简单记录spring 实现IOC的流程

IOC: Inversion of Control， 指由spring容器来生成对象并且完成对象的装配。

 

下面来看下这两个问题：（下面是以 ClassPathXmlApplicationContext为起点）

1， spring如何生成对象？

2， spring如何完成对象组装？

 

完成这个的核心是通过BeanFactory，下面是BeanFactory的代码：

public interface BeanFactory {	
	String FACTORY_BEAN_PREFIX = "&";

	Object getBean(String name) throws BeansException;

	<T> T getBean(String name, Class<T> requiredType) throws BeansException;

	<T> T getBean(Class<T> requiredType) throws BeansException;

	Object getBean(String name, Object... args) throws BeansException;

	boolean containsBean(String name);

	boolean isSingleton(String name) throws NoSuchBeanDefinitionException;

	boolean isTypeMatch(String name, Class<?> targetType) throws NoSuchBeanDefinitionException;

	Class<?> getType(String name) throws NoSuchBeanDefinitionException;

	String[] getAliases(String name);
}

 从上面可知，如果有了BeanFactory，就可以通过getBean的方法去获取想要的对象，后面组装的事情也就顺理成章了。

 

下面是简单的spring的demo，代码可以通过附件下载，部分测试代码如下：

public class SpringDemo {
	public static void main(String[] args) {
		ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath*:/beans.xml");
		AppleService appleService = context.getBean("appleService", AppleService.class);
		Apple apple = appleService.createApple();
		System.out.println(apple);
	}
}

 

代码的入口从ClassPathXmlApplicationContext开始，下面是ClassPathXmlApplicationContext的代码：

public ClassPathXmlApplicationContext(String[] paths, Class clazz, ApplicationContext parent)
			throws BeansException {
		super(parent);
		Assert.notNull(paths, "Path array must not be null");
		Assert.notNull(clazz, "Class argument must not be null");
		this.configResources = new Resource[paths.length];
		for (int i = 0; i < paths.length; i++) {
			this.configResources[i] = new ClassPathResource(paths[i], clazz);
		}
		refresh();
	}

调用父类AbstractApplicationContext的refresh方法，

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);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}
		}
	}

 在obtainFreshBeanFactory方法中，创建DefaultListableBeanFactory，然后通过DocumentBuilderFactory将spring配置文件转成Document对象，然后根据不同的节点名进行处理，解析元素代码如下：

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
                //IMPORT_ELEMENT = "import"
		if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
			importBeanDefinitionResource(ele);
		}
                //ALIAS_ELEMENT = "alias"
		else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
			processAliasRegistration(ele);
		}
                //BEAN_ELEMENT="bean"
		else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
			processBeanDefinition(ele, delegate);
		}
                //NESTED_BEANS_ELEMENT = "beans"
		else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
			// recurse
			doRegisterBeanDefinitions(ele);
		}
	}

 以bean节点解析为例，看下parseBeanDefinitionElement的方法

 

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
                //解析bean节点
		BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
		if (bdHolder != null) {
			bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
			try {
				// Register the final decorated instance.
                                //将BeanDefinition注册到BeanFactory中（后面有解释）
				BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
			}
			catch (BeanDefinitionStoreException ex) {
				getReaderContext().error("Failed to register bean definition with name '" +
						bdHolder.getBeanName() + "'", ele, ex);
			}
			// Send registration event.
			getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
		}
	}

 下面是调用BeanDefinitionParserDelegate的parseBeanDefinitionAttributes方法，将xml配置的bean的属性（lazy-init, scope, destroy-method等属性）设置到BeanDefinition中。

 

 

public AbstractBeanDefinition parseBeanDefinitionAttributes(Element ele, String beanName,
			BeanDefinition containingBean, AbstractBeanDefinition bd) {

		if (ele.hasAttribute(SCOPE_ATTRIBUTE)) {
			// Spring 2.x "scope" attribute
			bd.setScope(ele.getAttribute(SCOPE_ATTRIBUTE));
			if (ele.hasAttribute(SINGLETON_ATTRIBUTE)) {
				error("Specify either 'scope' or 'singleton', not both", ele);
			}
		}
		else if (ele.hasAttribute(SINGLETON_ATTRIBUTE)) {
			// Spring 1.x "singleton" attribute
			bd.setScope(TRUE_VALUE.equals(ele.getAttribute(SINGLETON_ATTRIBUTE)) ?
					BeanDefinition.SCOPE_SINGLETON : BeanDefinition.SCOPE_PROTOTYPE);
		}
		else if (containingBean != null) {
			// Take default from containing bean in case of an inner bean definition.
			bd.setScope(containingBean.getScope());
		}

		if (ele.hasAttribute(ABSTRACT_ATTRIBUTE)) {
			bd.setAbstract(TRUE_VALUE.equals(ele.getAttribute(ABSTRACT_ATTRIBUTE)));
		}

