Clarify default value of ConditionalOnMissingBean on bean methods

Closes gh-9387

Author: snicoll

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/ConditionalOnBean.java

@@ -30,6 +30,23 @@
 /**
  * {@link Conditional} that only matches when the specified bean classes and/or names are
  * already contained in the {@link BeanFactory}.
+ * When placed on a {@code @Bean} method, the bean class default to the return type of
+ * the factory method:
+ *
+ * <pre class="code">
+ * &#064;Configuration
+ * public class MyAutoConfiguration {
+ *
+ *     &#064;ConditionalOnBean
+ *     &#064;Bean
+ *     public MyService myService() {
+ *         ...
+ *     }
+ *
+ * }</pre>
+ * <p>
+ * In the sample above the condition will match if a bean of type {@code MyService} is
+ * already contained in the {@link BeanFactory}.
  * <p>
  * The condition can only match the bean definitions that have been processed by the
  * application context so far and, as such, it is strongly recommended to use this

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/ConditionalOnMissingBean.java

@@ -31,6 +31,24 @@
  * {@link Conditional} that only matches when the specified bean classes and/or names are
  * not already contained in the {@link BeanFactory}.
  * <p>
+ * When placed on a {@code @Bean} method, the bean class default to the return type of
+ * the factory method:
+ *
+ * <pre class="code">
+ * &#064;Configuration
+ * public class MyAutoConfiguration {
+ *
+ *     &#064;ConditionalOnMissingBean
+ *     &#064;Bean
+ *     public MyService myService() {
+ *         ...
+ *     }
+ *
+ * }</pre>
+ * <p>
+ * In the sample above the condition will match if no bean of type {@code MyService} is
+ * already contained in the {@link BeanFactory}.
+ * <p>
  * The condition can only match the bean definitions that have been processed by the
  * application context so far and, as such, it is strongly recommended to use this
  * condition on auto-configuration classes only. If a candidate bean may be created by

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

@@ -6231,6 +6231,24 @@ attribute to specify beans by type, or `name` to specify beans by name. The `sea
 attribute allows you to limit the `ApplicationContext` hierarchy that should be considered
 when searching for beans.
 
+When placed on a `@Bean` method, the target type defaults to the return type of the
+method, for instance:
+
+[source,java,indent=0]
+----
+	@Configuration
+	public class MyAutoConfiguration {
+
+		@Bean
+		@ConditionalOnMissingBean
+		public MyService myService() { ... }
+
+	}
+----
+
+In the example above, the `myService` bean is going to be created if no bean of type
+`MyService` is already contained in the `ApplicationContext`.
+
 TIP: You need to be very careful about the order that bean definitions are added as these
 conditions are evaluated based on what has been processed so far. For this reason,
 we recommend only using `@ConditionalOnBean` and `@ConditionalOnMissingBean` annotations