Document common use cases for @order vs @priority vs @dependsOn

Issue: SPR-16213

(cherry picked from commit 84699c8)

Author: jhoeller

spring-beans/src/main/java/org/springframework/beans/factory/annotation/Autowired.java

@@ -23,31 +23,33 @@
 import java.lang.annotation.Target;
 
 /**
- * Marks a constructor, field, setter method or config method as to be
- * autowired by Spring's dependency injection facilities.
+ * Marks a constructor, field, setter method or config method as to be autowired
+ * by Spring's dependency injection facilities.
  *
- * <p>Only one constructor (at max) of any given bean class may carry this
- * annotation, indicating the constructor to autowire when used as a Spring
- * bean. Such a constructor does not have to be public.
+ * <p>Only one constructor (at max) of any given bean class may carry this annotation,
+ * indicating the constructor to autowire when used as a Spring bean. Such a
+ * constructor does not have to be public.
  *
- * <p>Fields are injected right after construction of a bean, before any
- * config methods are invoked. Such a config field does not have to be public.
+ * <p>Fields are injected right after construction of a bean, before any config
+ * methods are invoked. Such a config field does not have to be public.
  *
- * <p>Config methods may have an arbitrary name and any number of arguments;
- * each of those arguments will be autowired with a matching bean in the
- * Spring container. Bean property setter methods are effectively just
- * a special case of such a general config method. Such config methods
- * do not have to be public.
+ * <p>Config methods may have an arbitrary name and any number of arguments; each of
+ * those arguments will be autowired with a matching bean in the Spring container.
+ * Bean property setter methods are effectively just a special case of such a general
+ * config method. Such config methods do not have to be public.
  *
- * <p>In the case of multiple argument methods, the 'required' parameter is
- * applicable for all arguments.
+ * <p>In the case of a multi-arg constructor or method, the 'required' parameter is
+ * applicable to all arguments. Individual parameters may be declared as Java-8-style
+ * {@link java.util.Optional}, overriding the base required semantics.
  *
- * <p>In case of a {@link java.util.Collection} or {@link java.util.Map}
- * dependency type, the container can autowire all beans matching the
- * declared value type. For such purposes, the map keys must be declared
- * as type String and will be resolved to the corresponding bean names.
- * Alternatively, a target bean may also be of type {@code Collection} or
- * {@code Map} itself, getting injected as such.
+ * <p>In case of a {@link java.util.Collection} or {@link java.util.Map} dependency type,
+ * the container autowires all beans matching the declared value type. For such purposes,
+ * the map keys must be declared as type String which will be resolved to the corresponding
+ * bean names. Such a container-provided collection will be ordered, taking into account
+ * {@link org.springframework.core.Ordered}/{@link org.springframework.core.annotation.Order}
+ * values of the target components, otherwise following their registration order in the
+ * container. Alternatively, a single matching target bean may also be a generally typed
+ * {@code Collection} or {@code Map} itself, getting injected as such.
  *
  * <p>Note that actual injection is performed through a
  * {@link org.springframework.beans.factory.config.BeanPostProcessor

spring-context/src/main/java/org/springframework/context/annotation/Bean.java

@@ -60,22 +60,50 @@
  *     }
  * </pre>
  *
- * <h3>Scope, DependsOn, Primary, and Lazy</h3>
+ * <h3>Profile, Scope, Lazy, DependsOn, Primary, Order</h3>
  *
- * <p>Note that the {@code @Bean} annotation does not provide attributes for scope,
- * depends-on, primary, or lazy. Rather, it should be used in conjunction with
- * {@link Scope @Scope}, {@link DependsOn @DependsOn}, {@link Primary @Primary},
- * and {@link Lazy @Lazy} annotations to achieve those semantics. For example:
+ * <p>Note that the {@code @Bean} annotation does not provide attributes for profile,
+ * scope, lazy, depends-on or primary. Rather, it should be used in conjunction with
+ * {@link Scope @Scope}, {@link Lazy @Lazy}, {@link DependsOn @DependsOn} and
+ * {@link Primary @Primary} annotations to declare those semantics. For example:
  *
  * <pre class="code">
  *     &#064;Bean
+ *     &#064;Profile("production")
  *     &#064;Scope("prototype")
  *     public MyBean myBean() {
  *         // instantiate and configure MyBean obj
  *         return obj;
  *     }
  * </pre>
  *
+ * The semantics of the above-mentioned annotations match their use at the component
+ * class level: {@code Profile} allows for selective inclusion of certain beans.
+ * {@code @Scope} changes the bean's scope from singleton to the specified scope.
+ * {@code @Lazy} only has an actual effect in case of the default singleton scope.
+ * {@code @DependsOn} enforces the creation of specific other beans before this
+ * bean will be created, in addition to any dependencies that the bean expressed
+ * through direct references, which is typically helpful for singleton startup.
+ * {@code @Primary} is a mechanism to resolve ambiguity at the injection point level
+ * if a single target component needs to be injected but several beans match by type.
+ *
+ * <p>Additionally, {@code @Bean} methods may also declare qualifier annotations
+ * and {@link org.springframework.core.annotation.Order @Order} values, to be
+ * taken into account during injection point resolution just like corresponding
+ * annotations on the corresponding component classes but potentially being very
+ * individual per bean definition (in case of multiple definitions with the same
+ * bean class). Qualifiers narrow the set of candidates after the initial type match;
+ * order values determine the order of resolved elements in case of collection
+ * injection points (with several target beans matching by type and qualifier).
+ *
+ * <p><b>NOTE:</b> {@code @Order} values may influence priorities at injection points
+ * but please be aware that they do not influence singleton startup order which is an
+ * orthogonal concern determined by dependency relationships and {@code @DependsOn}
+ * declarations as mentioned above. Also, {@link javax.annotation.Priority} is not
+ * available at this level since it cannot be declared on methods; its semantics can
+ * be modelled through {@code @Order} values in combination with {@code @Primary} on
+ * a single bean per type.
+ *
  * <h3>{@code @Bean} Methods in {@code @Configuration} Classes</h3>
  *
  * <p>Typically, {@code @Bean} methods are declared within {@code @Configuration}
@@ -143,7 +171,7 @@
  *
  * <h3>Bootstrapping</h3>
  *
- * <p>See @{@link Configuration} Javadoc for further details including how to bootstrap
+ * <p>See the @{@link Configuration} javadoc for further details including how to bootstrap
  * the container using {@link AnnotationConfigApplicationContext} and friends.
  *
  * <h3>{@code BeanFactoryPostProcessor}-returning {@code @Bean} methods</h3>

