Allow multiple listeners on ResponseBodyEmitter

Prior to this commit, `ResponseBodyEmitter` woud accept a single
`Runnable` callback on each of its `onTimeout`, `onError` or
`onCompletion` methods. This would limit the developers' ability to
register multiple sets of callbacks: one for managing the publication of
streaming values, another one for managing other concerns like
keep-alive signals to maintain the connection.

This commit now allows multiple calls to `onTimeout`, `onError` and
`onCompletion` and will register all callbacks accordingly.

Closes gh-33356

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitterTests.java

@@ -17,6 +17,7 @@
 package org.springframework.web.servlet.mvc.method.annotation;
 
 import java.io.IOException;
+import java.util.function.Consumer;
 
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -31,6 +32,7 @@ import static org.assertj.core.api.Assertions.assertThatIOException;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anySet;
+import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.BDDMockito.willThrow;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
@@ -41,6 +43,7 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
  *
  * @author Rossen Stoyanchev
  * @author Tomasz Nurkiewicz
+ * @author Brian Clozel
  */
 @ExtendWith(MockitoExtension.class)
 public class ResponseBodyEmitterTests {
@@ -197,6 +200,25 @@ public class ResponseBodyEmitterTests {
 		verify(runnable).run();
 	}
 
+	@Test
+	void multipleOnTimeoutCallbacks() throws Exception {
+		this.emitter.initialize(this.handler);
+
+		ArgumentCaptor<Runnable> captor = ArgumentCaptor.forClass(Runnable.class);
+		verify(this.handler).onTimeout(captor.capture());
+		verify(this.handler).onCompletion(any());
+
+		Runnable first = mock();
+		Runnable second = mock();
+		this.emitter.onTimeout(first);
+		this.emitter.onTimeout(second);
+
+		assertThat(captor.getValue()).isNotNull();
+		captor.getValue().run();
+		verify(first).run();
+		verify(second).run();
+	}
+
 	@Test
 	void onCompletionBeforeHandlerInitialized() throws Exception {
 		Runnable runnable = mock();
@@ -228,4 +250,42 @@ public class ResponseBodyEmitterTests {
 		verify(runnable).run();
 	}
 
+	@Test
+	void multipleOnCompletionCallbacks() throws Exception {
+		this.emitter.initialize(this.handler);
+
+		ArgumentCaptor<Runnable> captor = ArgumentCaptor.forClass(Runnable.class);
+		verify(this.handler).onTimeout(any());
+		verify(this.handler).onCompletion(captor.capture());
+
+		Runnable first = mock();
+		Runnable second = mock();
+		this.emitter.onCompletion(first);
+		this.emitter.onCompletion(second);
+
+		assertThat(captor.getValue()).isNotNull();
+		captor.getValue().run();
+		verify(first).run();
+		verify(second).run();
+	}
+
+	@Test
+	void multipleOnErrorCallbacks() throws Exception {
+		this.emitter.initialize(this.handler);
+
+		ArgumentCaptor<Consumer<Throwable>> captor = ArgumentCaptor.<Consumer<Throwable>, Consumer>forClass(Consumer.class);
+		verify(this.handler).onError(captor.capture());
+
+		Consumer<Throwable> first = mock();
+		Consumer<Throwable> second = mock();
+		this.emitter.onError(first);
+		this.emitter.onError(second);
+
+		assertThat(captor.getValue()).isNotNull();
+		IllegalStateException illegalStateException = new IllegalStateException();
+		captor.getValue().accept(illegalStateException);
+		verify(first).accept(eq(illegalStateException));
+		verify(second).accept(eq(illegalStateException));
+	}
+
 }