Reinstate RedisCacheConfiguration.getTtl().

Reintroduce and deprecate RedisCacheConfiguration.getTtl for backwards compatibility.

Closes #2628
Original Pull Request: #2597

src/main/java/org/springframework/data/redis/cache/RedisCacheConfiguration.java

@@ -326,6 +326,26 @@ public class RedisCacheConfiguration {
 		return this.valueSerializationPair;
 	}
 
+	/**
+	 * Returns a computed {@link Duration TTL expiration timeout} based on cache entry key/value
+	 * if a {@link TtlFunction} was confiugred using {@link #entryTtl(TtlFunction)}.
+	 * <p>
+	 * Otherwise, returns the user-provided, fixed {@link Duration} if {@link #entryTtl(Duration)}
+	 * was called during cache configuration.
+	 *
+	 * @return the configured {@link Duration TTL expiration}.
+	 * @deprecated use {@link #getTtlFunction()} instead.
+	 */
+	@Deprecated
+	@SuppressWarnings("all")
+	public Duration getTtl() {
+
+		TtlFunction ttlFunction = getTtlFunction();
+
+		return ttlFunction instanceof FixedDurationTtlFunction it ? it.duration()
+			: ttlFunction.getTimeToLive(null, null);
+	}
+
 	/**
 	 * Gets the {@link TtlFunction} used to compute a cache key {@literal time-to-live (TTL) expiration}.
 	 *

src/test/java/org/springframework/data/redis/cache/RedisCacheConfigurationUnitTests.java

@@ -15,9 +15,18 @@
  */
 package org.springframework.data.redis.cache;
 
-import org.junit.jupiter.api.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.isNull;
+import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
 
-import static org.assertj.core.api.Assertions.*;
+import java.time.Duration;
+
+import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.core.convert.converter.Converter;
@@ -28,6 +37,7 @@ import org.springframework.lang.Nullable;
  * Unit tests for {@link RedisCacheConfiguration}.
  *
  * @author Mark Paluch
+ * @author John Blum
  */
 class RedisCacheConfigurationUnitTests {
 
@@ -56,6 +66,42 @@ class RedisCacheConfigurationUnitTests {
 		assertThat(config.getConversionService().canConvert(DomainType.class, String.class)).isTrue();
 	}
 
+	@Test // GH-2628
+	@SuppressWarnings("deprecation")
+	void getTtlReturnsFixedDuration() {
+
+		Duration sixtySeconds = Duration.ofSeconds(60);
+
+		RedisCacheConfiguration cacheConfiguration = RedisCacheConfiguration.defaultCacheConfig()
+			.entryTtl(sixtySeconds);
+
+
+		assertThat(cacheConfiguration).isNotNull();
+		assertThat(cacheConfiguration.getTtl()).isEqualByComparingTo(sixtySeconds);
+		assertThat(cacheConfiguration.getTtl()).isEqualByComparingTo(sixtySeconds); // does not change!
+	}
+
+	@Test // GH-2628
+	@SuppressWarnings("deprecation")
+	public void getTtlCanReturnDynamicDuration() {
+
+		Duration thirtyMinutes = Duration.ofMinutes(30);
+		Duration twoHours = Duration.ofHours(2);
+
+		RedisCacheWriter.TtlFunction mockTtlFunction = mock(RedisCacheWriter.TtlFunction.class);
+
+		doReturn(thirtyMinutes).doReturn(twoHours).when(mockTtlFunction).getTimeToLive(any(), any());
+
+		RedisCacheConfiguration cacheConfiguration = RedisCacheConfiguration.defaultCacheConfig()
+				.entryTtl(mockTtlFunction);
+
+		assertThat(cacheConfiguration.getTtl()).isEqualTo(thirtyMinutes);
+		assertThat(cacheConfiguration.getTtl()).isEqualTo(twoHours);
+
+		verify(mockTtlFunction, times(2)).getTimeToLive(isNull(), isNull());
+		verifyNoMoreInteractions(mockTtlFunction);
+	}
+
 	private static class DomainType {
 
 	}