Container Overview
1.2. 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.
org.springframework.context.ApplicationContext接口表示Spring IoC容器，并负责实例化、配置和组装bean。容器通过读取配置元数据来获取关于实例化、配置和组装哪些对象的指令。配置元数据用XML、Java注释或Java代码表示。它允许您表达组成应用程序的对象以及这些对象之间丰富的相互依赖关系。
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.
Figure 1. The Spring IoC container
1.2.1. 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: Spring 2.5 introduced support for annotation-based configuration metadata.
- Java-based configuration: Starting with Spring 3.0, many features provided by the Spring JavaConfig project became part of the core Spring Framework. Thus, you can define beans external to your application classes by using Java rather than XML files. To use these new features, see the
@Configuration,@Bean,@Import, and@DependsOnannotations.
- 基于注释的配置:Spring 2.5引入了对基于注释的配置元数据的支持。
- 基于java的配置:从Spring 3.0开始，Spring JavaConfig项目提供的许多特性成为核心Spring框架的一部分。因此，您可以使用Java而不是XML文件来定义应用程序类外部的bean。要使用这些新特性，请参阅@Configuration、@Bean、@Import和@DependsOn注释。
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, data access objects (DAOs), presentation objects such as Struts Action instances, infrastructure objects such as Hibernate SessionFactories, JMS Queues, and so forth. Typically, one does not configure fine-grained domain objects in the container, because it is usually the responsibility of DAOs and business logic to create and load domain objects. However, you can use Spring’s integration with AspectJ to configure objects that have been created outside the control of an IoC container. See Using AspectJ to dependency-inject domain objects with Spring.
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="...">
<!-- 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>
- The
idattribute is a string that identifies the individual bean definition. - The
classattribute defines the type of the bean and uses the fully qualified classname.
- id属性是标识各个bean定义的字符串。
- class属性定义bean的类型，并使用完全限定的类名。
The value of the id attribute refers to collaborating objects. The XML for referring to collaborating objects is not shown in this example. See Dependencies for more information.
1.2.2. 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.
ApplicationContext context = new ClassPathXmlApplicationContext("services.xml", "daos.xml");
After you learn about Spring’s IoC container, you may want to know more about Spring’s
Resourceabstraction (as described in Resources), which provides a convenient mechanism for reading an InputStream from locations defined in a URI syntax. In particular,Resourcepaths are used to construct applications contexts, as described in Application Contexts and Resource Paths.
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
classpath:URLs (for example,classpath:../services.xml), where the runtime resolution process chooses the “nearest” classpath root and then looks into its parent directory. Classpath configuration changes may lead to the choice of a different, incorrect directory.You can always use fully qualified resource locations instead of relative paths: for example,file:C:/config/services.xmlorclasspath:/config/services.xml. However, be aware that you are coupling your application’s configuration to specific absolute locations. It is generally preferable to keep an indirection for such absolute locations — for example, through "${…}" placeholders that are resolved against JVM system properties at runtime.
file:C:/config/services.xml或classpath:/config/services.xml。但是，请注意，您正在将应用程序的配置耦合到特定的绝对位置。通常更可取的做法是对这种绝对位置保持间接——例如，通过运行时针对/VM系统属性解析的“$(...)”占位符。
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.
1.2.3. 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.
T getBean(String name，Class requiredType) ，您可以检索bean的实例。
The ApplicationContext lets you read bean definitions and access them, as the following example shows:
// 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();
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:
ApplicationContext context = new 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:
GenericApplicationContext context = new GenericApplicationContext();
new XmlBeanDefinitionReader(context).loadBeanDefinitions("services.xml", "daos.xml");
context.refresh();
You can also use the GroovyBeanDefinitionReader for Groovy files, as the following example shows:
GenericApplicationContext context = new GenericApplicationContext();
new 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).
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