		String lazyInit = ele.getAttribute(LAZY_INIT_ATTRIBUTE);
		if (DEFAULT_VALUE.equals(lazyInit)) {
			lazyInit = this.defaults.getLazyInit();
		}
		bd.setLazyInit(TRUE_VALUE.equals(lazyInit));

		String autowire = ele.getAttribute(AUTOWIRE_ATTRIBUTE);
		bd.setAutowireMode(getAutowireMode(autowire));

		String dependencyCheck = ele.getAttribute(DEPENDENCY_CHECK_ATTRIBUTE);
		bd.setDependencyCheck(getDependencyCheck(dependencyCheck));

		if (ele.hasAttribute(DEPENDS_ON_ATTRIBUTE)) {
			String dependsOn = ele.getAttribute(DEPENDS_ON_ATTRIBUTE);
			bd.setDependsOn(StringUtils.tokenizeToStringArray(dependsOn, MULTI_VALUE_ATTRIBUTE_DELIMITERS));
		}

		String autowireCandidate = ele.getAttribute(AUTOWIRE_CANDIDATE_ATTRIBUTE);
		if ("".equals(autowireCandidate) || DEFAULT_VALUE.equals(autowireCandidate)) {
			String candidatePattern = this.defaults.getAutowireCandidates();
			if (candidatePattern != null) {
				String[] patterns = StringUtils.commaDelimitedListToStringArray(candidatePattern);
				bd.setAutowireCandidate(PatternMatchUtils.simpleMatch(patterns, beanName));
			}
		}
		else {
			bd.setAutowireCandidate(TRUE_VALUE.equals(autowireCandidate));
		}

		if (ele.hasAttribute(PRIMARY_ATTRIBUTE)) {
			bd.setPrimary(TRUE_VALUE.equals(ele.getAttribute(PRIMARY_ATTRIBUTE)));
		}

		if (ele.hasAttribute(INIT_METHOD_ATTRIBUTE)) {
			String initMethodName = ele.getAttribute(INIT_METHOD_ATTRIBUTE);
			if (!"".equals(initMethodName)) {
				bd.setInitMethodName(initMethodName);
			}
		}
		else {
			if (this.defaults.getInitMethod() != null) {
				bd.setInitMethodName(this.defaults.getInitMethod());
				bd.setEnforceInitMethod(false);
			}
		}

		if (ele.hasAttribute(DESTROY_METHOD_ATTRIBUTE)) {
			String destroyMethodName = ele.getAttribute(DESTROY_METHOD_ATTRIBUTE);
			if (!"".equals(destroyMethodName)) {
				bd.setDestroyMethodName(destroyMethodName);
			}
		}
		else {
			if (this.defaults.getDestroyMethod() != null) {
				bd.setDestroyMethodName(this.defaults.getDestroyMethod());
				bd.setEnforceDestroyMethod(false);
			}
		}

		if (ele.hasAttribute(FACTORY_METHOD_ATTRIBUTE)) {
			bd.setFactoryMethodName(ele.getAttribute(FACTORY_METHOD_ATTRIBUTE));
		}
		if (ele.hasAttribute(FACTORY_BEAN_ATTRIBUTE)) {
			bd.setFactoryBeanName(ele.getAttribute(FACTORY_BEAN_ATTRIBUTE));
		}

		return bd;
	}

 在上面的processBeanDefinition方法中，解析完某个节点，生成BeanDefinition时，会调用BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry())方法，然后向BeanFactory注册该BeanDefinition对象，代码如下：

 

 

//DefaultListableBeanFactory.java
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException {

		...
		synchronized (this.beanDefinitionMap) {
			Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
			if (oldBeanDefinition != null) {
				if (!this.allowBeanDefinitionOverriding) {
					throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
							"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
							"': There is already [" + oldBeanDefinition + "] bound.");
				}
			}
			else {
				this.beanDefinitionNames.add(beanName);
				this.frozenBeanDefinitionNames = null;
			}
			this.beanDefinitionMap.put(beanName, beanDefinition);
		}

		resetBeanDefinition(beanName);
	}

 然后重新回到AbstractApplicationContext的refresh方法，调用invokeBeanFactoryPostProcessors（spring提供了BeanFactoryPostProcessor扩展点，可以通过实现BeanFactoryPostProcess来初始化某些参数），再通过调用refresh方法中的finishBeanFactoryInitialization来完成对象的创建。代码如下：

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		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));
		}

		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
                //创建并且组装对象
		beanFactory.preInstantiateSingletons();
	}

然后进入DefaultListableBeanFactory的preInstantiateSingletons方法，代码如下，beanDefinitionMap存放的是前面解析配置文件得到的<beanName, BeanDefinition>的map数据。

public void preInstantiateSingletons() throws BeansException {
		...
		List<String> beanNames;
		synchronized (this.beanDefinitionMap) {
			// Iterate over a copy to allow for init methods which in turn register new bean definitions.
			// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
			beanNames = new ArrayList<String>(this.beanDefinitionNames);
		}
		for (String beanName : beanNames) {
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				if (isFactoryBean(beanName)) {
					final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
					boolean isEagerInit;
					if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
						isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
							public Boolean run() {
								return ((SmartFactoryBean<?>) factory).isEagerInit();
							}
						}, getAccessControlContext());
					}
					else {
						isEagerInit = (factory instanceof SmartFactoryBean &&
								((SmartFactoryBean<?>) factory).isEagerInit());
					}
					if (isEagerInit) {
						getBean(beanName);
					}
				}
				else {
					getBean(beanName);
				}
			}
		}
	}

