Core Technologies

1. The IoC Container

1.1 Introduction to the Spring IoC Container and Beans

Service Locator pattern
The BeanFactory interface provides an advanced configuration mechanism capable of managing any type of object.
- ApplicationContext is a sub-interface of BeanFactory. It adds:
  - Easier integration with Spring’s AOP features
  - Message resource handling (for use in internationalization)
  - Event publication
  - Application-layer specific contexts such as the WebApplicationContext for use in web applications.

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 configuration metadata represented in:
- XML
- Java annotations
- Java code
ClassPathXmlApplicationContext and FileSystemXmlApplicationContext is implementations of the ApplicationContext which is commonly used by stand-alone applications

1.2.1. 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.
note the differences?:
- 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.

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>

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");
```
In particular, Resource paths are used to construct applications contexts, as described in Application Contexts and Resource Paths.