以XmlBeanFactory為例，最簡單的取bean方式是：

BeanFactory factory = new XmlBeanFactory(new FileSystemResource("D:\\workspace\\JavaApplication2\\src\\javaapplication2\\spring\\beans.xml")); Car obj = (Car)factory.getBean("car");

Bean的配置文件內容也很簡單：

<bean id="vehicle" abstract="true"> <property name="wheel" value="Four wheeler"/> </bean> <bean id="car" class="javaapplication2.spring.Car" parent="vehicle"> <property name="dicky" value="Flat dicky"/> </bean>

先看起始點，載入先走AbstractBeanFactory

public Object getBean(String name) throws BeansException { return doGetBean(name, null, null, false); }

doGetBean方法中：

// Create bean instance. if (mbd.isSingleton()) { //傳入一個內聯類ObjectFactory并實現了getObject方法。 sharedInstance = getSingleton(beanName, new ObjectFactory() { 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); //有異常則銷毀bean throw ex; } } }); bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); //此處開始實例化bean

通過new ObjectFactory()的回調方法，回調當前類繼承的createBean方法，該方法在父類AbstractAutowireCapableBeanFactory中：

AbstractAutowireCapableBeanFactory->

protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) throws BeanCreationException { // Make sure bean class is actually resolved at this point. resolveBeanClass(mbd, beanName); //載入該bean的class，并放置到mbd里面,bean的生成不在這里。 // Prepare method overrides. try { mbd.prepareMethodOverrides(); } catch (BeanDefinitionValidationException ex) { throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName, "Validation of method overrides failed", ex); } try { // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance. Object bean = resolveBeforeInstantiation(beanName, mbd); //嘗試獲取一個proxy，普通bean這里一般是空的返回 if (bean != null) { return bean; } } catch (Throwable ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "BeanPostProcessor before instantiation of bean failed", ex); } Object beanInstance = doCreateBean(beanName, mbd, args); //開始create bean的實例，mbd中包括了需要的class if (logger.isDebugEnabled()) { logger.debug("Finished creating instance of bean '" + beanName + "'"); } return beanInstance; }

進入AbstractBeanFactory中的protected Class resolveBeanClass方法：

try { if (mbd.hasBeanClass()) { return mbd.getBeanClass(); } if (System.getSecurityManager() != null) { return AccessController.doPrivileged(new PrivilegedExceptionAction<Class>() { public Class run() throws Exception { return doResolveBeanClass(mbd, typesToMatch); } }, getAccessControlContext()); } else { return doResolveBeanClass(mbd, typesToMatch); <---還要繼續進去看生成方法。 } }

轉入doResolveBeanClass：

private Class doResolveBeanClass(RootBeanDefinition mbd, Class... typesToMatch) throws ClassNotFoundException { if (!ObjectUtils.isEmpty(typesToMatch)) { ClassLoader tempClassLoader = getTempClassLoader(); if (tempClassLoader != null) { if (tempClassLoader instanceof DecoratingClassLoader) { DecoratingClassLoader dcl = (DecoratingClassLoader) tempClassLoader; for (Class<?> typeToMatch : typesToMatch) { dcl.excludeClass(typeToMatch.getName()) } } String className = mbd.getBeanClassName(); return (className != null ? ClassUtils.forName(className, tempClassLoader) : null); //通過自己的ClassUtils的forName方法來實例化class } } return mbd.resolveBeanClass(getBeanClassLoader()); <----這里傳入了bean的classloader，下面繼續看這里 }

AbstractBeanDefinition->resolveBeanClass

public Class resolveBeanClass(ClassLoader classLoader) throws ClassNotFoundException { String className = getBeanClassName(); if (className == null) { return null; } Class resolvedClass = ClassUtils.forName(className, classLoader);//classloader傳入后，仍然是用forName方法加載class this.beanClass = resolvedClass; return resolvedClass; }

再來看forName做了些什么

ClassUtils ->

ClassLoader classLoaderToUse = classLoader; if (classLoaderToUse == null) { classLoaderToUse = getDefaultClassLoader(); } try { return classLoaderToUse.loadClass(name); //也比較簡單，直接調用loadClass方法加載 }

最終將class load進來。

進入

AbstractAutowireCapableBeanFactory->instantiateBean

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) ...這里省略沒用的 else { beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);//下面看這里的實例化 } BeanWrapper bw = new BeanWrapperImpl(beanInstance); //返回一個包裝類對象 initBeanWrapper(bw); return bw;

SimpleInstantiationStrategy->instantiate

public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) synchronized (beanDefinition.constructorArgumentLock) { constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod; ... } return BeanUtils.instantiateClass(constructorToUse); //BeanUtils來初始化實例 ，給出了實例化需要的構造函數

再來看BeanUtils的實例化方法，比較簡單，直接用反射的構造函數來newInstance。

BeanUtils->

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException { try { ReflectionUtils.makeAccessible(ctor); return ctor.newInstance(args); }

AbstractAutowireCapableBeanFactory->

Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) // Initialize the bean instance. Object exposedObject = bean; try { populateBean(beanName, mbd, instanceWrapper); if (exposedObject != null) { exposedObject = initializeBean(beanName, exposedObject, mbd); } } return exposedObject; //返回給AbstractBeanFactory

總結

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