.example.demo.FooController$foo$1.invoke(DemoApplication.kt:28) ~[main/:na]
        at com.example.demo.FooController$foo$1.invoke(DemoApplication.kt:23) ~[main/:na]
        at kotlin.concurrent.ThreadsKt$thread$thread$1.run(Thread.kt:30) ~[kotlin-stdlib-1.9.25.jar:1.9.25-release-852]
Caused by: org.apache.catalina.connector.ClientAbortException: java.io.IOException: An established connection was aborted by the software in your host machine
        at org.apache.catalina.connector.OutputBuffer.doFlush(OutputBuffer.java:304) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.catalina.connector.OutputBuffer.flush(OutputBuffer.java:266) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.catalina.connector.CoyoteOutputStream.flush(CoyoteOutputStream.java:133) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.springframework.web.context.request.async.StandardServletAsyncWebRequest$LifecycleServletOutputStream.flush(StandardServletAsyncWebRequest.java:415) ~[spring-web-6.2.8.jar:6.2.8]
        ... 13 common frames omitted
Caused by: java.io.IOException: An established connection was aborted by the software in your host machine
        at java.base/sun.nio.ch.SocketDispatcher.write0(Native Method) ~[na:na]
        at java.base/sun.nio.ch.SocketDispatcher.write(SocketDispatcher.java:54) ~[na:na]
        at java.base/sun.nio.ch.IOUtil.writeFromNativeBuffer(IOUtil.java:132) ~[na:na]
        at java.base/sun.nio.ch.IOUtil.write(IOUtil.java:97) ~[na:na]
        at java.base/sun.nio.ch.IOUtil.write(IOUtil.java:53) ~[na:na]
        at java.base/sun.nio.ch.SocketChannelImpl.write(SocketChannelImpl.java:532) ~[na:na]
        at org.apache.tomcat.util.net.NioChannel.write(NioChannel.java:125) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.tomcat.util.net.NioEndpoint$NioSocketWrapper.doWrite(NioEndpoint.java:1411) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.tomcat.util.net.SocketWrapperBase.doWrite(SocketWrapperBase.java:732) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.tomcat.util.net.SocketWrapperBase.flushBlocking(SocketWrapperBase.java:698) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.tomcat.util.net.SocketWrapperBase.flush(SocketWrapperBase.java:683) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.http11.Http11OutputBuffer$SocketOutputBuffer.flush(Http11OutputBuffer.java:574) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.http11.filters.ChunkedOutputFilter.flush(ChunkedOutputFilter.java:156) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.http11.Http11OutputBuffer.flush(Http11OutputBuffer.java:216) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.http11.Http11Processor.flush(Http11Processor.java:1271) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.AbstractProcessor.action(AbstractProcessor.java:408) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.Response.action(Response.java:208) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.catalina.connector.OutputBuffer.doFlush(OutputBuffer.java:300) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        ... 16 common frames omitted

2025-07-01T09:26:03.553+02:00 ERROR 24452 --- [demo] [nio-8080-exec-2] com.example.demo.FooController           : Error in exception handler

