fix: Avoid HttpMessageNotWritableException when an exception happens in an SSE stream

Fixes #126

Author: wimdeblauwe

src/main/java/io/github/wimdeblauwe/errorhandlingspringbootstarter/servlet/ErrorHandlingControllerAdvice.java

@@ -5,6 +5,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.ControllerAdvice;
 import org.springframework.web.bind.annotation.ExceptionHandler;
@@ -33,6 +34,7 @@ public ResponseEntity<?> handleException(Throwable exception, WebRequest webRequ
         ApiErrorResponse errorResponse = errorHandlingFacade.handle(exception);
 
         return ResponseEntity.status(Objects.requireNonNull(errorResponse.getHttpStatus()))
+                             .contentType(MediaType.APPLICATION_JSON)
                              .body(errorResponse);
     }
 }

src/test/java/io/github/wimdeblauwe/errorhandlingspringbootstarter/IntegrationTest.java

@@ -2,22 +2,25 @@
 
 import io.github.wimdeblauwe.errorhandlingspringbootstarter.exception.MyCustomHttpResponseStatusException;
 import io.github.wimdeblauwe.errorhandlingspringbootstarter.mapper.HttpResponseStatusFromExceptionMapper;
+import org.hamcrest.Matchers;
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.webmvc.test.autoconfigure.WebMvcTest;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Import;
 import org.springframework.http.HttpStatusCode;
+import org.springframework.http.MediaType;
 import org.springframework.security.config.annotation.web.builders.HttpSecurity;
 import org.springframework.security.web.SecurityFilterChain;
 import org.springframework.test.web.servlet.MockMvc;
+import org.springframework.test.web.servlet.MvcResult;
 
 import java.time.Instant;
 
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.asyncDispatch;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
 import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print;
-import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
-import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
 
 @WebMvcTest(value = IntegrationTestRestController.class,
         properties = {"spring.main.allow-bean-definition-overriding=true",
@@ -58,6 +61,18 @@ void testExceptionWithCustomStatus() throws Exception {
         ;
     }
 
+    @Test
+    void testSse() throws Exception {
+        MvcResult result = mockMvc.perform(get("/integration-test/sse")
+                                                      .accept(MediaType.TEXT_EVENT_STREAM))
+                                     .andExpect(request().asyncStarted())
+                                     .andReturn();
+
+        mockMvc.perform(asyncDispatch(result))
+               .andExpect(content().contentType(MediaType.APPLICATION_JSON))
+               .andExpect(content().string(Matchers.containsString("Simulated SSE error")));
+    }
+
     static class WebSecurityConfig {
         @Bean
         public SecurityFilterChain securityFilterChain(HttpSecurity http) {

src/test/java/io/github/wimdeblauwe/errorhandlingspringbootstarter/IntegrationTestRestController.java

@@ -6,6 +6,9 @@
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;
+
+import java.io.IOException;
 
 @RestController
 @RequestMapping("/integration-test")
@@ -25,4 +28,22 @@ void throwExceptionWithBadRequestStatus() {
     void throwMyCustomHttpResponseStatusException() {
         throw new MyCustomHttpResponseStatusException(HttpStatus.I_AM_A_TEAPOT);
     }
+
+    @GetMapping(value = "sse", produces = "text/event-stream")
+    SseEmitter sseEndpoint() throws IOException {
+        SseEmitter emitter = new SseEmitter();
+        emitter.send("test");
+
+        // Simulate error during stream processing
+        new Thread(() -> {
+            try {
+                Thread.sleep(10);
+                emitter.completeWithError(new RuntimeException("Simulated SSE error"));
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+            }
+        }).start();
+
+        return emitter;
+    }
 }