spring-core/src/main/java/org/springframework/core/annotation/Order.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,21 +27,29 @@
 /**
  * {@code @Order} defines the sort order for an annotated component.
  *
- * <p>The {@link #value} is optional and represents an order value as defined
- * in the {@link Ordered} interface. Lower values have higher priority. The
- * default value is {@code Ordered.LOWEST_PRECEDENCE}, indicating
- * lowest priority (losing to any other specified order value).
+ * <p>The {@link #value} is optional and represents an order value as defined in the
+ * {@link Ordered} interface. Lower values have higher priority. The default value is
+ * {@code Ordered.LOWEST_PRECEDENCE}, indicating lowest priority (losing to any other
+ * specified order value).
  *
- * <p>Since Spring 4.1, the standard {@link javax.annotation.Priority}
- * annotation can be used as a drop-in replacement for this annotation.
+ * <p><b>NOTE:</b> Since Spring 4.0, annotation-based ordering is supported for many
+ * kinds of components in Spring, even for collection injection where the order values
+ * of the target components are taken into account (either from their target class or
+ * from their {@code @Bean} method). While such order values may influence priorities
+ * at injection points, please be aware that they do not influence singleton startup
+ * order which is an orthogonal concern determined by dependency relationships and
+ * {@code @DependsOn} declarations (influencing a runtime-determined dependency graph).
  *
- * <p><b>NOTE</b>: Annotation-based ordering is supported for specific kinds
- * of components only &mdash; for example, for annotation-based AspectJ
- * aspects. Ordering strategies within the Spring container, on the other
- * hand, are typically based on the {@link Ordered} interface in order to
- * allow for programmatically configurable ordering of each <i>instance</i>.
+ * <p>Since Spring 4.1, the standard {@link javax.annotation.Priority} annotation
+ * can be used as a drop-in replacement for this annotation in ordering scenarios.
+ * Note that {@code Priority} may have additional semantics when a single element
+ * has to be picked (see {@link AnnotationAwareOrderComparator#getPriority}).
  *
- * <p>Consult the Javadoc for {@link org.springframework.core.OrderComparator
+ * <p>Alternatively, order values may also be determined on a per-instance basis
+ * through the {@link Ordered} interface, allowing for configuration-determined
+ * instance values instead of hard-coded values attached to a particular class.
+ *
+ * <p>Consult the javadoc for {@link org.springframework.core.OrderComparator
  * OrderComparator} for details on the sort semantics for non-ordered objects.
  *
  * @author Rod Johnson

src/asciidoc/core-beans.adoc

@@ -4468,9 +4468,20 @@ The same applies for typed collections:
 
 [TIP]
 ====
-Your beans can implement the `org.springframework.core.Ordered` interface or either use
+Your target beans can implement the `org.springframework.core.Ordered` interface or use
 the `@Order` or standard `@Priority` annotation if you want items in the array or list
-to be sorted into a specific order.
+to be sorted into a specific order. Otherwise their order will follow the registration
+order of the corresponding target bean definitions in the container.
+
+The `@Order` annotation may be declared at target class level but also on `@Bean` methods,
+potentially being very individual per bean definition (in case of multiple definitions
+with the same bean class). `@Order` values may influence priorities at injection points
+but please be aware that they do not influence singleton startup order which is an
+orthogonal concern determined by dependency relationships and `@DependsOn` declarations.
+
+Note that the standard `javax.annotation.Priority` annotation is not available at the
+`@Bean` level since it cannot be declared on methods. Its semantics can be modelled
+through `@Order` values in combination with `@Primary` on a single bean per type.
 ====
 
 Even typed Maps can be autowired as long as the expected key type is `String`. The Map
@@ -6968,7 +6979,6 @@ another configuration class:
 		public A a() {
 			return new A();
 		}
-
 	}
 
 	@Configuration
@@ -7231,6 +7241,14 @@ way, navigating `@Configuration` classes and their dependencies becomes no diffe
 than the usual process of navigating interface-based code.
 --
 
+[TIP]
+====
+If you would like to influence the startup creation order of certain beans, consider
+declaring some of them as `@Lazy` (for creation on first access instead of on startup)
+or as `@DependsOn` on certain other beans (making sure that specific other beans will
+be created before the current bean, beyond what the latter's direct dependencies imply).
+====
+
 
 [[beans-java-conditional]]
 ==== Conditionally include @Configuration classes or @Bean methods