Restore zero capacity support in ConcurrentLruCache

Since the rewrite of ConcurrentLruCache in Spring Framework 6.0, an
attempt to create a ConcurrentLruCache with zero capacity results in an
IllegalArgumentException even though the documentation states that zero
capacity indicates "no caching, always generating a new value".

This commit restores the ability to configure a ConcurrentLruCache with
zero capacity and introduces corresponding tests (which were first
verified against the 5.3.x branch to ensure backward compatibility).

See gh-26320
Closes gh-31317

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -81,7 +81,7 @@ public ConcurrentLruCache(int capacity, Function<K, V> generator) {
 	}
 
 	private ConcurrentLruCache(int capacity, Function<K, V> generator, int concurrencyLevel) {
-		Assert.isTrue(capacity > 0, "Capacity must be > 0");
+		Assert.isTrue(capacity >= 0, "Capacity must be >= 0");
 		this.capacity = capacity;
 		this.cache = new ConcurrentHashMap<>(16, 0.75f, concurrencyLevel);
 		this.generator = generator;
@@ -95,6 +95,9 @@ private ConcurrentLruCache(int capacity, Function<K, V> generator, int concurren
 	 * @return the cached or newly generated value
 	 */
 	public V get(K key) {
+		if (this.capacity == 0) {
+			return this.generator.apply(key);
+		}
 		final Node<K, V> node = this.cache.get(key);
 		if (node == null) {
 			V value = this.generator.apply(key);

spring-core/src/test/java/org/springframework/util/ConcurrentLruCacheTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -22,13 +22,38 @@
 
 /**
  * Tests for {@link ConcurrentLruCache}.
+ *
  * @author Juergen Hoeller
+ * @author Sam Brannen
  */
 class ConcurrentLruCacheTests {
 
 	private final ConcurrentLruCache<String, String> cache = new ConcurrentLruCache<>(2, key -> key + "value");
 
 
+	@Test
+	void zeroCapacity() {
+		ConcurrentLruCache<String, String> cache = new ConcurrentLruCache<>(0, key -> key + "value");
+
+		assertThat(cache.capacity()).isZero();
+		assertThat(cache.size()).isZero();
+
+		assertThat(cache.get("k1")).isEqualTo("k1value");
+		assertThat(cache.size()).isZero();
+		assertThat(cache.contains("k1")).isFalse();
+
+		assertThat(cache.get("k2")).isEqualTo("k2value");
+		assertThat(cache.size()).isZero();
+		assertThat(cache.contains("k1")).isFalse();
+		assertThat(cache.contains("k2")).isFalse();
+
+		assertThat(cache.get("k3")).isEqualTo("k3value");
+		assertThat(cache.size()).isZero();
+		assertThat(cache.contains("k1")).isFalse();
+		assertThat(cache.contains("k2")).isFalse();
+		assertThat(cache.contains("k3")).isFalse();
+	}
+
 	@Test
 	void getAndSize() {
 		assertThat(this.cache.capacity()).isEqualTo(2);