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Bean Overview
A Spring IoC container manages one or more beans. These beans are created with the
configuration metadata that you supply to the container (for example, in the form of XML
<bean/>
definitions).
Within the container itself, these bean definitions are represented as BeanDefinition
objects, which contain (among other information) the following metadata:
-
A package-qualified class name: typically, the actual implementation class of the bean being defined.
-
Bean behavioral configuration elements, which state how the bean should behave in the container (scope, lifecycle callbacks, and so forth).
-
References to other beans that are needed for the bean to do its work. These references are also called collaborators or dependencies.
-
Other configuration settings to set in the newly created object — for example, the size limit of the pool or the number of connections to use in a bean that manages a connection pool.
This metadata translates to a set of properties that make up each bean definition. The following table describes these properties:
Property | Explained in… |
---|---|
Class |
|
Name |
|
Scope |
|
Constructor arguments |
|
Properties |
|
Autowiring mode |
|
Lazy initialization mode |
|
Initialization method |
|
Destruction method |
In addition to bean definitions that contain information on how to create a specific
bean, the ApplicationContext
implementations also permit the registration of existing
objects that are created outside the container (by users). This is done by accessing the
ApplicationContext’s BeanFactory
through the getBeanFactory()
method, which returns
the DefaultListableBeanFactory
implementation. DefaultListableBeanFactory
supports
this registration through the registerSingleton(..)
and registerBeanDefinition(..)
methods. However, typical applications work solely with beans defined through regular
bean definition metadata.
Bean metadata and manually supplied singleton instances need to be registered as early as possible, in order for the container to properly reason about them during autowiring and other introspection steps. While overriding existing metadata and existing singleton instances is supported to some degree, the registration of new beans at runtime (concurrently with live access to the factory) is not officially supported and may lead to concurrent access exceptions, inconsistent state in the bean container, or both. |
Naming Beans
Every bean has one or more identifiers. These identifiers must be unique within the container that hosts the bean. A bean usually has only one identifier. However, if it requires more than one, the extra ones can be considered aliases.
In XML-based configuration metadata, you use the id
attribute, the name
attribute, or
both to specify bean identifiers. The id
attribute lets you specify exactly one id
.
Conventionally, these names are alphanumeric ('myBean', 'someService', etc.), but they
can contain special characters as well. If you want to introduce other aliases for the
bean, you can also specify them in the name
attribute, separated by a comma (,
),
semicolon (;
), or white space. Although the id
attribute is defined as an
xsd:string
type, bean id
uniqueness is enforced by the container, though not by XML
parsers.
You are not required to supply a name
or an id
for a bean. If you do not supply a
name
or id
explicitly, the container generates a unique name for that bean. However,
if you want to refer to that bean by name, through the use of the ref
element or a
Service Locator style lookup, you must provide a name.
Motivations for not supplying a name are related to using inner beans
and autowiring collaborators.
With component scanning in the classpath, Spring generates bean names for unnamed
components, following the rules described earlier: essentially, taking the simple class name
and turning its initial character to lower-case. However, in the (unusual) special
case when there is more than one character and both the first and second characters
are upper case, the original casing gets preserved. These are the same rules as
defined by java.beans.Introspector.decapitalize (which Spring uses here).
|
Aliasing a Bean outside the Bean Definition
In a bean definition itself, you can supply more than one name for the bean, by using a
combination of up to one name specified by the id
attribute and any number of other
names in the name
attribute. These names can be equivalent aliases to the same bean
and are useful for some situations, such as letting each component in an application
refer to a common dependency by using a bean name that is specific to that component
itself.
Specifying all aliases where the bean is actually defined is not always adequate,
however. It is sometimes desirable to introduce an alias for a bean that is defined
elsewhere. This is commonly the case in large systems where configuration is split
amongst each subsystem, with each subsystem having its own set of object definitions.
In XML-based configuration metadata, you can use the <alias/>
element to accomplish
this. The following example shows how to do so:
<alias name="fromName" alias="toName"/>
In this case, a bean (in the same container) named fromName
may also,
after the use of this alias definition, be referred to as toName
.
For example, the configuration metadata for subsystem A may refer to a DataSource by the
name of subsystemA-dataSource
. The configuration metadata for subsystem B may refer to
a DataSource by the name of subsystemB-dataSource
. When composing the main application
that uses both these subsystems, the main application refers to the DataSource by the
name of myApp-dataSource
. To have all three names refer to the same object, you can
add the following alias definitions to the configuration metadata:
<alias name="myApp-dataSource" alias="subsystemA-dataSource"/>
<alias name="myApp-dataSource" alias="subsystemB-dataSource"/>
Now each component and the main application can refer to the dataSource through a name that is unique and guaranteed not to clash with any other definition (effectively creating a namespace), yet they refer to the same bean.
Instantiating Beans
A bean definition is essentially a recipe for creating one or more objects. The container looks at the recipe for a named bean when asked and uses the configuration metadata encapsulated by that bean definition to create (or acquire) an actual object.
If you use XML-based configuration metadata, you specify the type (or class) of object
that is to be instantiated in the class
attribute of the <bean/>
element. This
class
attribute (which, internally, is a Class
property on a BeanDefinition
instance) is usually mandatory. (For exceptions, see
Instantiation by Using an Instance Factory Method and Bean Definition Inheritance.)
You can use the Class
property in one of two ways:
-
Typically, to specify the bean class to be constructed in the case where the container itself directly creates the bean by calling its constructor reflectively, somewhat equivalent to Java code with the
new
operator. -
To specify the actual class containing the
static
factory method that is invoked to create the object, in the less common case where the container invokes astatic
factory method on a class to create the bean. The object type returned from the invocation of thestatic
factory method may be the same class or another class entirely.
