@Target(value={METHOD,ANNOTATION_TYPE})
@Retention(value=RUNTIME)
@Documented
public @interface Bean
The names and semantics of the attributes to this annotation are intentionally
similar to those of the <bean/>
element in the Spring XML schema. For
example:
@Bean public MyBean myBean() { // instantiate and configure MyBean obj return obj; }
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({"b1", "b2"}) // bean available as 'b1' and 'b2', but not 'myBean' public MyBean myBean() { // instantiate and configure MyBean obj return obj; }
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
ClassesTypically, @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.
For example:
@Component public class Calculator { public int sum(int a, int b) { return a+b; } @Bean public MyBean myBean() { return new MyBean(); } }
See the @Configuration
javadoc for further details including how to bootstrap
the container using AnnotationConfigApplicationContext
and friends.
BeanFactoryPostProcessor
-returning @Bean
methodsSpecial 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, a WARN-level log message will be
issued for any non-static @Bean
methods having a return type assignable to
BeanFactoryPostProcessor
.Modifier and Type | Optional Element and Description |
---|---|
Autowire |
autowire
Are dependencies to be injected via convention-based autowiring by name or type?
|
java.lang.String |
destroyMethod
The optional name of a method to call on the bean instance upon closing the
application context, for example a
close() method on a JDBC
DataSource implementation, or a Hibernate SessionFactory object. |
java.lang.String |
initMethod
The optional name of a method to call on the bean instance during initialization.
|
java.lang.String[] |
name
The name of this bean, or if several names, a primary bean name plus aliases.
|
java.lang.String[] |
value
Alias for
name() . |
@AliasFor(value="value") public abstract java.lang.String[] name
If left unspecified, the name of the bean is the name of the annotated method. If specified, the method name is ignored.
The bean name and aliases may also be configured via the value()
attribute if no other attributes are declared.
value()
public abstract Autowire autowire
Note that this autowire mode is just about externally driven autowiring based on bean property setter methods by convention, analogous to XML bean definitions.
The default mode does allow for annotation-driven autowiring. "no" refers to externally driven autowiring only, not affecting any autowiring demands that the bean class itself expresses through annotations.
Autowire.BY_NAME
,
Autowire.BY_TYPE
public abstract java.lang.String initMethod
The default value is ""
, indicating no init method to be called.
public abstract java.lang.String destroyMethod
close()
method on a JDBC
DataSource
implementation, or a Hibernate SessionFactory
object.
The method must have no arguments but may throw any exception.
As a convenience to the user, the container will attempt to infer a destroy
method against an object returned from the @Bean
method. For example, given
an @Bean
method returning an Apache Commons DBCP BasicDataSource
,
the container will notice the close()
method available on that object and
automatically register it as the destroyMethod
. This 'destroy method
inference' is currently limited to detecting only public, no-arg methods named
'close' or 'shutdown'. The method may be declared at any level of the inheritance
hierarchy and will be detected regardless of the return type of the @Bean
method (i.e., detection occurs reflectively against the bean instance itself at
creation time).
To disable destroy method inference for a particular @Bean
, specify an
empty string as the value, e.g. @Bean(destroyMethod="")
. Note that the
DisposableBean
callback interface will
nevertheless get detected and the corresponding destroy method invoked: In other
words, destroyMethod=""
only affects custom close/shutdown methods and
Closeable
/AutoCloseable
declared close methods.
Note: Only invoked on beans whose lifecycle is under the full control of the factory, which is always the case for singletons but not guaranteed for any other scope.