Add rejectTasksWhenLimitReached option for concurrency limit

Closes gh-34727

Author: jhoeller

spring-core/src/main/java/org/springframework/core/task/SimpleAsyncTaskExecutor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -92,6 +92,8 @@ public class SimpleAsyncTaskExecutor extends CustomizableThreadCreator
 	@Nullable
 	private Set<Thread> activeThreads;
 
+	private boolean rejectTasksWhenLimitReached = false;
+
 	private volatile boolean active = true;
 
 
@@ -190,6 +192,17 @@ public void setTaskTerminationTimeout(long timeout) {
 		this.activeThreads = (timeout > 0 ? ConcurrentHashMap.newKeySet() : null);
 	}
 
+	/**
+	 * Specify whether to reject tasks when the concurrency limit has been reached,
+	 * throwing {@link TaskRejectedException} on any further submission attempts.
+	 * <p>The default is {@code false}, blocking the caller until the submission can
+	 * be accepted. Switch this to {@code true} for immediate rejection instead.
+	 * @since 6.2.6
+	 */
+	public void setRejectTasksWhenLimitReached(boolean rejectTasksWhenLimitReached) {
+		this.rejectTasksWhenLimitReached = rejectTasksWhenLimitReached;
+	}
+
 	/**
 	 * Set the maximum number of parallel task executions allowed.
 	 * The default of -1 indicates no concurrency limit at all.
@@ -372,13 +385,21 @@ public void close() {
 	 * making {@code beforeAccess()} and {@code afterAccess()}
 	 * visible to the surrounding class.
 	 */
-	private static class ConcurrencyThrottleAdapter extends ConcurrencyThrottleSupport {
+	private class ConcurrencyThrottleAdapter extends ConcurrencyThrottleSupport {
 
 		@Override
 		protected void beforeAccess() {
 			super.beforeAccess();
 		}
 
+		@Override
+		protected void onLimitReached() {
+			if (rejectTasksWhenLimitReached) {
+				throw new TaskRejectedException("Concurrency limit reached: " + getConcurrencyLimit());
+			}
+			super.onLimitReached();
+		}
+
 		@Override
 		protected void afterAccess() {
 			super.afterAccess();

spring-core/src/main/java/org/springframework/util/ConcurrencyThrottleSupport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -105,6 +105,7 @@ public boolean isThrottleActive() {
 	/**
 	 * To be invoked before the main execution logic of concrete subclasses.
 	 * <p>This implementation applies the concurrency throttle.
+	 * @see #onLimitReached()
 	 * @see #afterAccess()
 	 */
 	protected void beforeAccess() {
@@ -113,29 +114,12 @@ protected void beforeAccess() {
 					"Currently no invocations allowed - concurrency limit set to NO_CONCURRENCY");
 		}
 		if (this.concurrencyLimit > 0) {
-			boolean debug = logger.isDebugEnabled();
 			this.concurrencyLock.lock();
 			try {
-				boolean interrupted = false;
-				while (this.concurrencyCount >= this.concurrencyLimit) {
-					if (interrupted) {
-						throw new IllegalStateException("Thread was interrupted while waiting for invocation access, " +
-								"but concurrency limit still does not allow for entering");
-					}
-					if (debug) {
-						logger.debug("Concurrency count " + this.concurrencyCount +
-								" has reached limit " + this.concurrencyLimit + " - blocking");
-					}
-					try {
-						this.concurrencyCondition.await();
-					}
-					catch (InterruptedException ex) {
-						// Re-interrupt current thread, to allow other threads to react.
-						Thread.currentThread().interrupt();
-						interrupted = true;
-					}
+				if (this.concurrencyCount >= this.concurrencyLimit) {
+					onLimitReached();
 				}
-				if (debug) {
+				if (logger.isDebugEnabled()) {
 					logger.debug("Entering throttle at concurrency count " + this.concurrencyCount);
 				}
 				this.concurrencyCount++;
@@ -146,6 +130,33 @@ protected void beforeAccess() {
 		}
 	}
 