 在getBean方法中，会调用AbstractBeanFactory的doGetBean来创建对象，方法如下：

protected <T> T doGetBean(
			final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {
                ...
		if (mbd.isSingleton()) {
			sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
				public Object getObject() throws BeansException {
					try {
						return createBean(beanName, mbd, args);
					}
					catch (BeansException ex) {
						// Explicitly remove instance from singleton cache: It might have been put there
						// eagerly by the creation process, to allow for circular reference resolution.
						// Also remove any beans that received a temporary reference to the bean.
						destroySingleton(beanName);
						throw ex;
					}
				}
			});
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
		}

		else if (mbd.isPrototype()) {
			// It's a prototype -> create a new instance.
			Object prototypeInstance = null;
			try {
				beforePrototypeCreation(beanName);
				prototypeInstance = createBean(beanName, mbd, args);
			}
			finally {
				afterPrototypeCreation(beanName);
			}
			bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
		}
                ...
		return (T) bean;
	}

 在createBean中，会调用AbstractAutowireCapableBeanFactory的doCreateBean方法，代码如下：（这个方法显示了对象的创建以及后续初始化的流程）

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
                //实例化对象
		if (instanceWrapper == null) {
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
		Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

		// Allow post-processors to modify the merged bean definition.
		synchronized (mbd.postProcessingLock) {
			if (!mbd.postProcessed) {
                               //调用BeanPostProcessor处理
				applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				mbd.postProcessed = true;
			}
		}

		// Eagerly cache singletons to be able to resolve circular references
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isDebugEnabled()) {
				logger.debug("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			addSingletonFactory(beanName, new ObjectFactory() {
				public Object getObject() throws BeansException {
					return getEarlyBeanReference(beanName, mbd, bean);
				}
			});
		}

		// Initialize the bean instance.
		Object exposedObject = bean;
		try {
                        //设置bean的参数（通过property标签配置）
			populateBean(beanName, mbd, instanceWrapper);
			if (exposedObject != null) {
                                //调用init-method
				exposedObject = initializeBean(beanName, exposedObject, mbd);
			}
		}
		catch (Throwable ex) {
			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
				throw (BeanCreationException) ex;
			}
			else {
				throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
			}
		}

		...
		return exposedObject;
	}

 继续往下看createBeanInstance方法，通过调用instantiateBean实例化对象，代码如下：

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
		try {
			Object beanInstance;
			final BeanFactory parent = this;
			if (System.getSecurityManager() != null) {
				beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
					public Object run() {
						return getInstantiationStrategy().instantiate(mbd, beanName, parent);
					}
				}, getAccessControlContext());
			}
			else {
				beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
			}
			BeanWrapper bw = new BeanWrapperImpl(beanInstance);
			initBeanWrapper(bw);
			return bw;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
		}
	}

 InstantiationStrategy有两种，SimpleInstantiationStrategy和CglibSubclassingInstantiationStrategy。关系如下：

 

spring对CglibSubclassingInstantiationStrategy的使用场景注释：Using Method Injection features requires CGLIB on the classpath. However, the core IoC container will still run without CGLIB being available.

下面再来看SimpleInstantiationStrategy的instantiate方法，代码如下：

public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
		// Don't override the class with CGLIB if no overrides.
		if (beanDefinition.getMethodOverrides().isEmpty()) {
			Constructor<?> constructorToUse;
			synchronized (beanDefinition.constructorArgumentLock) {
				constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod;
				if (constructorToUse == null) {
					final Class clazz = beanDefinition.getBeanClass();
					if (clazz.isInterface()) {
						throw new BeanInstantiationException(clazz, "Specified class is an interface");
					}
					try {
						if (System.getSecurityManager() != null) {
							constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor>() {
								public Constructor run() throws Exception {
									return clazz.getDeclaredConstructor((Class[]) null);
								}
							});
						}
						else {
							constructorToUse =	clazz.getDeclaredConstructor((Class[]) null);
						}
						beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
					}
					catch (Exception ex) {
						throw new BeanInstantiationException(clazz, "No default constructor found", ex);
					}
				}
			}
			return BeanUtils.instantiateClass(constructorToUse);
		}
		else {
			// Must generate CGLIB subclass.
			return instantiateWithMethodInjection(beanDefinition, beanName, owner);
		}
	}

 通过上面的流程，这样对象的创建就完成了。

 

受篇幅所限，只是介绍了对象的生成，对象的装配具体过程，下篇来写了～