【Review】Spring源码分析之bean加载 川长思鸟来 2022-09-09 06:18 173阅读 0赞 ### 1、背景 ### IOC容器,是Spring的两大特性之一。作为容器,他可以方便地管理bean,以及bean与bean之间的依赖关系。 本篇文章,主要从源码角度介绍在ioc容器中,bean的加载流程。 ### 2、源码 ### #### (1)容器的重要接口/类 #### ![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAeXlzeA_size_20_color_FFFFFF_t_70_g_se_x_16] #### (2) Bean Definition #### 入口:XmlBeanFactory 构造函数中调用 XmlBeanDefinitionReader的loadBeanDefinitions(resource) **大致分为三个步骤: 验证xml文件的正确性; 加载xml,转为Document对象; 注册bean定义。** public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException { super(parentBeanFactory); this.reader.loadBeanDefinitions(resource); } protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException { //加载xml,转为Document对象 Document doc = doLoadDocument(inputSource, resource); //注册bean定义 return registerBeanDefinitions(doc, resource); } 以下是注册bean定义的详细代码,可以看到,最后返回了BeanDefinition对象。 public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException { BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader(); int countBefore = getRegistry().getBeanDefinitionCount(); documentReader.registerBeanDefinitions(doc, createReaderContext(resource)); return getRegistry().getBeanDefinitionCount() - countBefore; } public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) { this.readerContext = readerContext; logger.debug("Loading bean definitions"); Element root = doc.getDocumentElement(); doRegisterBeanDefinitions(root); } protected void doRegisterBeanDefinitions(Element root) { preProcessXml(root); parseBeanDefinitions(root, this.delegate); postProcessXml(root); } 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); } } public BeanDefinition parseCustomElement(Element ele) { return parseCustomElement(ele, null); } public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) { String namespaceUri = getNamespaceURI(ele); 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)); } #### (3) Bean Loading–normal #### 以AbstractBeanFactory的getBean()为例: **获取beanName; 尝试从缓存中获取(单例模式下); 合并bean定义,merge; 实例化; 填充属性; 初始化。** public Object getBean(String name) throws BeansException { return doGetBean(name, null, null, false); } protected <T> T doGetBean( final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException { //获取beanName final String beanName = transformedBeanName(name); Object bean; //尝试从缓存中获取 Object sharedInstance = getSingleton(beanName); if (sharedInstance != null && args == null) { bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); }else { ...... //合并bean定义 final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); checkMergedBeanDefinition(mbd, beanName, args); //处理bean依赖 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); } } //创建bean实例,根据不同的scope if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { @Override 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); } else { String scopeName = mbd.getScope(); final Scope scope = this.scopes.get(scopeName); if (scope == null) { throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'"); } try { Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { beforePrototypeCreation(beanName); try { return createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } } }); bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd); } catch (IllegalStateException ex) { throw new BeanCreationException(beanName, "Scope '" + scopeName + "' is not active for the current thread; consider " + "defining a scoped proxy for this bean if you intend to refer to it from a singleton", ex); } } } catch (BeansException ex) { cleanupAfterBeanCreationFailure(beanName); throw ex; } } return (T) bean; } 我们继续看createBean()方法的细节。 protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException { Object beanInstance = doCreateBean(beanName, mbdToUse, args); return beanInstance; } protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args){ //实例化bean. BeanWrapper instanceWrapper = null; if (mbd.isSingleton()) { instanceWrapper = this.factoryBeanInstanceCache.remove(beanName); } if (instanceWrapper == null) { instanceWrapper = createBeanInstance(beanName, mbd, args); } final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null); Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null); mbd.resolvedTargetType = beanType; // Allow post-processors to modify the merged bean definition. synchronized (mbd.postProcessingLock) { if (!mbd.postProcessed) { 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) { addSingletonFactory(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { return getEarlyBeanReference(beanName, mbd, bean); } }); } //【重要!!!】初始化bean 实例 Object exposedObject = bean; //填充属性 populateBean(beanName, mbd, instanceWrapper); if (exposedObject != null) { exposedObject = initializeBean(beanName, exposedObject, mbd); } if (earlySingletonExposure) { Object earlySingletonReference = getSingleton(beanName, false); if (earlySingletonReference != null) { if (exposedObject == bean) { exposedObject = earlySingletonReference; } else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { String[] dependentBeans = getDependentBeans(beanName); Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length); for (String dependentBean : dependentBeans) { if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) { actualDependentBeans.add(dependentBean); } } } } } //注册bean try { registerDisposableBeanIfNecessary(beanName, bean, mbd); } catch (BeanDefinitionValidationException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex); } return exposedObject; } //实例化bean 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>() { @Override 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); } } public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) { if (bd.getMethodOverrides().isEmpty()) { //通过构造函数实例化bean Constructor<?