Container Overview
The org.springframework.context.ApplicationContext
interface represents the Spring IoC
container and is responsible for instantiating, configuring, and assembling the
beans. The container gets its instructions on what objects to
instantiate, configure, and assemble by reading configuration metadata. The
configuration metadata is represented in XML, Java annotations, or Java code. It lets
you express the objects that compose your application and the rich interdependencies
between those objects.
Several implementations of the ApplicationContext
interface are supplied
with Spring. In stand-alone applications, it is common to create an
instance of
ClassPathXmlApplicationContext
or FileSystemXmlApplicationContext
.
While XML has been the traditional format for defining configuration metadata, you can
instruct the container to use Java annotations or code as the metadata format by
providing a small amount of XML configuration to declaratively enable support for these
additional metadata formats.
In most application scenarios, explicit user code is not required to instantiate one or
more instances of a Spring IoC container. For example, in a web application scenario, a
simple eight (or so) lines of boilerplate web descriptor XML in the web.xml
file
of the application typically suffices (see
Convenient ApplicationContext Instantiation for Web Applications).
If you use the Spring Tools for Eclipse (an Eclipse-powered
development environment), you can easily create this boilerplate configuration with a
few mouse clicks or keystrokes.
The following diagram shows a high-level view of how Spring works. Your application classes
are combined with configuration metadata so that, after the ApplicationContext
is
created and initialized, you have a fully configured and executable system or
application.
Configuration Metadata
As the preceding diagram shows, the Spring IoC container consumes a form of configuration metadata. This configuration metadata represents how you, as an application developer, tell the Spring container to instantiate, configure, and assemble the objects in your application.
Configuration metadata is traditionally supplied in a simple and intuitive XML format, which is what most of this chapter uses to convey key concepts and features of the Spring IoC container.
XML-based metadata is not the only allowed form of configuration metadata. The Spring IoC container itself is totally decoupled from the format in which this configuration metadata is actually written. These days, many developers choose Java-based configuration for their Spring applications. |
For information about using other forms of metadata with the Spring container, see:
-
Annotation-based configuration: define beans using annotation-based configuration metadata.
-
Java-based configuration: define beans external to your application classes by using Java rather than XML files. To use these features, see the
@Configuration
,@Bean
,@Import
, and@DependsOn
annotations.
Spring configuration consists of at least one and typically more than one bean
definition that the container must manage. XML-based configuration metadata configures these
beans as <bean/>
elements inside a top-level <beans/>
element. Java
configuration typically uses @Bean
-annotated methods within a @Configuration
class.
These bean definitions correspond to the actual objects that make up your application.
Typically, you define service layer objects, persistence layer objects such as
repositories or data access objects (DAOs), presentation objects such as Web controllers,
infrastructure objects such as a JPA EntityManagerFactory
, JMS queues, and so forth.
Typically, one does not configure fine-grained domain objects in the container, because
it is usually the responsibility of repositories and business logic to create and load
domain objects.
The following example shows the basic structure of XML-based configuration metadata:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="..." class="..."> (1) (2)
<!-- collaborators and configuration for this bean go here -->
</bean>
<bean id="..." class="...">
<!-- collaborators and configuration for this bean go here -->
</bean>
<!-- more bean definitions go here -->
</beans>
1 | The id attribute is a string that identifies the individual bean definition. |
2 | The class attribute defines the type of the bean and uses the fully qualified
class name. |
The value of the id
attribute can be used to refer to collaborating objects. The XML
for referring to collaborating objects is not shown in this example. See
Dependencies for more information.
Instantiating a Container
The location path or paths
supplied to an ApplicationContext
constructor are resource strings that let
the container load configuration metadata from a variety of external resources, such
as the local file system, the Java CLASSPATH
, and so on.
-
Java
-
Kotlin
ApplicationContext context = new ClassPathXmlApplicationContext("services.xml", "daos.xml");
val context = ClassPathXmlApplicationContext("services.xml", "daos.xml")
After you learn about Spring’s IoC container, you may want to know more about Spring’s
|
The following example shows the service layer objects (services.xml)
configuration file:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd">
<!-- services -->
<bean id="petStore" class="org.springframework.samples.jpetstore.services.PetStoreServiceImpl">
<property name="accountDao" ref="accountDao"/>
<property name="itemDao" ref="itemDao"/>
<!-- additional collaborators and configuration for this bean go here -->
</bean>
<!-- more bean definitions for services go here -->
</beans>
The following example shows the data access objects daos.xml
file:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id="accountDao"
class="org.springframework.samples.jpetstore.dao.jpa.JpaAccountDao">
<!-- additional collaborators and configuration for this bean go here -->
</bean>
<bean id="itemDao" class="org.springframework.samples.jpetstore.dao.jpa.JpaItemDao">
<!-- additional collaborators and configuration for this bean go here -->
</bean>
<!-- more bean definitions for data access objects go here -->
</beans>
In the preceding example, the service layer consists of the PetStoreServiceImpl
class
and two data access objects of the types JpaAccountDao
and JpaItemDao
(based
on the JPA Object-Relational Mapping standard). The property name
element refers to the
name of the JavaBean property, and the ref
element refers to the name of another bean
definition. This linkage between id
and ref
elements expresses the dependency between
collaborating objects. For details of configuring an object’s dependencies, see
Dependencies.
