What is IOC
IOC stands for Inversion of Control. Instead of manually instantiating objects using new, the responsibility for object creation is delegated to the Spring framework. This approach significantly reduces application complexity and minimizes coupling between system components.
Application Entry Point
public static void main(String[] args) {
ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:application.xml");
IMessageService service = ctx.getBean(IMessageService.class);
service.say("hello world");
}
XML Configuration
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
<bean id="messageService" class="org.springframework.test.web.test.service.impl.MessageServiceImpl"/>
<context:component-scan base-package="org.springframework.test.web.test"/>
</beans>
IOC Implementation Flow
Entry Point
The ClassPathXmlApplicationContext constructor accepts configuration file locations and triggers the container refresh process:
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
Core Refresh Logic
The AbstractApplicationContext.refresh() method orchestrates the entire container lifecycle:
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
prepareRefresh();
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
prepareBeanFactory(beanFactory);
try {
postProcessBeanFactory(beanFactory);
invokeBeanFactoryPostProcessors(beanFactory);
registerBeanPostProcessors(beanFactory);
initMessageSource();
initApplicationEventMulticaster();
onRefresh();
registerListeners();
finishBeanFactoryInitialization(beanFactory);
finishRefresh();
}
catch (BeansException ex) {
destroyBeans();
cancelRefresh(ex);
throw ex;
}
finally {
resetCommonCaches();
}
}
}
BeanFactory Creation
The AbstractRefreshableApplicationContext.refreshBeanFactory() method handles BeanFactory instantiation and bean definition loading:
@Override
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
DefaultListableBeanFactory factory = createBeanFactory();
factory.setSerializationId(getId());
customizeBeanFactory(factory);
loadBeanDefinitions(factory);
this.beanFactory = factory;
}
catch (IOException ex) {
throw new ApplicationContextException(
"I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
Bean Definition Registration
The DefaultBeanDefinitionDocumentReader processes XML elements and registers bean definitions:
protected void doRegisterBeanDefinitions(Element root) {
BeanDefinitionParserDelegate previous = this.delegate;
this.delegate = createDelegate(getReaderContext(), root, previous);
if (this.delegate.isDefaultNamespace(root)) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
return;
}
}
}
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = previous;
}
Element Parsing
The DefaultBeanDefinitionDocumentReader.parseBeanDefinitions() iterates through XML child nodes:
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element) {
Element ele = (Element) node;
if (delegate.isDefaultNamespace(ele)) {
parseDefaultElement(ele, delegate);
}
else {
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}
Custom Namespace Handling
The BeanDefinitionParserDelegate.parseCustomElement() resolves namespace handlers:
public BeanDefinition parseCustomElement(Element ele, @Nullable BeanDefinition containingBd) {
String namespaceUri = getNamespaceURI(ele);
if (namespaceUri == null) {
return null;
}
NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver()
.resolve(namespaceUri);
if (handler == null) {
error("Unable to locate Spring NamespaceHandler for XML schema namespace ["
+ namespaceUri + "]", ele);
return null;
}
return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}
The NamespaceHandlerSupport.parse() delegates to the appropriate parser:
@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
BeanDefinitionParser parser = findParserForElement(element, parserContext);
return (parser != null ? parser.parse(element, parserContext) : null);
}
Component Scanning
The ComponentScanBeanDefinitionParser handles package scenning for annotated components:
@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
String basePackage = element.getAttribute(BASE_PACKAGE_ATTRIBUTE);
basePackage = parserContext.getReaderContext().getEnvironment()
.resolvePlaceholders(basePackage);
String[] basePackages = StringUtils.tokenizeToStringArray(basePackage,
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);
registerComponents(parserContext.getReaderContext(), beanDefinitions, element);
return null;
}
Singleton Initialization
The AbstractApplicationContext.finishBeanFactoryInitialization() completes the BeanFactory setup:
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(
strVal -> getEnvironment().resolvePlaceholders(strVal));
}
String[] weaverAwareNames = beanFactory.getBeanNamesForType(
LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
beanFactory.setTempClassLoader(null);
beanFactory.freezeConfiguration();
beanFactory.preInstantiateSingletons();
}
The DefaultListableBeanFactory.preInstantiateSingletons() iterates through all bean definitions:
public void preInstantiateSingletons() throws BeansException {
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
getBean(beanName);
}
}
}
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
SmartInitializingSingleton smartSingleton =
(SmartInitializingSingleton) singletonInstance;
smartSingleton.afterSingletonsInstantiated();
}
}
}
Bean Retrieval and Creation
The AbstractBeanFactory.doGetBean() method handles the actual bean creation:
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType,
@Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
String beanName = transformedBeanName(name);
Object bean;
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
return (T) parentBeanFactory.getBean(nameToLookup, requiredType);
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '"
+ beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
getBean(dep);
}
}
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
Object prototypeInstance = createBean(beanName, mbd, args);
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
Scope scope = this.scopes.get(scopeName);
Object scopedInstance = scope.get(beanName, () -> {
return createBean(beanName, mbd, args);
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
if (requiredType != null && !requiredType.isInstance(bean)) {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
return (T) bean;
}
Singleton Registration
The DefaultSingletonBeanRegistry.getSingleton() manages singleton storage:
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed during destruction");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
try {
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
throw ex;
}
finally {
afterSingletonCreation(beanName);
}
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
The DefaultSingletonBeanRegistry.addSingleton() stores the created bean:
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}