3.3 Bean overview

3.3 Bean overview
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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 number of connections to use in a bean that manages a connection pool, or the size limit of the pool.

This metadata translates to a set of properties that make up each bean definition.

Table 3.1. The bean definition

Property	Explained in...
class	Section 3.3.2, “Instantiating beans”
name	Section 3.3.1, “Naming beans”
scope	Section 3.5, “Bean scopes”
constructor arguments	Section 3.4.1, “Dependency injection”
properties	Section 3.4.1, “Dependency injection”
autowiring mode	Section 3.4.5, “Autowiring collaborators”
dependency checking mode	Section 3.4.6, “Checking for dependencies”
lazy-initialization mode	Section 3.4.4, “Lazy-initialized beans”
initialization method	Section 3.6.1.1, “Initialization callbacks”
destruction method	Section 3.6.1.2, “Destruction callbacks”

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 via the method getBeanFactory which returns the BeanFactory implementation DefaultListableBeanFactory. DefaultListableBeanFactory supports this registration through the methods registerSingleton(..) and registerBeanDefinition(..). However, typical applications work solely with beans defined through metadata bean definitions.

3.3.1 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, but if it requires more than one, the extra ones can be considered aliases.

In XML-based configuration metadata, you use the id and/or name attributes to specify the bean identifier(s). The id attribute allows you to specify exactly one id, and because it is a real XML element ID attribute, the XML parser can do some extra validation when other elements reference the id. As such, it is the preferred way to specify a bean identifier. However, the XML specification does limit the characters that are legal in XML ids. This is usually not a constraint, but if you need to use one of these special XML characters, or want to introduce other aliases to the bean, you can also specify them in the name attribute, separated by a comma (,), semicolon (;), or white space.

You are not required to supply a name or id for a bean. If no name or id is supplied 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 Service Location style lookup, you must provide a name. Motivations for not supplying a name are related to using inner beans and autowiring collaborators.

Bean naming conventions

The convention is to use the standard Java convention for instance field names when naming beans. That is, bean names start with a lowercase letter, and are camel-cased from then on. Examples of such names would be (without quotes) 'accountManager', 'accountService', 'userDao', 'loginController', and so forth.

Naming beans consistently makes your configuration easier to read and understand, and if you are using Spring AOP it helps a lot when applying advice to a set of beans related by name.

3.3.1.1 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 allowing each component in an application to 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, each subsystem having its own set of object defintions. In XML-based configuration metadata, you can use of the <alias/> element to accomplish this.

<alias name="fromName" alias="toName"/>

In this case, a bean in the same container which is 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 via the name 'subsystemA-dataSource. The configuration metadata for subsystem B may refer to a DataSource via the name 'subsystemB-dataSource'. When composing the main application that uses both these subsystems the main application refers to the DattaSource via the name 'myApp-dataSource'. To have all three names refer to the same object you add to the MyApp configuration metadata the following aliases definitions:

<alias name="subsystemA-dataSource" alias="subsystemB-dataSource"/>
<alias name="subsystemA-dataSource" alias="myApp-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.

3.3.2 Instantiating beans

A bean definition essentially is 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 Section 3.3.2.3, “Instantiation using an instance factory method” and Section 3.7, “Bean definition inheritance”.) You 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 using the new operator.

To specify the actual class containing the static factory method that will be invoked to create the object, in the less common case where the container invokes a static, factory method on a class to create the bean. The object type returned from the invocation of the static factory method may be the same class or another class entirely.

Inner class names

If you want to configure a bean definition for a static inner class, you have to use the binary name of the inner class.

For example, if you have a class called Foo in the com.example package, and this Foo class has a static inner class called Bar, the value of the 'class' attribute on a bean definition would be...

com.example.Foo$Bar

Notice the use of the $ character in the name to separate the inner class name from the outer class name.

3.3.2.1 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.

3.3.2.2 Instantiation with a static factory method

When defining a bean that you create with a static factory method, you use the class attribute to specify the class containing 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, only the class containing the factory method. In this example, the createInstance() method must be a static method.

<bean id="exampleBean"
      class="examples.ExampleBean2"
      factory-method="createInstance"/>

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.

3.3.2.3 Instantiation 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 classattribute empty, and in the factory-bean attribute, specify the name of a bean in the current (or parent/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 factory bean, which contains a method called createInstance() -->
<bean id="serviceLocator" class="com.foo.DefaultServiceLocator">
  <!-- inject any dependencies required by this locator bean -->
</bean>

<!-- the bean to be created via the factory bean -->
<bean id="exampleBean"
      factory-bean="serviceLocator"
      factory-method="createInstance"/>

This approach shows that the factory bean itself can be managed and configured through dependency injection (DI). See Dependencies and configuration in detail .

	Note
	In Spring documentation, factory bean refers to a bean that is configured in the Spring container that will create objects through an instance or static factory method. By contrast, `FactoryBean` (notice the capitalization) refers to a Spring-specific `FactoryBean` .

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