java.io.IOException: An established connection was aborted by the software in your host machine
        at java.base/sun.nio.ch.SocketDispatcher.write0(Native Method) ~[na:na]
        at java.base/sun.nio.ch.SocketDispatcher.write(SocketDispatcher.java:54) ~[na:na]
        at java.base/sun.nio.ch.IOUtil.writeFromNativeBuffer(IOUtil.java:132) ~[na:na]
        at java.base/sun.nio.ch.IOUtil.write(IOUtil.java:97) ~[na:na]
        at java.base/sun.nio.ch.IOUtil.write(IOUtil.java:53) ~[na:na]
        at java.base/sun.nio.ch.SocketChannelImpl.write(SocketChannelImpl.java:532) ~[na:na]
        at org.apache.tomcat.util.net.NioChannel.write(NioChannel.java:125) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.tomcat.util.net.NioEndpoint$NioSocketWrapper.doWrite(NioEndpoint.java:1411) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.tomcat.util.net.SocketWrapperBase.doWrite(SocketWrapperBase.java:732) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.tomcat.util.net.SocketWrapperBase.flushBlocking(SocketWrapperBase.java:698) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.tomcat.util.net.SocketWrapperBase.flush(SocketWrapperBase.java:683) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.http11.Http11OutputBuffer$SocketOutputBuffer.flush(Http11OutputBuffer.java:574) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.http11.filters.ChunkedOutputFilter.flush(ChunkedOutputFilter.java:156) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.http11.Http11OutputBuffer.flush(Http11OutputBuffer.java:216) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.http11.Http11Processor.flush(Http11Processor.java:1271) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.AbstractProcessor.action(AbstractProcessor.java:408) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.coyote.Response.action(Response.java:208) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.catalina.connector.OutputBuffer.doFlush(OutputBuffer.java:300) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.catalina.connector.OutputBuffer.flush(OutputBuffer.java:266) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.apache.catalina.connector.CoyoteOutputStream.flush(CoyoteOutputStream.java:133) ~[tomcat-embed-core-10.1.42.jar:10.1.42]
        at org.springframework.web.context.request.async.StandardServletAsyncWebRequest$LifecycleServletOutputStream.flush(StandardServletAsyncWebRequest.java:415) ~[spring-web-6.2.8.jar:6.2.8]
        at java.base/java.io.FilterOutputStream.flush(FilterOutputStream.java:153) ~[na:na]
        at com.fasterxml.jackson.core.json.UTF8JsonGenerator.flush(UTF8JsonGenerator.java:1205) ~[jackson-core-2.18.4.1.jar:2.18.4.1]
        at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:1063) ~[jackson-databind-2.18.4.jar:2.18.4]
        at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:485) ~[spring-web-6.2.8.jar:6.2.8]
        at org.springframework.http.converter.AbstractGenericHttpMessageConverter.writeInternal(AbstractGenericHttpMessageConverter.java:135) ~[spring-web-6.2.8.jar:6.2.8]
        at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:235) ~[spring-web-6.2.8.jar:6.2.8]
        at org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitterReturnValueHandler$DefaultSseEmitterHandler.sendInternal(ResponseBodyEmitterReturnValueHandler.java:310) ~[spring-webmvc-6.2.8.jar:6.2.8]
        at org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitterReturnValueHandler$DefaultSseEmitterHandler.send(ResponseBodyEmitterReturnValueHandler.java:297) ~[spring-webmvc-6.2.8.jar:6.2.8]
        at org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitter.sendInternal(ResponseBodyEmitter.java:226) ~[spring-webmvc-6.2.8.jar:6.2.8]
        at org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitter.send(ResponseBodyEmitter.java:217) ~[spring-webmvc-6.2.8.jar:6.2.8]
        at com.example.demo.FooController$foo$1.invoke(DemoApplication.kt:28) ~[main/:na]
        at com.example.demo.FooController$foo$1.invoke(DemoApplication.kt:23) ~[main/:na]
        at kotlin.concurrent.ThreadsKt$thread$thread$1.run(Thread.kt:30) ~[kotlin-stdlib-1.9.25.jar:1.9.25-release-852]

My problem is that in the exception handler method only the original IOException shows up which:

• makes it cumbersome to detect cases when the user disconnected because the ClientAbortException disappears
• might even be considered incorrect behavior given that the exception was handled by the caller

My question is: Is this the intended behavior? If so how can I gracefully handle the exceptions resulting from the client disconnecting?

This issue might be related: #32509

I've tried to investigate why the ClientAbortException doesn't show up in the exception handler. What I found was that in this class the original exception is stored in the request attributes and then it gets rethrown here thereby ignoring the fact that it has been handled on the caller side.

Here's the repo with my code

[rstoyanchev]

In this scenario with an async request in progress on an application thread "Thread-1", Tomcat notifies of the exception via AsyncListener.onError#onError in a Servlet container thread "nio-8080-exec-2". The framework saves the exception in a request attribute and performs an ASYNC dispatch to allow the exception to be handled by the application.

I don't know why Tomcat passes the IOException, and not the ClientAbortException, and that's not anything we can influence or change.

There is a change d910091 in main (towards 7.0) that wraps every IOException received via AsyncListener.onError#onError as AsyncRequestNotUsableException to make it easier to recognize and handle.

To do something similar before 7.0 you could register a DeferredResultProcessingInterceptor that implements handleError, wraps the exception the same way and calls DeferredResult#setErrorResult.

I'm closing the as the exception from Tomcat is not in our control, but feel free to comment.