【Spring源码解析】—— 依赖注入结合SpringMVC Demo-xml配置理解
在IOC容器初始化的梳理之后，对依赖注入做一个总结，就是bean实例化的过程，bean的定义有两种方式，一种是xml文件配置，一种是注解，这里是对xml配置文件的依赖注入的介绍，后续对bean与该部分的不同会再做介绍。
先对整个过程做一个整体说明：从refresh()的入口进入之后，因为是注入依赖实例化bean的过程，所以在这个过程中，会不断判断是否有可用的快捷方式，是什么类型的，是singleton的，还是protype类型的，进行getBean获取bean的操作时，针对已经创建的可以直接返回的直接结束，若无则进行创建，创建中会要判断是否有parentFactory，是否有依赖bean，set创建过程中可能需要的条件，例如method overrides的验证和准备等，获取可用的constructor，有可用的构造器，有参数的情况等，都要走自主选择的逻辑，否则全部最后走兜底策略，就是默认构造器，注意，在对constructor选择的过程中，可能会出现无可用，或者是Ambiguous的情况，会抛出异常；具体最终创建bean要用什么策略，提供两种选择
一、Demo部分的修改：
首先是xml配置相关，首先在cn.lx.controller包中新建一个TestController2.java，将之前配置的注解的形式全部注释掉，在xml中将bean配置清楚，TestController2的类内容如下：
//@Controller
public class TestController2 {
// @RequestMapping("/test.form")
public void execute(){
return ;
}
}
修改applicationContext.xml文件，为如下：
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="simpleUrlHandlerMapping" class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping"> <property name="mappings"> <props> <prop key="/test.form">startControl</prop> </props> </property> </bean> <bean id="startControl" class="cn.lx.controller.TestController2"></bean> </beans>
二、依赖注入过程介绍：
依赖注入会发生两种场景，一种是：IOC容器初始化到最后阶段，会将none-Lazy-init的bean全部都实例化，其他的会在具体用到的时候实例化；先从第一种情况IOC容器初始化最后阶段来看实例化的整个过程，实例化的过程后续阶段都是相同的，只是入口不同而已
现在，正文开始，共分为三个步骤：入口-xml配置方式的实例化入口、获取bean、new-bean（实例化bean）
入口部分-xml配置方式的实例化入口：
从AbstractApplicationContext类的refresh()方法中，之前在IOC初始化中主要从obtainFreshBeanFactory()方法调用后得到了ConfigurableListableBeanFactory类型的beanFactory，在beanFactry中已经对xml文件做了解析并且将需要的信息都存储到了beanDefinitionMap中，之后将beanFactory作为参数传递给finishBeanFactoryInitialization方法，进入到其具体实现中：
//完成beanfactory的初始化，初始化所有保留的单例bean protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) { // Initialize conversion service for this context. //类型转换服务，用于提供线程安全的类型转换 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)); } // Register a default embedded value resolver if no bean post-processor // (such as a PropertyPlaceholderConfigurer bean) registered any before: // at this point, primarily for resolution in annotation attribute values. if (!beanFactory.hasEmbeddedValueResolver()) { beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal)); } // 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. //预实例化，对未设置lazy-init=true的进行预实例化处理 beanFactory.preInstantiateSingletons(); }
之后，之后看最后一步，通过beanFactory.preInstantiateSingletons()进入到：DefaultListableBeanFactory类中，该方法会进行判断，会判断是否不是抽象且为singleton且是non lazy-init的，就会进入到是否为FactoryBean的判断，最终发现不是FactoryBean,就进入了else的逻辑分支，即：getBean(beanName)了，从这个部分就进入到下一个关键阶段：获取Bean阶段
@Override public void preInstantiateSingletons() throws BeansException { if (logger.isTraceEnabled()) { logger.trace("Pre-instantiating singletons in " + this); } // 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. List<String> beanNames = new ArrayList<>(this.beanDefinitionNames); // Trigger initialization of all non-lazy singleton beans... //触发对于所有非懒加载的singleton beans的实例化 //通过getBean()方法重点执行，其中getBean会找到doGetBean for (String beanName : beanNames) { RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); //判断是否不为abstract、为singleton、lazyinit属性为false，则进入到下一步 if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) { //判断是否是FactoryBean if (isFactoryBean(beanName)) { Object bean = getBean(FACTORY_BEAN_PREFIX + beanName); if (bean instanceof FactoryBean) { final FactoryBean<?> factory = (FactoryBean<?>) bean; boolean isEagerInit; if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) { isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit, getAccessControlContext()); } else { isEagerInit = (factory instanceof SmartFactoryBean && ((SmartFactoryBean<?>) factory).isEagerInit()); } if (isEagerInit) { getBean(beanName); } } } //判断不是FactoryBean，直接进入getBean逻辑 else { getBean(beanName); } } }
