3.The IoC container

http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html

The ApplicationContext is the interface for an advanced factory capable of maintaining a registry of different beans and their dependencies. Using the method T getBean(Stringname, Class<T> requiredType) you can retrieve instances of your beans.

The ApplicationContext enables you to read bean definitions and access them as follows:

// create and configure beans
ApplicationContext context =
    new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"});

// retrieve configured instance
PetStoreServiceImpl service = context.getBean("petStore", PetStoreServiceImpl.class);

// use configured instance
List userList service.getUsernameList();

接口ApplicationContext中还有其它的方法来取回beans实例，但是理想的情况是你的应用应该不使用它们。从而使应用不dependency on Spring APIs。For example, Spring's integration with web frameworks provides for dependency injection for various web framework classes such as controllers and JSF-managed beans.

3.3.2.2 Instantiation with a static factory method

<bean id="clientService"
      class="examples.ClientService"
      factory-method="createInstance"/>

public class ClientService {
  private static ClientService clientService = new ClientService();
  private ClientService() {}

  public static ClientService createInstance() {
    return clientService;
  }
}

3.3.2.3 Instantiation using an instance factory method

<!-- the factory bean, which contains a method called createInstance() -->
<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"/>

public class DefaultServiceLocator {
  private static ClientService clientService = new ClientServiceImpl();
  private DefaultServiceLocator() {}

  public ClientService createClientServiceInstance() {
    return clientService;
  }
}

Dependency injection (DI)的两种方法：Constructor-based dependency injection and Setter-based dependency injection.

3.4.1.1 Constructor-based dependency injection

package x.y;

public class Foo {

  public Foo(Bar bar, Baz baz) {
      // ...
  }
}

<beans>
  <bean id="foo" class="x.y.Foo">
      <constructor-arg ref="bar"/>
      <constructor-arg ref="baz"/>
  </bean>

  <bean id="bar" class="x.y.Bar"/>
  <bean id="baz" class="x.y.Baz"/>

</beans>

//////////////////////////////////////////////////////////////////
package examples;

public class ExampleBean {

  // No. of years to the calculate the Ultimate Answer
  private int years;

  // The Answer to Life, the Universe, and Everything
  private String ultimateAnswer;

  public ExampleBean(int years, String ultimateAnswer) {
      this.years = years;
      this.ultimateAnswer = ultimateAnswer;
  }
}

<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg type="int" value="7500000"/>
<constructor-arg type="java.lang.String" value="42"/>
</bean>

<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg index="0" value="7500000"/>
<constructor-arg index="1" value="42"/>
</bean>

<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg name="years" value="7500000"/>
<constructor-arg name="ultimateanswer" value="42"/>
</bean>

package examples;

public class ExampleBean {

  // Fields omitted

  @ConstructorProperties({"years", "ultimateAnswer"})
  public ExampleBean(int years, String ultimateAnswer) {
      this.years = years;
      this.ultimateAnswer = ultimateAnswer;
  }
}

3.4.1.2 Setter-based dependency injection

Spring容器验证每个bean的配置当容器被创建的时候。然而bean的属性只有bean创建时才被设置。

只有被设置为singleton-scoped并且pre-instantiated (默认)的bean才在容器被创建时被创建。否则bean直到被请求时才被创建。

Creation of a bean potentially causes a graph of beans to be created, as the bean's dependencies and its dependencies' dependencies (and so on) are created and assigned.

<bean id="lazy" class="com.foo.ExpensiveToCreateBean" lazy-init="true"/>

 

<beans default-lazy-init="true">
  <!-- no beans will be pre-instantiated... -->
</beans>