+	/**
+	 * Triggered by {@link #beforeAccess()} when the concurrency limit has been reached.
+	 * The default implementation blocks until the concurrency count allows for entering.
+	 * @since 6.2.6
+	 */
+	protected void onLimitReached() {
+		boolean interrupted = false;
+		while (this.concurrencyCount >= this.concurrencyLimit) {
+			if (interrupted) {
+				throw new IllegalStateException("Thread was interrupted while waiting for invocation access, " +
+						"but concurrency limit still does not allow for entering");
+			}
+			if (logger.isDebugEnabled()) {
+				logger.debug("Concurrency count " + this.concurrencyCount +
+						" has reached limit " + this.concurrencyLimit + " - blocking");
+			}
+			try {
+				this.concurrencyCondition.await();
+			}
+			catch (InterruptedException ex) {
+				// Re-interrupt current thread, to allow other threads to react.
+				Thread.currentThread().interrupt();
+				interrupted = true;
+			}
+		}
+	}
+
 	/**
 	 * To be invoked after the main execution logic of concrete subclasses.
 	 * @see #beforeAccess()

spring-core/src/test/java/org/springframework/core/task/SimpleAsyncTaskExecutorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2025 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.
@@ -21,6 +21,7 @@
 import org.springframework.util.ConcurrencyThrottleSupport;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
@@ -31,6 +32,23 @@
  */
 class SimpleAsyncTaskExecutorTests {
 
+	@Test
+	void isActiveUntilClose() {
+		SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor();
+		assertThat(executor.isActive()).isTrue();
+		assertThat(executor.isThrottleActive()).isFalse();
+		executor.close();
+		assertThat(executor.isActive()).isFalse();
+		assertThat(executor.isThrottleActive()).isFalse();
+	}
+
+	@Test
+	void throwsExceptionWhenSuppliedWithNullRunnable() {
+		try (SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor()) {
+			assertThatIllegalArgumentException().isThrownBy(() -> executor.execute(null));
+		}
+	}
+
 	@Test
 	void cannotExecuteWhenConcurrencyIsSwitchedOff() {
 		try (SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor()) {
@@ -41,35 +59,34 @@ void cannotExecuteWhenConcurrencyIsSwitchedOff() {
 	}
 
 	@Test
-	void throttleIsNotActiveByDefault() {
+	void taskRejectedWhenConcurrencyLimitReached() {
 		try (SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor()) {
-			assertThat(executor.isThrottleActive()).as("Concurrency throttle must not default to being active (on)").isFalse();
+			executor.setConcurrencyLimit(1);
+			executor.setRejectTasksWhenLimitReached(true);
+			assertThat(executor.isThrottleActive()).isTrue();
+			executor.execute(new NoOpRunnable());
+			assertThatExceptionOfType(TaskRejectedException.class).isThrownBy(() -> executor.execute(new NoOpRunnable()));
 		}
 	}
 
 	@Test
 	void threadNameGetsSetCorrectly() {
-		final String customPrefix = "chankPop#";
-		final Object monitor = new Object();
-		SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor(customPrefix);
-		ThreadNameHarvester task = new ThreadNameHarvester(monitor);
-		executeAndWait(executor, task, monitor);
-		assertThat(task.getThreadName()).startsWith(customPrefix);
+		String customPrefix = "chankPop#";
+		Object monitor = new Object();
+		try (SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor(customPrefix)) {
+			ThreadNameHarvester task = new ThreadNameHarvester(monitor);
+			executeAndWait(executor, task, monitor);
+			assertThat(task.getThreadName()).startsWith(customPrefix);
+		}
 	}
 
 	@Test
 	void threadFactoryOverridesDefaults() {
-		final Object monitor = new Object();
-		SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor(runnable -> new Thread(runnable, "test"));
-		ThreadNameHarvester task = new ThreadNameHarvester(monitor);
-		executeAndWait(executor, task, monitor);
-		assertThat(task.getThreadName()).isEqualTo("test");
-	}
-
-	@Test
-	void throwsExceptionWhenSuppliedWithNullRunnable() {
-		try (SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor()) {
-			assertThatIllegalArgumentException().isThrownBy(() -> executor.execute(null));
+		Object monitor = new Object();
+		try (SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor(runnable -> new Thread(runnable, "test"))) {
+			ThreadNameHarvester task = new ThreadNameHarvester(monitor);
+			executeAndWait(executor, task, monitor);
+			assertThat(task.getThreadName()).isEqualTo("test");
 		}
 	}
 
@@ -89,7 +106,12 @@ private static final class NoOpRunnable implements Runnable {
 
 		@Override
 		public void run() {
-			// no-op
+			try {
+				Thread.sleep(1000);
+			}
+			catch (InterruptedException ex) {
+				Thread.currentThread().interrupt();
+			}
 		}
 	}