Spring 4

Spring IoC Container Part 1

Let’s look a simple code within Spring below:

1
2
3
4
5
6
7
8
9
 // create and configure beans
ApplicationContext context =
    new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"});

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

// use configured instance
List<String> userList = service.getUsernameList();

First of all, we should know the class hierarchy in org.springframework.context.support:

ClassPathXmlApplicationContext implements ApplicationContext and calls its constructor as below: 

1
2
3
4
5
6
7
8
9
 /**
 * Create a new ClassPathXmlApplicationContext, loading the definitions
 * from the given XML file and automatically refreshing the context.
 * @param configLocation resource location
 * @throws BeansException if context creation failed
 **/
public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
	this(new String[] {configLocation}, true, null);
}

Step into ClassPathXmlApplicationContext(String[], boolean, ApplicationContext)：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
/**
 * Create a new ClassPathXmlApplicationContext with the given parent,
 * loading the definitions from the given XML files.
 * @param configLocations array of resource locations
 * @param refresh whether to automatically refresh the context,
 * loading all bean definitions and creating all singletons.
 * Alternatively, call refresh manually after further configuring the context.
 * @param parent the parent context
 * @throws BeansException if context creation failed
 * @see #refresh()
 **/
public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
		throws BeansException {

	super(parent);
	setConfigLocations(configLocations);
	if (refresh) {
		refresh();
	}
}

In this method, it first call super(parent), which is AbstractXmlApplicationContext(parent) –> AbstractRefreshableConfigApplicationContext(parent) –> AbstractRefreshableApplicationContext(parent) –> AbstractApplicationContext(parent).

Then, for the setConfigLocations(configLocations), we need move to AbstractRefreshableConfigApplicationContext, the superclass of ClassPathXmlApplicationContext. It has setConfigLocations(configLocations) that deals with the locations String and save to its private value String[] configLocations.

Now, we get to the refresh() method belong to AbstractApplicationContext that implements the method in the interface ConfigurableApplicationContext

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
/**
 * Load or refresh the persistent representation of the configuration,
 * which might an XML file, properties file, or relational database schema.
 * <p>As this is a startup method, it should destroy already created singletons
 * if it fails, to avoid dangling resources. In other words, after invocation
 * of that method, either all or no singletons at all should be instantiated.
 * @throws BeansException if the bean factory could not be initialized
 * @throws IllegalStateException if already initialized and multiple refresh
 * attempts are not supported
 */
 @Override
public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
		// Prepare this context for refreshing.
		prepareRefresh();

		// Tell the subclass to refresh the internal bean factory.
		ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

		// Prepare the bean factory for use in this context.
		prepareBeanFactory(beanFactory);
		// remainder...
		}
}

For this line call obtainFreshBeanFactory() that then calls refreshBeanFactory(), subclass AbstractXmlApplicationContext to refresh bean factory and load bean definitions by calling loadBeanDefinitions(beanFactory), and then it calls loadBeanDefinitions(reader), in which reader is XmlBeanDefinitionReader implements BeanDefinitionReader. This class loads a DOM document and applies the reader to it. The document reader will register each bean definition with the given bean factory, talking to the latter’s implementation of the BeanDefinitionRegistry interface.

1
2
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

The reader has the loadBeanDefinitions(configLocations) method and the inner method of XmlBeanDefinitionReader provides a loadBeanDefinitions(encodedResource) where EncodedResource is the resource descriptor for the XML file, allowing to specify an encoding to use for parsing the file.

Method doLoadDocument(inputSource, resource) actually load bean definitions from the specified XML file. DefaultDocumentLoader that implements DocumentLoader loads a Document document from the supplied InputSource source by calling method loadDocument(InputSource, EntityResolver, ...) (that is a classical design pattern).

Further topics: InputSource of Spring Resource,ThreadLocal of Java SE concurrency, context.getBean(...) of how to get instantialization of service.