获取Bean阶段：
从亮色的：getBean方法进入到类：AbstractBeanFactory中，然后就看到do真正做事情的来啦：
@Override public Object getBean(String name) throws BeansException { //实际执行 return doGetBean(name, null, null, false); }
兴奋地点击doGetBean，去看看它究竟干了啥？进入到类：AbstractBeanFactory类中，doGetBean所完成的内容主要包含：判断是否已经有存在singleton的beanName，若有可以直接返回，若无则获取此bean所依赖的beans-dependsOn，然后通过一个for循环判断，是否这个中的所有dependsOn中的某个bean是否存在环形依赖，若存在的话，直接抛出异常，无异常则为给定的bean注册依赖bean（依赖bean也会在给定bean销毁之前被销毁），最后走到createBean逻辑，至此进入到下一个阶段：new-bean真正创建的过程了
doGetBean部分的重要代码如下：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException { //规范化beanName final String beanName = transformedBeanName(name); //类型:Object Object bean; // Eagerly check singleton cache for manually registered singletons. //获取已经创建的singleton的bean的实例对象，将之赋给Object的sharedInstance Object sharedInstance = getSingleton(beanName); if (sharedInstance != null && args == null) { if (logger.isTraceEnabled()) { if (isSingletonCurrentlyInCreation(beanName)) { logger.trace("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { logger.trace("Returning cached instance of singleton bean '" + beanName + "'"); } } //对于已经存在的直接返回 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } //没有已经存在的singleton的bean的实例对象 else { // Fail if we're already creating this bean instance: // We're assumably within a circular reference. if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } // Check if bean definition exists in this factory. BeanFactory parentBeanFactory = getParentBeanFactory(); if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // Not found -> check parent. String nameToLookup = originalBeanName(name); if (parentBeanFactory instanceof AbstractBeanFactory) { return ((AbstractBeanFactory) parentBeanFactory).doGetBean( nameToLookup, requiredType, args, typeCheckOnly); } else if (args != null) { // Delegation to parent with explicit args. return (T) parentBeanFactory.getBean(nameToLookup, args); } else if (requiredType != null) { // No args -> delegate to standard getBean method. return parentBeanFactory.getBean(nameToLookup, requiredType); } else { return (T) parentBeanFactory.getBean(nameToLookup); } } if (!typeCheckOnly) { markBeanAsCreated(beanName); } try { final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); checkMergedBeanDefinition(mbd, beanName, args); // Guarantee initialization of beans that the current bean depends on. //确保被依赖的bean要初始化之后，其他的才能继续初始化 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 + "'"); } //给给定bean注册依赖bean registerDependentBean(dep, beanName); try { getBean(dep); } catch (NoSuchBeanDefinitionException ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "'" + beanName + "' depends on missing bean '" + dep + "'", ex); } } } // Create bean instance. //实例化 if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, ()-> { try { //实例化 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, () -> { 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; } } // Check if required type matches the type of the actual bean instance. if (requiredType != null && !requiredType.isInstance(bean)) { try { T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType); if (convertedBean == null) { throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } return convertedBean; } catch (TypeMismatchException ex) { if (logger.isTraceEnabled()) { logger.trace("Failed to convert bean '" + name + "' to required type '" + ClassUtils.getQualifiedName(requiredType) + "'", ex); } throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } return (T) bean; }
