Better document #result semantic

Commit 240f254 has introduced support for `java.util.Optional` in the
cache abstraction. If such type is present, the contained value is cached
if it is present.

This new feature slightly changed the semantic of `#result` that was
documented up till this commit as the "return value of the method
invocation". This is no longer true as `#result` for `Optional<T>`
refers to the `T` instance and not the `Optional` instance.

This commit clarifies both the javadoc and the documentation.

Issue: SPR-14587

Author: snicoll

spring-context/src/main/java/org/springframework/cache/annotation/CacheEvict.java

@@ -69,7 +69,9 @@
 	 * following meta-data:
 	 * <ul>
 	 * <li>{@code #result} for a reference to the result of the method invocation, which
-	 * can only be used if {@link #beforeInvocation()} is {@code false}.</li>
+	 * can only be used if {@link #beforeInvocation()} is {@code false}. For supported
+	 * wrappers such as {@code Optional}, {@code #result} refers to the actual object,
+	 * not the wrapper</li>
 	 * <li>{@code #root.method}, {@code #root.target}, and {@code #root.caches} for
 	 * references to the {@link java.lang.reflect.Method method}, target object, and
 	 * affected cache(s) respectively.</li>

spring-context/src/main/java/org/springframework/cache/annotation/CachePut.java

@@ -31,7 +31,9 @@
  *
  * <p>In contrast to the {@link Cacheable @Cacheable} annotation, this annotation
  * does not cause the advised method to be skipped. Rather, it always causes the
- * method to be invoked and its result to be stored in the associated cache.
+ * method to be invoked and its result to be stored in the associated cache. Note
+ * that Java8's {@code Optional} return types are automatically handled and its
+ * content is stored in the cache if present.
  *
  * <p>This annotation may be used as a <em>meta-annotation</em> to create custom
  * <em>composed annotations</em> with attribute overrides.
@@ -73,7 +75,9 @@
 	 * <p>The SpEL expression evaluates against a dedicated context that provides the
 	 * following meta-data:
 	 * <ul>
-	 * <li>{@code #result} for a reference to the result of the method invocation.</li>
+	 * <li>{@code #result} for a reference to the result of the method invocation. For
+	 * supported wrappers such as {@code Optional}, {@code #result} refers to the actual
+	 * object, not the wrapper</li>
 	 * <li>{@code #root.method}, {@code #root.target}, and {@code #root.caches} for
 	 * references to the {@link java.lang.reflect.Method method}, target object, and
 	 * affected cache(s) respectively.</li>
@@ -138,7 +142,9 @@
 	 * <p>The SpEL expression evaluates against a dedicated context that provides the
 	 * following meta-data:
 	 * <ul>
-	 * <li>{@code #result} for a reference to the result of the method invocation.</li>
+	 * <li>{@code #result} for a reference to the result of the method invocation. For
+	 * supported wrappers such as {@code Optional}, {@code #result} refers to the actual
+	 * object, not the wrapper</li>
 	 * <li>{@code #root.method}, {@code #root.target}, and {@code #root.caches} for
 	 * references to the {@link java.lang.reflect.Method method}, target object, and
 	 * affected cache(s) respectively.</li>

spring-context/src/main/java/org/springframework/cache/annotation/Cacheable.java

@@ -38,7 +38,9 @@
  * replace the default one (see {@link #keyGenerator}).
  *
  * <p>If no value is found in the cache for the computed key, the target method
- * will be invoked and the returned value stored in the associated cache.
+ * will be invoked and the returned value stored in the associated cache. Note
+ * that Java8's {@code Optional} return types are automatically handled and its
+ * content is stored in the cache if present.
  *
  * <p>This annotation may be used as a <em>meta-annotation</em> to create custom
  * <em>composed annotations</em> with attribute overrides.
@@ -144,7 +146,9 @@
 	 * <p>The SpEL expression evaluates against a dedicated context that provides the
 	 * following meta-data:
 	 * <ul>
-	 * <li>{@code #result} for a reference to the result of the method invocation.</li>
+	 * <li>{@code #result} for a reference to the result of the method invocation. For
+	 * supported wrappers such as {@code Optional}, {@code #result} refers to the actual
+	 * object, not the wrapper</li>
 	 * <li>{@code #root.method}, {@code #root.target}, and {@code #root.caches} for
 	 * references to the {@link java.lang.reflect.Method method}, target object, and
 	 * affected cache(s) respectively.</li>

src/asciidoc/integration.adoc

@@ -8459,6 +8459,18 @@ only want to cache paperback books:
 	public Book findBook(String name)
 ----
 
+The cache abstraction supports `java.util.Optional`, using its content as cached value
+only if it present. `#result` always refers to the business entity and never on a
+supported wrapper so the previous example can be rewritten as follows:
+
+[source,java,indent=0]
+[subs="verbatim,quotes"]
+----
+	@Cacheable(cacheNames="book", condition="#name.length < 32", **unless="#result.hardback"**)
+	public Optional<Book> findBook(String name)
+----
+
+Note that `result` still refers to `Book` and not `Optional`.
 
 [[cache-spel-context]]
 ===== Available caching SpEL evaluation context
@@ -8515,7 +8527,8 @@ conditional computations:
 | evaluation context
 | The result of the method call (the value to be cached). Only available in `unless`
   expressions, `cache put` expressions (to compute the `key`), or `cache evict`
-  expressions (when `beforeInvocation` is `false`).
+  expressions (when `beforeInvocation` is `false`). For supported wrappers such as
+  `Optional`, `#result` refers to the actual object, not the wrapper.
 | `#result`
 |===