Implement JCacheCache#putIfAbsent as atomic operation

This commit modifies putIfAbsent to use an EntryProcessor that
guarantees that the operation is atomic.

Closes gh-21591

spring-context-support/src/main/java/org/springframework/cache/jcache/JCacheCache.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 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.
@@ -98,8 +98,8 @@ public void put(Object key, @Nullable Object value) {
 	@Override
 	@Nullable
 	public ValueWrapper putIfAbsent(Object key, @Nullable Object value) {
-		boolean set = this.cache.putIfAbsent(key, toStoreValue(value));
-		return (set ? null : get(key));
+		Object previous = this.cache.invoke(key, PutIfAbsentEntryProcessor.INSTANCE, toStoreValue(value));
+		return (previous != null ? toValueWrapper(previous) : null);
 	}
 
 	@Override
@@ -125,6 +125,22 @@ public boolean invalidate() {
 	}
 
 
+	private static class PutIfAbsentEntryProcessor implements EntryProcessor<Object, Object, Object> {
+
+		private static final PutIfAbsentEntryProcessor INSTANCE = new PutIfAbsentEntryProcessor();
+
+		@Override
+		@Nullable
+		public Object process(MutableEntry<Object, Object> entry, Object... arguments) throws EntryProcessorException {
+			Object existingValue = entry.getValue();
+			if (existingValue == null) {
+				entry.setValue(arguments[0]);
+			}
+			return existingValue;
+		}
+	}
+
+
 	private class ValueLoaderEntryProcessor<T> implements EntryProcessor<Object, Object, T> {
 
 		@SuppressWarnings("unchecked")

spring-context-support/src/test/java/org/springframework/cache/jcache/JCacheEhCacheApiTests.java

@@ -24,9 +24,12 @@
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.context.testfixture.cache.AbstractValueAdaptingCacheTests;
 
+import static org.assertj.core.api.Assertions.assertThat;
+
 /**
  * @author Stephane Nicoll
  */
@@ -79,4 +82,22 @@ protected Object getNativeCache() {
 		return this.nativeCache;
 	}
 
+	@Test
+	void testPutIfAbsentNullValue() {
+		JCacheCache cache = getCache(true);
+
+		String key = createRandomKey();
+		String value = null;
+
+		assertThat(cache.get(key)).isNull();
+		assertThat(cache.putIfAbsent(key, value)).isNull();
+		assertThat(cache.get(key).get()).isEqualTo(value);
+		org.springframework.cache.Cache.ValueWrapper wrapper = cache.putIfAbsent(key, "anotherValue");
+		// A value is set but is 'null'
+		assertThat(wrapper).isNotNull();
+		assertThat(wrapper.get()).isNull();
+		// not changed
+		assertThat(cache.get(key).get()).isEqualTo(value);
+	}
+
 }