Use optimistic locking where possible in ResponseBodyEmitter

Closes gh-33831

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitter.java

@@ -21,6 +21,7 @@ import java.util.ArrayList;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Consumer;
 
 import org.springframework.http.MediaType;
@@ -76,7 +77,7 @@ public class ResponseBodyEmitter {
 	private final Set<DataWithMediaType> earlySendAttempts = new LinkedHashSet<>(8);
 
 	/** Store successful completion before the handler is initialized. */
-	private boolean complete;
+	private final AtomicBoolean complete = new AtomicBoolean();
 
 	/** Store an error before the handler is initialized. */
 	@Nullable
@@ -127,7 +128,7 @@ public class ResponseBodyEmitter {
 			this.earlySendAttempts.clear();
 		}
 
-		if (this.complete) {
+		if (this.complete.get()) {
 			if (this.failure != null) {
 				this.handler.completeWithError(this.failure);
 			}
@@ -142,11 +143,12 @@ public class ResponseBodyEmitter {
 		}
 	}
 
-	synchronized void initializeWithError(Throwable ex) {
-		this.complete = true;
-		this.failure = ex;
-		this.earlySendAttempts.clear();
-		this.errorCallback.accept(ex);
+	void initializeWithError(Throwable ex) {
+		if (this.complete.compareAndSet(false, true)) {
+			this.failure = ex;
+			this.earlySendAttempts.clear();
+			this.errorCallback.accept(ex);
+		}
 	}
 
 	/**
@@ -184,7 +186,7 @@ public class ResponseBodyEmitter {
 	 * @throws java.lang.IllegalStateException wraps any other errors
 	 */
 	public synchronized void send(Object object, @Nullable MediaType mediaType) throws IOException {
-		Assert.state(!this.complete, () -> "ResponseBodyEmitter has already completed" +
+		Assert.state(!this.complete.get(), () -> "ResponseBodyEmitter has already completed" +
 				(this.failure != null ? " with error: " + this.failure : ""));
 		if (this.handler != null) {
 			try {
@@ -212,7 +214,7 @@ public class ResponseBodyEmitter {
 	 * @since 6.0.12
 	 */
 	public synchronized void send(Set<DataWithMediaType> items) throws IOException {
-		Assert.state(!this.complete, () -> "ResponseBodyEmitter has already completed" +
+		Assert.state(!this.complete.get(), () -> "ResponseBodyEmitter has already completed" +
 				(this.failure != null ? " with error: " + this.failure : ""));
 		sendInternal(items);
 	}
@@ -245,9 +247,8 @@ public class ResponseBodyEmitter {
 	 * to complete request processing. It should not be used after container
 	 * related events such as an error while {@link #send(Object) sending}.
 	 */
-	public synchronized void complete() {
-		this.complete = true;
-		if (this.handler != null) {
+	public void complete() {
+		if (this.complete.compareAndSet(false, true) && this.handler != null) {
 			this.handler.complete();
 		}
 	}
@@ -263,11 +264,12 @@ public class ResponseBodyEmitter {
 	 * container related events such as an error while
 	 * {@link #send(Object) sending}.
 	 */
-	public synchronized void completeWithError(Throwable ex) {
-		this.complete = true;
-		this.failure = ex;
-		if (this.handler != null) {
-			this.handler.completeWithError(ex);
+	public void completeWithError(Throwable ex) {
+		if (this.complete.compareAndSet(false, true)) {
+			this.failure = ex;
+			if (this.handler != null) {
+				this.handler.completeWithError(ex);
+			}
 		}
 	}
 
@@ -276,7 +278,7 @@ public class ResponseBodyEmitter {
 	 * called from a container thread when an async request times out.
 	 * <p>As of 6.2, one can register multiple callbacks for this event.
 	 */
-	public synchronized void onTimeout(Runnable callback) {
+	public void onTimeout(Runnable callback) {
 		this.timeoutCallback.addDelegate(callback);
 	}
 
@@ -287,7 +289,7 @@ public class ResponseBodyEmitter {
 	 * <p>As of 6.2, one can register multiple callbacks for this event.
 	 * @since 5.0
 	 */
-	public synchronized void onError(Consumer<Throwable> callback) {
+	public void onError(Consumer<Throwable> callback) {
 		this.errorCallback.addDelegate(callback);
 	}
 
@@ -298,7 +300,7 @@ public class ResponseBodyEmitter {
 	 * detecting that a {@code ResponseBodyEmitter} instance is no longer usable.
 	 * <p>As of 6.2, one can register multiple callbacks for this event.
 	 */
-	public synchronized void onCompletion(Runnable callback) {
+	public void onCompletion(Runnable callback) {
 		this.completionCallback.addDelegate(callback);
 	}
 
@@ -369,15 +371,15 @@ public class ResponseBodyEmitter {
 
 	private class DefaultCallback implements Runnable {
 
-		private List<Runnable> delegates = new ArrayList<>(1);
+		private final List<Runnable> delegates = new ArrayList<>(1);
 
-		public void addDelegate(Runnable delegate) {
+		public synchronized void addDelegate(Runnable delegate) {
 			this.delegates.add(delegate);
 		}
 
 		@Override
 		public void run() {
-			ResponseBodyEmitter.this.complete = true;
+			ResponseBodyEmitter.this.complete.compareAndSet(false, true);
 			for (Runnable delegate : this.delegates) {
 				delegate.run();
 			}
@@ -387,15 +389,15 @@ public class ResponseBodyEmitter {
 
 	private class ErrorCallback implements Consumer<Throwable> {
 
-		private List<Consumer<Throwable>> delegates = new ArrayList<>(1);
+		private final List<Consumer<Throwable>> delegates = new ArrayList<>(1);
 
-		public void addDelegate(Consumer<Throwable> callback) {
+		public synchronized void addDelegate(Consumer<Throwable> callback) {
 			this.delegates.add(callback);
 		}
 
 		@Override
 		public void accept(Throwable t) {
-			ResponseBodyEmitter.this.complete = true;
+			ResponseBodyEmitter.this.complete.compareAndSet(false, true);
 			for(Consumer<Throwable> delegate : this.delegates) {
 				delegate.accept(t);
 			}