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 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:
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.
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 there. Examples of such names include accountManager , accountService , userDao , loginController , and so forth.
Naming beans consistently makes your configuration easier to read and understand. Also, if you use Spring AOP, it helps a lot when applying advice to a set of beans related by name.
| 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 element to accomplish this. The following example shows how to do so:
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:
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.
If you use Java Configuration, the @Bean annotation can be used to provide aliases. See Using the @Bean Annotation for details.
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 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 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.
If you want to configure a bean definition for a nested class, you may use either the binary name or the source name of the nested class.
For example, if you have a class called SomeThing in the com.example package, and this SomeThing class has a static nested class called OtherThing , they can be separated by a dollar sign ( $ ) or a dot ( . ). So the value of the class attribute in a bean definition would be com.example.SomeThing$OtherThing or com.example.SomeThing.OtherThing .
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:
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:
The following example shows a class that would work with the preceding bean definition:
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 following example shows the corresponding class: