Ability to view micronaut-cache value from key/parameter (#647)

* Implement new endpoint to retrieve a cache entry

* Add tests for the new endpoint

* Add documentation

cache-management/src/main/java/io/micronaut/cache/management/CachesEndpoint.java

@@ -93,6 +93,25 @@ public Mono<Map<String, Object>> getCache(@NotBlank @Selector String name) {
                 .onErrorResume((Throwable e) -> Mono.empty());
     }
 
+    /**
+     * Returns a cache entry as a {@link Mono}, given a cache and a key within the provided cache.
+     *
+     * @param name The name of the cache
+     * @param key the key within the cache to retrieve
+     * @return The cache entry as a {@link Mono}
+     */
+    @Read
+    public Mono<Map<String, Object>> getCacheEntry(@NotBlank @Selector String name, @NotBlank @Selector String key) {
+        return Mono.just(name)
+            .map(cacheManager::getCache)
+            .map(SyncCache::async)
+            .map(asyncCache -> asyncCache.get(key, Object.class))
+            .flatMap(Mono::fromFuture)
+            .flatMap(optional -> optional.map(Mono::just).orElseGet(Mono::empty))
+            .map(value -> Map.of("key", key, "value", value))
+            .onErrorResume(e -> Mono.empty());
+    }
+
     /**
      * Invalidates all the caches.
      *

cache-management/src/test/groovy/io/micronaut/cache/management/CachesEndpointSpec.groovy

@@ -22,6 +22,7 @@ import io.micronaut.context.env.Environment
 import io.micronaut.http.HttpRequest
 import io.micronaut.http.HttpStatus
 import io.micronaut.http.client.HttpClient
+import io.micronaut.http.client.exceptions.HttpClientResponseException
 import io.micronaut.runtime.server.EmbeddedServer
 import spock.lang.Specification
 
@@ -117,6 +118,22 @@ class CachesEndpointSpec extends Specification {
         result.caffeine.stats.evictionCount == 0
         result.caffeine.stats.evictionWeight == 0
 
+        when:
+        response = client.toBlocking().exchange("/caches/foo-cache/foo1", Map)
+        result = response.body()
+
+        then:
+        response.code() == HttpStatus.OK.code
+        result.key == "foo1"
+        result.value == "value1"
+
+        when:
+        client.toBlocking().exchange("/caches/foo-cache/no-such-key", Map)
+
+        then:
+        HttpClientResponseException error = thrown()
+        error.status == HttpStatus.NOT_FOUND
+
         cleanup:
         client.close()
         embeddedServer?.close()

src/main/docs/guide/endpoint.adoc

@@ -29,6 +29,14 @@ example, to access the configuration of the cache 'book-cache':
 $ curl http://localhost:8080/caches/book-cache
 ----
 
+To retrieve a specific cache entry within a single cache, include both cache name and key in your GET request.
+For example, to access the entry under key '123' in cache 'book-cache':
+
+[source,bash]
+----
+$ curl http://localhost:8080/caches/book-cache/123
+----
+
 To invalidate a specific cache entry within a single cache, send a DELETE request to the named cache URL with the desired key.
 
 NOTE: This only works for caches which have keys of type `String`.