Add redis: Boot 4.1 + Spring Data Redis 4.1, the JDK-serialisation default vs Jackson 3 serializers, @RedisHash TTL and keyspace events, @RedisListener, and Redis as the Spring cache

Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
2026-09-21 15:46:55 +00:00
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+[Home](../README.md) | Prev: [TTL](08-ttl.md) | Next: [Production checklist](10-production-checklist.md)
+
+# 9. Redis as the Spring cache
+
+The annotations (`@Cacheable`, `@CacheEvict`, keys, the self-invocation trap) are covered in
+[The Spring Cache Abstraction](https://ankurm.com/spring-cache-abstraction-cacheable-cacheevict-self-invocation-trap/) and its
+[companion project](../../caching). This chapter is only about what changes when the cache is Redis.
+
+With `spring-boot-starter-cache` and the Redis starter on the classpath and `@EnableCaching`, Boot
+picks `org.springframework.data.redis.cache.RedisCacheManager`
+([14-cache-default.txt](output/14-cache-default.txt)). [UserService](../src/main/java/com/ankurm/redis/cache/UserService.java)
+has two cached methods that differ only in whether the result is `Serializable`.
+
+## What the defaults store
+
+Keys are readable: `<cacheName>::<key>`, here `legacy::1`. The value is JDK-serialised, exactly as in
+[chapter 2](02-default-serialization.md):
+
+```
+$ redis-cli -p 6390 --no-raw GET legacy::1
+"\xac\xed\x00\x05sr\x00'com.ankurm.redis.model.SerializableUser..."
+```
+
+`spring.cache.redis.time-to-live=10m` works: `TTL legacy::1` is `600`.
+
+A result that is **not** `Serializable` is not cached, and the *method call fails*:
+
+```
+java.lang.IllegalStateException
+  message: Cannot serialize value of type com.ankurm.redis.model.User without a serializer
+method body ran 2 time(s) in total
+```
+
+The body ran (twice in total: once for the good call, once for the failing one), the write to Redis threw, and
+the caller sees the exception. A `@Cacheable` on a method returning a non-Serializable type breaks that method.
+
+## Switching to JSON
+
+Declare a `RedisCacheConfiguration` bean. [JsonCacheConfig](../src/main/java/com/ankurm/redis/config/JsonCacheConfig.java)
+(active under the `json-cache` profile) does it with the same allow-listed Jackson 3 serializer as
+[chapter 4](04-json-serializers.md) ([15-cache-json.txt](output/15-cache-json.txt)):
+
+```
+$ redis-cli -p 6390 GET users::1
+{"@class":"com.ankurm.redis.model.User","id":1,"name":"Ankur"}
+```
+
+The non-Serializable record now caches, and the method body ran once for two calls.
+
+## The property that stops applying
+
+The test sets `spring.cache.redis.time-to-live=10m` **and** declares a bean with
+`entryTtl(Duration.ofMinutes(5))`. The key's TTL is `300`. When you define your own
+`RedisCacheConfiguration`, Boot used that bean and the `spring.cache.redis.time-to-live` property had no effect
+on the TTL. Put the TTL in the bean.
+
+## Going deeper
+
+- [RedisCacheDefaultTest](../src/test/java/com/ankurm/redis/RedisCacheDefaultTest.java) and [RedisCacheJsonTest](../src/test/java/com/ankurm/redis/RedisCacheJsonTest.java)
+- [Spring Boot: Caching, Redis](https://docs.spring.io/spring-boot/reference/io/caching.html#io.caching.provider.redis)
+- [Spring Data Redis: Redis Cache](https://docs.spring.io/spring-data/redis/reference/redis/redis-cache.html)
+
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