> constructorToUse; Constructor<?> constructorToUse; synchronized (bd.constructorArgumentLock) { constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod; if (constructorToUse == null) { final Class<?> clazz = bd.getBeanClass(); if (clazz.isInterface()) { throw new BeanInstantiationException(clazz, "Specified class is an interface"); } try { if (System.getSecurityManager() != null) { constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() { @Override public Constructor<?> run() throws Exception { return clazz.getDeclaredConstructor((Class[]) null); } }); } else { constructorToUse = clazz.getDeclaredConstructor((Class[]) null); } bd.resolvedConstructorOrFactoryMethod = constructorToUse; } catch (Throwable ex) { throw new BeanInstantiationException(clazz, "No default constructor found", ex); } } } return BeanUtils.instantiateClass(constructorToUse); }else { return instantiateWithMethodInjection(bd, beanName, owner); } } //填充属性 protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) { ...... if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) { MutablePropertyValues newPvs = new MutablePropertyValues(pvs); // Add property values based on autowire by name if applicable. if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) { autowireByName(beanName, mbd, bw, newPvs); } // Add property values based on autowire by type if applicable. if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) { autowireByType(beanName, mbd, bw, newPvs); } pvs = newPvs; } ...... applyPropertyValues(beanName, mbd, bw, pvs); } #### (4) Bean Loading–FactoryBean #### 在(3)中,我们介绍了普通bean加载的流程。但有时候,对于一个有复杂的定义的bean,如果扔通过上述方式创建bean,就会比较繁琐了。Spring提供了工厂类来处理这种情况,即FactoryBean。 实现了FactoryBean接口的bean,在加载时,容器会进行判断,若要加载的bean是FactoryBean类型的,就会根据其定义,创建一个新的bean,并将新的bean返回。 源码分析,以\*\*DefaultListableBeanFactory类的getBean(Class)\*\*为例。 public <T> T getBean(Class<T> requiredType, Object... args) throws BeansException { NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args); if (namedBean != null) { return namedBean.getBeanInstance(); } BeanFactory parent = getParentBeanFactory(); if (parent != null) { return parent.getBean(requiredType, args); } throw new NoSuchBeanDefinitionException(requiredType); } private <T> NamedBeanHolder<T> resolveNamedBean(Class<T> requiredType, Object... args) throws BeansException { //根据类型加载beanName String[] candidateNames = getBeanNamesForType(requiredType); ...... //根据获取到的beanName加载bean getBean(); } private String[] doGetBeanNamesForType(ResolvableType type, boolean includeNonSingletons, boolean allowEagerInit) { List<String> result = new ArrayList<String>(); //校验所有bean的定义 for (String beanName : this.beanDefinitionNames) { // Only consider bean as eligible if the bean name // is not defined as alias for some other bean. if (!isAlias(beanName)) { //为了方便查看,把try catch去掉了 //获取根的bean定义 RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); // Only check bean definition if it is complete. if (!mbd.isAbstract() && (allowEagerInit || ((mbd.hasBeanClass() || !mbd.isLazyInit() || isAllowEagerClassLoading())) && !requiresEagerInitForType(mbd.getFactoryBeanName()))) { // 【重要!!!】判断是否是FactoryBean类型 boolean isFactoryBean = isFactoryBean(beanName, mbd); BeanDefinitionHolder dbd = mbd.getDecoratedDefinition(); boolean matchFound = (allowEagerInit || !isFactoryBean || (dbd != null && !mbd.isLazyInit()) || containsSingleton(beanName)) && (includeNonSingletons || (dbd != null ? mbd.isSingleton() : isSingleton(beanName))) && isTypeMatch(beanName, type); if (!matchFound && isFactoryBean) { // 如果是FactoryBean,修改BeanName,加上"&"前缀。 beanName = FACTORY_BEAN_PREFIX + beanName; matchFound = (includeNonSingletons || mbd.isSingleton()) && isTypeMatch(beanName, type); } if (matchFound) { result.add(beanName); } } } } // 校验已注册的实例 for (String beanName : this.manualSingletonNames) { //和上述的处理类似 // In case of FactoryBean, match object created by FactoryBean. if (isFactoryBean(beanName)) { if ((includeNonSingletons || isSingleton(beanName)) && isTypeMatch(beanName, type)) { result.add(beanName); // Match found for this bean: do not match FactoryBean itself anymore. continue; } // In case of FactoryBean, try to match FactoryBean itself next. beanName = FACTORY_BEAN_PREFIX + beanName; } // Match raw bean instance (might be raw FactoryBean). if (isTypeMatch(beanName, type)) { result.add(beanName); } } return StringUtils.toStringArray(result); } protected boolean isFactoryBean(String beanName, RootBeanDefinition mbd) { Class<?> beanType = predictBeanType(beanName, mbd, FactoryBean.class); return (beanType != null && FactoryBean.class.isAssignableFrom(beanType)); } 附: bean加载的详细流程图(图源网络) ![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAeXlzeA_size_20_color_FFFFFF_t_70_g_se_x_16 1] ### 3、小结 ### * 容器的bean和FactoryBean: 大多数框架、容器、工具类,都会提供至少两种模式。一是适用于一般情况的处理方式,另一类是用户自定义模式,方便扩展和适用其他场景。 * Spring中用于存储bean定义的map和BeanUtils 虽然做一件事的流程很复杂,但最根本的实现方式是很简单的。要深入研究,也要大胆设想,然后去验证。分清主次,剔除一些细节的问题,会更方便的掌握主要逻辑。 * populateBean()中的ByName/ByType,判断是否单例等 看源码时,一定要和实践结合起来,分析思考。不要孤立。 [watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAeXlzeA_size_20_color_FFFFFF_t_70_g_se_x_16]: /images/20220829/4d47314c00944b9cb5974eb34c7197bb.png [watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBAeXlzeA_size_20_color_FFFFFF_t_70_g_se_x_16 1]: /images/20220829/f446970d84d24822827a16ea7383d30d.png
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