Add configurable throttle policy to @ConcurrencyLimit annotation

Closes gh-36109

spring-context/src/main/java/org/springframework/resilience/InvocationRejectedException.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2002-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.resilience;
+
+import java.util.concurrent.RejectedExecutionException;
+
+/**
+ * Exception thrown when a target will not get invoked due to a resilience policy,
+ * such as the concurrency limit having been reached for a class/method annotated with
+ * {@link org.springframework.resilience.annotation.ConcurrencyLimit @ConcurrencyLimit}.
+ *
+ * @author Juergen Hoeller
+ * @since 7.0.3
+ * @see org.springframework.resilience.annotation.ConcurrencyLimit#rejectOnExcess()
+ */
+@SuppressWarnings("serial")
+public class InvocationRejectedException extends RejectedExecutionException {
+
+	private final Object target;
+
+
+	/**
+	 * Create a new {@code InvocationRejectedException}
+	 * with the specified detail message and target instance.
+	 * @param msg the detail message
+	 * @param target the target instance that was about to be invoked
+	 */
+	public InvocationRejectedException(String msg, Object target) {
+		super(msg);
+		this.target = target;
+	}
+
+
+	/**
+	 * Return the target instance that was about to be invoked.
+	 */
+	public Object getTarget() {
+		return this.target;
+	}
+
+}

spring-context/src/main/java/org/springframework/resilience/annotation/ConcurrencyLimit.java

@@ -28,7 +28,9 @@ import org.springframework.core.annotation.AliasFor;
 /**
  * A common annotation specifying a concurrency limit for an individual method,
  * or for all proxy-invoked methods in a given class hierarchy if annotated at
- * the type level.
+ * the type level. The default behavior is to block further method invocations
+ * when the limit has been reached. Alternatively, further invocations can be
+ * rejected through configuring {@link #policy()} as {@code policy = REJECT}.
  *
  * <p>In the type-level case, all methods inheriting the concurrency limit
  * from the type level share a common concurrency throttle, with any mix
@@ -95,4 +97,35 @@ public @interface ConcurrencyLimit {
 	 */
 	String limitString() default "";
 
+	/**
+	 * The policy for throttling method invocations when the limit has been reached.
+	 * <p>The default behavior is to block further concurrent invocations once the
+	 * specified limit has been reached: {@link ThrottlePolicy#BLOCK}.
+	 * <p>Switch this policy to {@code REJECT} for rejecting further invocations instead,
+	 * throwing {@link org.springframework.resilience.InvocationRejectedException}
+	 * (which extends the common {@link java.util.concurrent.RejectedExecutionException})
+	 * on any further concurrent invocation attempts: {@link ThrottlePolicy#REJECT}.
+	 * @since 7.0.3
+	 */
+	ThrottlePolicy policy() default ThrottlePolicy.BLOCK;
+
+
+	/**
+	 * Policy to apply for throttling method invocations when the limit has been reached.
+	 * @since 7.0.3
+	 */
+	enum ThrottlePolicy {
+
+		/**
+		 * The default: block until we can invoke the method within the configured limit.
+		 */
+		BLOCK,
+
+		/**
+		 * Alternative: reject further method invocations once the limit has been reached.
+		 * @see org.springframework.resilience.InvocationRejectedException
+		 */
+		REJECT
+	}
+
 }

spring-context/src/main/java/org/springframework/resilience/annotation/ConcurrencyLimitBeanPostProcessor.java

@@ -35,7 +35,9 @@ import org.springframework.aop.support.DefaultPointcutAdvisor;
 import org.springframework.aop.support.annotation.AnnotationMatchingPointcut;
 import org.springframework.context.EmbeddedValueResolverAware;
 import org.springframework.core.annotation.AnnotatedElementUtils;
+import org.springframework.resilience.InvocationRejectedException;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.util.StringValueResolver;
 
@@ -115,7 +117,11 @@ public class ConcurrencyLimitBeanPostProcessor extends AbstractBeanFactoryAwareA
 							if (concurrencyLimit < -1) {
 								throw new IllegalStateException(annotation + " must be configured with a valid limit");
 							}
-							interceptor = new ConcurrencyThrottleInterceptor(concurrencyLimit);
+							interceptor = (annotation.policy() == ConcurrencyLimit.ThrottlePolicy.REJECT ?
+									new RejectingConcurrencyThrottleInterceptor(concurrencyLimit,
+											(perMethod ? ClassUtils.getQualifiedMethodName(method) : targetClass.getName()),
+											instance) :
+									new ConcurrencyThrottleInterceptor(concurrencyLimit));
 							if (!perMethod) {
 								holder.classInterceptor = interceptor;
 							}
@@ -148,4 +154,24 @@ public class ConcurrencyLimitBeanPostProcessor extends AbstractBeanFactoryAwareA
 		@Nullable MethodInterceptor classInterceptor;
 	}
 
+
+	private static class RejectingConcurrencyThrottleInterceptor extends ConcurrencyThrottleInterceptor {
+
+		private final String identifier;
+
+		private final Object target;
+
+		public RejectingConcurrencyThrottleInterceptor(int concurrencyLimit, String identifier, Object target) {
+			super(concurrencyLimit);
+			this.identifier = identifier;
+			this.target = target;
+		}
+
+		@Override
+		protected void onLimitReached() {
+			throw new InvocationRejectedException(
+					"Concurrency limit reached for " + this.identifier + ": " + getConcurrencyLimit(), this.target);
+		}
+	}
+
 }

spring-context/src/main/java/org/springframework/resilience/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Common exceptions thrown by Spring's resilience facilities.
+ */
+@NullMarked
+package org.springframework.resilience;
+
+import org.jspecify.annotations.NullMarked;