Instantiation with a Constructor
When you create a bean by the constructor approach, all normal classes are usable by and compatible with Spring. That is, the class being developed does not need to implement any specific interfaces or to be coded in a specific fashion. Simply specifying the bean class should suffice. However, depending on what type of IoC you use for that specific bean, you may need a default (empty) constructor.
The Spring IoC container can manage virtually any class you want it to manage. It is not limited to managing true JavaBeans. Most Spring users prefer actual JavaBeans with only a default (no-argument) constructor and appropriate setters and getters modeled after the properties in the container. You can also have more exotic non-bean-style classes in your container. If, for example, you need to use a legacy connection pool that absolutely does not adhere to the JavaBean specification, Spring can manage it as well.
With XML-based configuration metadata you can specify your bean class as follows:
<bean id="exampleBean" class="examples.ExampleBean"/>
<bean name="anotherExample" class="examples.ExampleBeanTwo"/>
For details about the mechanism for supplying arguments to the constructor (if required) and setting object instance properties after the object is constructed, see Injecting Dependencies.
Instantiation with a Static Factory Method
When defining a bean that you create with a static factory method, use the class
attribute to specify the class that contains the static
factory method and an attribute
named factory-method
to specify the name of the factory method itself. You should be
able to call this method (with optional arguments, as described later) and return a live
object, which subsequently is treated as if it had been created through a constructor.
One use for such a bean definition is to call static
factories in legacy code.
The following bean definition specifies that the bean will be created by calling a
factory method. The definition does not specify the type (class) of the returned object,
but rather the class containing the factory method. In this example, the
createInstance()
method must be a static
method. The following example shows how to
specify a factory method:
<bean id="clientService"
class="examples.ClientService"
factory-method="createInstance"/>
The following example shows a class that would work with the preceding bean definition:
-
Java
-
Kotlin
public class ClientService {
private static ClientService clientService = new ClientService();
private ClientService() {}
public static ClientService createInstance() {
return clientService;
}
}
class ClientService private constructor() {
companion object {
private val clientService = ClientService()
@JvmStatic
fun createInstance() = clientService
}
}
For details about the mechanism for supplying (optional) arguments to the factory method and setting object instance properties after the object is returned from the factory, see Dependencies and Configuration in Detail.
Instantiation by Using an Instance Factory Method
Similar to instantiation through a static factory method
, instantiation with an instance factory method invokes a non-static
method of an existing bean from the container to create a new bean. To use this
mechanism, leave the class
attribute empty and, in the factory-bean
attribute,
specify the name of a bean in the current (or parent or ancestor) container that contains
the instance method that is to be invoked to create the object. Set the name of the
factory method itself with the factory-method
attribute. The following example shows
how to configure such a bean:
<!-- the factory bean, which contains a method called createClientServiceInstance() -->
<bean id="serviceLocator" class="examples.DefaultServiceLocator">
<!-- inject any dependencies required by this locator bean -->
</bean>
<!-- the bean to be created via the factory bean -->
<bean id="clientService"
factory-bean="serviceLocator"
factory-method="createClientServiceInstance"/>
The following example shows the corresponding class:
-
Java
-
Kotlin
public class DefaultServiceLocator {
private static ClientService clientService = new ClientServiceImpl();
public ClientService createClientServiceInstance() {
return clientService;
}
}
class DefaultServiceLocator {
companion object {
private val clientService = ClientServiceImpl()
}
fun createClientServiceInstance(): ClientService {
return clientService
}
}
One factory class can also hold more than one factory method, as the following example shows:
<bean id="serviceLocator" class="examples.DefaultServiceLocator">
<!-- inject any dependencies required by this locator bean -->
</bean>
<bean id="clientService"
factory-bean="serviceLocator"
factory-method="createClientServiceInstance"/>
<bean id="accountService"
factory-bean="serviceLocator"
factory-method="createAccountServiceInstance"/>
The following example shows the corresponding class:
-
Java
-
Kotlin
public class DefaultServiceLocator {
private static ClientService clientService = new ClientServiceImpl();
private static AccountService accountService = new AccountServiceImpl();
public ClientService createClientServiceInstance() {
return clientService;
}
public AccountService createAccountServiceInstance() {
return accountService;
}
}
class DefaultServiceLocator {
companion object {
private val clientService = ClientServiceImpl()
private val accountService = AccountServiceImpl()
}
fun createClientServiceInstance(): ClientService {
return clientService
}
fun createAccountServiceInstance(): AccountService {
return accountService
}
}
This approach shows that the factory bean itself can be managed and configured through dependency injection (DI). See Dependencies and Configuration in Detail.
In Spring documentation, "factory bean" refers to a bean that is configured in the
Spring container and that creates objects through an
instance or
static factory method. By contrast,
FactoryBean (notice the capitalization) refers to a Spring-specific
FactoryBean implementation class.
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Determining a Bean’s Runtime Type
The runtime type of a specific bean is non-trivial to determine. A specified class in
the bean metadata definition is just an initial class reference, potentially combined
with a declared factory method or being a FactoryBean
class which may lead to a
different runtime type of the bean, or not being set at all in case of an instance-level
factory method (which is resolved via the specified factory-bean
name instead).
Additionally, AOP proxying may wrap a bean instance with an interface-based proxy with
limited exposure of the target bean’s actual type (just its implemented interfaces).
The recommended way to find out about the actual runtime type of a particular bean is
a BeanFactory.getType
call for the specified bean name. This takes all of the above
cases into account and returns the type of object that a BeanFactory.getBean
call is
going to return for the same bean name.