createBean过程：
进入到AbstractAutowireCapableBeanFactory类的createBean方法中，该方法主要做了三件事情：首先将RootBeanDefinition（注：一个RootBeanDefinition类型的mbdToUse，RootBeanDefinition就是根bean定义，表示合并的bean定义，在运行时支持Spring Factory中定义的特定的bean）类的一个实例对其设置beanClass，并验证和准备method override，其次在真正进行create之前先判断是会否有short-cut快捷方式可以直接返回，最后，进入到doCreateBean()中，其中doCreateBean()方法的参数有三个，一个是：一个是beanName，一个是mbdToUse，一个是参数args；关键部分的代码如下：
//依赖注入，实例化一个bean @Override protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException { if (logger.isTraceEnabled()) { logger.trace("Creating instance of bean '" + beanName + "'"); } //一个RootBeanDefinition类型的mbdToUse //RootBeanDefinition就是根bean定义，表示合并的bean定义，在运行时支持Spring Factory中定义的特定的bean RootBeanDefinition mbdToUse = mbd; // Make sure bean class is actually resolved at this point, and // clone the bean definition in case of a dynamically resolved Class // which cannot be stored in the shared merged bean definition. //resolveBeanClass的作用是：根据特定的beanDefinition解析bean class，并得到一个class的引用；将已解析的类存储到bean定义中以供后续使用 Class<?> resolvedClass = resolveBeanClass(mbd, beanName); if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { //将解析后得到的resolvedCalss设置给RootBeanDefinition 根bean定义的 mbdToUse mbdToUse = new RootBeanDefinition(mbd); mbdToUse.setBeanClass(resolvedClass); } //在RootBeanDefinition中可以去准备Bean所需要的各种属性，例如：beanClass解析后的类信息的set，例如method override //这些信息在后续去进行实际的创建bean实例化的过程都是需要用到的 // Prepare method overrides. try { //要回头去看解析bean的存入到BeanDefinition中的时候，就已经设置过overrides的相关属性了 mbdToUse.prepareMethodOverrides(); } catch (BeanDefinitionValidationException ex) { throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(), beanName, "Validation of method overrides failed", ex); } try { // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance. //在实例化之前判断，是否有short-cut快捷方式可以应用 Object bean = resolveBeforeInstantiation(beanName, mbdToUse); if (bean != null) { return bean; } } catch (Throwable ex) { throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "BeanPostProcessor before instantiation of bean failed", ex); } try { //doCreateBean是真正做实例化操作的部分 //这里的三个参数，一个是beanName，一个是mbdToUse，一个是参数args Object beanInstance = doCreateBean(beanName, mbdToUse, args); if (logger.isTraceEnabled()) { logger.trace("Finished creating instance of bean '" + beanName + "'"); } return beanInstance; } catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) { // A previously detected exception with proper bean creation context already, // or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry. throw ex; } catch (Throwable ex) { throw new BeanCreationException( mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex); } }
继续：doCreateBean，会做三件事情：首先，判断是否是单例，如果单例的话，需要将其FactoryBeanInstanceCache中的该beanName取出，若为null，则需要继续创建，否则直接用cache中取出的接口；后面会重点说一下：创建createBeanInstance的过程，之后initializeBean，最后registerDisposableBeanIfNecessary，将bean注册为一次性的
主要部分代码代码见下方：
//方法说明：真正执行CreatBean的操作 protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException { // Instantiate the bean. BeanWrapper instanceWrapper = null; //判断是单例，则从factoryBeanInstanceCache中将其取出，若存在，则基于当前已经缓存的进行后续处理 //否则需要继续创建 if (mbd.isSingleton()) { instanceWrapper = this.factoryBeanInstanceCache.remove(beanName); } if (instanceWrapper == null) { //创建一个特定bean的new instance新的实例 instanceWrapper = createBeanInstance(beanName, mbd, args); } final Object bean = instanceWrapper.getWrappedInstance(); Class<?> beanType = instanceWrapper.getWrappedClass(); if (beanType != NullBean.class) { mbd.resolvedTargetType = beanType; } // Initialize the bean instance. //填充过程 Object exposedObject = bean; try { populateBean(beanName, mbd, instanceWrapper); //initializeBean 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); } } // Register bean as disposable. //将bean注册为一次性的 try { registerDisposableBeanIfNecessary(beanName, bean, mbd); } catch (BeanDefinitionValidationException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex); } return exposedObject; }
