Annotation Interface Bean
The names and semantics of the attributes to this annotation are intentionally similar to those of the element in the Spring XML schema. For example:
@Bean public MyBean myBean() < // instantiate and configure MyBean obj return obj; >
Bean Names
While a name() attribute is available, the default strategy for determining the name of a bean is to use the name of the @Bean method. This is convenient and intuitive, but if explicit naming is desired, the name attribute (or its alias value ) may be used. Also note that name accepts an array of Strings, allowing for multiple names (i.e. a primary bean name plus one or more aliases) for a single bean.
@Bean() // bean available as 'b1' and 'b2', but not 'myBean' public MyBean myBean() < // instantiate and configure MyBean obj return obj; >
Profile, Scope, Lazy, DependsOn, Primary, Order
Note that the @Bean annotation does not provide attributes for profile, scope, lazy, depends-on or primary. Rather, it should be used in conjunction with @Scope , @Lazy , @DependsOn and @Primary annotations to declare those semantics. For example:
@Bean @Profile("production") @Scope("prototype") public MyBean myBean() < // instantiate and configure MyBean obj return obj; > The semantics of the above-mentioned annotations match their use at the component class level: @Profile allows for selective inclusion of certain beans. @Scope changes the bean’s scope from singleton to the specified scope. @Lazy only has an actual effect in case of the default singleton scope. @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. @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.
Additionally, @Bean methods may also declare qualifier annotations and @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).
NOTE: @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 as mentioned above. Also, Priority is not available at this level since it cannot be declared on methods; its semantics can be modeled through @Order values in combination with @Primary on a single bean per type.
@Bean Methods in @Configuration Classes
Typically, @Bean methods are declared within @Configuration classes. In this case, bean methods may reference other @Bean methods in the same class by calling them directly. This ensures that references between beans are strongly typed and navigable. Such so-called ‘inter-bean references’ are guaranteed to respect scoping and AOP semantics, just like getBean() lookups would. These are the semantics known from the original ‘Spring JavaConfig’ project which require CGLIB subclassing of each such configuration class at runtime. As a consequence, @Configuration classes and their factory methods must not be marked as final or private in this mode. For example:
@Configuration public class AppConfig < @Bean public FooService fooService() < return new FooService(fooRepository()); >@Bean public FooRepository fooRepository() < return new JdbcFooRepository(dataSource()); >// . >
@Bean Lite Mode
@Bean methods may also be declared within classes that are not annotated with @Configuration . For example, bean methods may be declared in a @Component class or even in a plain old class. In such cases, a @Bean method will get processed in a so-called ‘lite’ mode.
Bean methods in lite mode will be treated as plain factory methods by the container (similar to factory-method declarations in XML), with scoping and lifecycle callbacks properly applied. The containing class remains unmodified in this case, and there are no unusual constraints for the containing class or the factory methods.
In contrast to the semantics for bean methods in @Configuration classes, ‘inter-bean references’ are not supported in lite mode. Instead, when one @Bean -method invokes another @Bean -method in lite mode, the invocation is a standard Java method invocation; Spring does not intercept the invocation via a CGLIB proxy. This is analogous to inter- @Transactional method calls where in proxy mode, Spring does not intercept the invocation — Spring does so only in AspectJ mode.
@Component public class Calculator < public int sum(int a, int b) < return a+b; >@Bean public MyBean myBean() < return new MyBean(); >>
Bootstrapping
See the @ Configuration javadoc for further details including how to bootstrap the container using AnnotationConfigApplicationContext and friends.
BeanFactoryPostProcessor -returning @Bean methods
Special consideration must be taken for @Bean methods that return Spring BeanFactoryPostProcessor ( BFPP ) types. Because BFPP objects must be instantiated very early in the container lifecycle, they can interfere with processing of annotations such as @Autowired , @Value , and @PostConstruct within @Configuration classes. To avoid these lifecycle issues, mark BFPP -returning @Bean methods as static . For example:
@Bean public static PropertySourcesPlaceholderConfigurer pspc() < // instantiate, configure and return pspc . >
By marking this method as static , it can be invoked without causing instantiation of its declaring @Configuration class, thus avoiding the above-mentioned lifecycle conflicts. Note however that static @Bean methods will not be enhanced for scoping and AOP semantics as mentioned above. This works out in BFPP cases, as they are not typically referenced by other @Bean methods. As a reminder, an INFO-level log message will be issued for any non-static @Bean methods having a return type assignable to BeanFactoryPostProcessor .
Basic Concepts: @Bean and @Configuration
The central artifacts in Spring’s Java configuration support are @Configuration -annotated classes and @Bean -annotated methods.
The @Bean annotation is used to indicate that a method instantiates, configures, and initializes a new object to be managed by the Spring IoC container. For those familiar with Spring’s XML configuration, the @Bean annotation plays the same role as the element. You can use @Bean -annotated methods with any Spring @Component . However, they are most often used with @Configuration beans.
Annotating a class with @Configuration indicates that its primary purpose is as a source of bean definitions. Furthermore, @Configuration classes let inter-bean dependencies be defined by calling other @Bean methods in the same class. The simplest possible @Configuration class reads as follows:
@Configuration public class AppConfig < @Bean public MyServiceImpl myService() < return new MyServiceImpl(); >>@Configuration class AppConfig < @Bean fun myService(): MyServiceImpl < return MyServiceImpl() >>The preceding AppConfig class is equivalent to the following Spring XML:
When @Bean methods are declared within classes that are not annotated with @Configuration , they are referred to as being processed in a “lite” mode. Bean methods declared in a @Component or even in a plain old class are considered to be “lite”, with a different primary purpose of the containing class and a @Bean method being a sort of bonus there. For example, service components may expose management views to the container through an additional @Bean method on each applicable component class. In such scenarios, @Bean methods are a general-purpose factory method mechanism.
Unlike full @Configuration , lite @Bean methods cannot declare inter-bean dependencies. Instead, they operate on their containing component’s internal state and, optionally, on arguments that they may declare. Such a @Bean method should therefore not invoke other @Bean methods. Each such method is literally only a factory method for a particular bean reference, without any special runtime semantics. The positive side-effect here is that no CGLIB subclassing has to be applied at runtime, so there are no limitations in terms of class design (that is, the containing class may be final and so forth).
In common scenarios, @Bean methods are to be declared within @Configuration classes, ensuring that “full” mode is always used and that cross-method references therefore get redirected to the container’s lifecycle management. This prevents the same @Bean method from accidentally being invoked through a regular Java call, which helps to reduce subtle bugs that can be hard to track down when operating in “lite” mode.
The @Bean and @Configuration annotations are discussed in depth in the following sections. First, however, we cover the various ways of creating a spring container by using Java-based configuration.