Enable prefix by default on RedisCacheManager

An overhaul of the `RedisCacheManager` is expected in Hopper (to be
consumed by Spring Boot 1.4). One of those changes is to make sure every
key have a decent prefix by default.

This commit enables the use of prefix as it is disabled by default.

Closes gh-5175

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/RedisCacheConfiguration.java

@@ -52,6 +52,7 @@ class RedisCacheConfiguration {
 	@Bean
 	public RedisCacheManager cacheManager(RedisTemplate<Object, Object> redisTemplate) {
 		RedisCacheManager cacheManager = new RedisCacheManager(redisTemplate);
+		cacheManager.setUsePrefix(true);
 		List<String> cacheNames = this.cacheProperties.getCacheNames();
 		if (!cacheNames.isEmpty()) {
 			cacheManager.setCacheNames(cacheNames);

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/cache/CacheAutoConfigurationTests.java

@@ -200,6 +200,8 @@ public class CacheAutoConfigurationTests {
 		load(RedisCacheConfiguration.class, "spring.cache.type=redis");
 		RedisCacheManager cacheManager = validateCacheManager(RedisCacheManager.class);
 		assertThat(cacheManager.getCacheNames()).isEmpty();
+		assertThat((Boolean) new DirectFieldAccessor(cacheManager)
+				.getPropertyValue("usePrefix")).isTrue();
 	}
 
 	@Test

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -3374,6 +3374,12 @@ If Redis is available and configured, the `RedisCacheManager` is auto-configured
 also possible to create additional caches on startup using the `spring.cache.cache-names`
 property.
 
+[NOTE]
+====
+By default, a key prefix is added to prevent that if two separate caches use the same
+key, Redis would have overlapping keys and be likely to return invalid values. We strongly
+recommend to keep this setting enabled if you create your own `RedisCacheManager`.
+====
 
 
 [[boot-features-caching-provider-guava]]