Composing XML-based Configuration Metadata
It can be useful to have bean definitions span multiple XML files. Often, each individual XML configuration file represents a logical layer or module in your architecture.
You can use the application context constructor to load bean definitions from all these
XML fragments. This constructor takes multiple Resource
locations, as was shown in the
previous section. Alternatively,
use one or more occurrences of the <import/>
element to load bean definitions from
another file or files. The following example shows how to do so:
<beans>
<import resource="services.xml"/>
<import resource="resources/messageSource.xml"/>
<import resource="/resources/themeSource.xml"/>
<bean id="bean1" class="..."/>
<bean id="bean2" class="..."/>
</beans>
In the preceding example, external bean definitions are loaded from three files:
services.xml
, messageSource.xml
, and themeSource.xml
. All location paths are
relative to the definition file doing the importing, so services.xml
must be in the
same directory or classpath location as the file doing the importing, while
messageSource.xml
and themeSource.xml
must be in a resources
location below the
location of the importing file. As you can see, a leading slash is ignored. However, given
that these paths are relative, it is better form not to use the slash at all. The
contents of the files being imported, including the top level <beans/>
element, must
be valid XML bean definitions, according to the Spring Schema.
It is possible, but not recommended, to reference files in parent directories using a
relative "../" path. Doing so creates a dependency on a file that is outside the current
application. In particular, this reference is not recommended for You can always use fully qualified resource locations instead of relative paths: for
example, |
The namespace itself provides the import directive feature. Further
configuration features beyond plain bean definitions are available in a selection
of XML namespaces provided by Spring — for example, the context
and util
namespaces.
The Groovy Bean Definition DSL
As a further example for externalized configuration metadata, bean definitions can also be expressed in Spring’s Groovy Bean Definition DSL, as known from the Grails framework. Typically, such configuration live in a ".groovy" file with the structure shown in the following example:
beans {
dataSource(BasicDataSource) {
driverClassName = "org.hsqldb.jdbcDriver"
url = "jdbc:hsqldb:mem:grailsDB"
username = "sa"
password = ""
settings = [mynew:"setting"]
}
sessionFactory(SessionFactory) {
dataSource = dataSource
}
myService(MyService) {
nestedBean = { AnotherBean bean ->
dataSource = dataSource
}
}
}
This configuration style is largely equivalent to XML bean definitions and even
supports Spring’s XML configuration namespaces. It also allows for importing XML
bean definition files through an importBeans
directive.
Using the Container
The ApplicationContext
is the interface for an advanced factory capable of maintaining
a registry of different beans and their dependencies. By using the method
T getBean(String name, Class<T> requiredType)
, you can retrieve instances of your beans.
The ApplicationContext
lets you read bean definitions and access them, as the following
example shows:
-
Java
-
Kotlin
// create and configure beans
ApplicationContext context = new ClassPathXmlApplicationContext("services.xml", "daos.xml");
// retrieve configured instance
PetStoreService service = context.getBean("petStore", PetStoreService.class);
// use configured instance
List<String> userList = service.getUsernameList();
import org.springframework.beans.factory.getBean
// create and configure beans
val context = ClassPathXmlApplicationContext("services.xml", "daos.xml")
// retrieve configured instance
val service = context.getBean<PetStoreService>("petStore")
// use configured instance
var userList = service.getUsernameList()
With Groovy configuration, bootstrapping looks very similar. It has a different context implementation class which is Groovy-aware (but also understands XML bean definitions). The following example shows Groovy configuration:
-
Java
-
Kotlin
ApplicationContext context = new GenericGroovyApplicationContext("services.groovy", "daos.groovy");
val context = GenericGroovyApplicationContext("services.groovy", "daos.groovy")
The most flexible variant is GenericApplicationContext
in combination with reader
delegates — for example, with XmlBeanDefinitionReader
for XML files, as the following
example shows:
-
Java
-
Kotlin
GenericApplicationContext context = new GenericApplicationContext();
new XmlBeanDefinitionReader(context).loadBeanDefinitions("services.xml", "daos.xml");
context.refresh();
val context = GenericApplicationContext()
XmlBeanDefinitionReader(context).loadBeanDefinitions("services.xml", "daos.xml")
context.refresh()
You can also use the GroovyBeanDefinitionReader
for Groovy files, as the following
example shows:
-
Java
-
Kotlin
GenericApplicationContext context = new GenericApplicationContext();
new GroovyBeanDefinitionReader(context).loadBeanDefinitions("services.groovy", "daos.groovy");
context.refresh();
val context = GenericApplicationContext()
GroovyBeanDefinitionReader(context).loadBeanDefinitions("services.groovy", "daos.groovy")
context.refresh()
You can mix and match such reader delegates on the same ApplicationContext
,
reading bean definitions from diverse configuration sources.
You can then use getBean
to retrieve instances of your beans. The ApplicationContext
interface has a few other methods for retrieving beans, but, ideally, your application
code should never use them. Indeed, your application code should have no calls to the
getBean()
method at all and thus have no dependency on Spring APIs at all. For example,
Spring’s integration with web frameworks provides dependency injection for various web
framework components such as controllers and JSF-managed beans, letting you declare
a dependency on a specific bean through metadata (such as an autowiring annotation).