下面重点说明：createBeanInstance过程，会根据有参和无参，分别进入到：autowireConstructor和instantiateBean中，重点代码见：
//创建一个特定bean的new instance新的实例 //会使用恰当的实例化策略：工厂方法、构造函数自动装配或简单实例化 protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) { // Make sure bean class is actually resolved at this point. Class<?> beanClass = resolveBeanClass(mbd, beanName); if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Bean class isn't public, and non-public access not allowed: " + beanClass.getName()); } Supplier<?> instanceSupplier = mbd.getInstanceSupplier(); if (instanceSupplier != null) { return obtainFromSupplier(instanceSupplier, beanName); } if (mbd.getFactoryMethodName() != null) { return instantiateUsingFactoryMethod(beanName, mbd, args); } // Shortcut when re-creating the same bean... //快捷方式 boolean resolved = false; boolean autowireNecessary = false; if (args == null) { synchronized (mbd.constructorArgumentLock) { if (mbd.resolvedConstructorOrFactoryMethod != null) { resolved = true; autowireNecessary = mbd.constructorArgumentsResolved; } } } if (resolved) { if (autowireNecessary) { //有参数的情况走到的 return autowireConstructor(beanName, mbd, null, null); } else { //无参的情况走到的 return instantiateBean(beanName, mbd); } } // Candidate constructors for autowiring? Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName); if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR || mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) { return autowireConstructor(beanName, mbd, ctors, args); } // Preferred constructors for default construction? ctors = mbd.getPreferredConstructors(); if (ctors != null) { return autowireConstructor(beanName, mbd, ctors, null); } // No special handling: simply use no-arg constructor. //调用无参构造器 return instantiateBean(beanName, mbd); }
如果是有参所用到的autowireConstructor,主要是：解析参数、获取constructorToUse，如果一直没有找到可用的，直接抛出异常，有可用的继续bw.setBeanInstance(instantiate(beanName, mbd, constructorToUse, argsToUse))；进入instantiate中，该方法是在：ContructorReSolver类中，该方法中可以看到：strategy的字眼，这里针对不同的情况进行了不同的策略选择，具体见下方：
/类内部的私有方法 //实例化，通过对应的Constructor和参数进行某个类的对象的实例化 //autowire的走到了这里 private Object instantiate( String beanName, RootBeanDefinition mbd, Constructor constructorToUse, Object[] argsToUse) { try { //获取初始化策略 InstantiationStrategy strategy = this.beanFactory.getInstantiationStrategy(); if (System.getSecurityManager() != null) { return AccessController.doPrivileged((PrivilegedAction<Object>) () -> strategy.instantiate(mbd, beanName, this.beanFactory, constructorToUse, argsToUse), this.beanFactory.getAccessControlContext()); } else { return strategy.instantiate(mbd, beanName, this.beanFactory, constructorToUse, argsToUse); } } catch (Throwable ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Bean instantiation via constructor failed", ex); } }
那么具体所给出的不同策略的支持是什么呢？进入到了：通过jdk反射和cglib来进行具体的实例化的选择
SimpleInstantiationStrategy类中，
//最终的很多实现的调到的都是：Simplexxx、或者是Defalutxxx之类的 //这是有参数的contructor的走的路径 @Override public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner, final Constructor<?> ctor, Object... args) { //没有method是override的情况，就是说不能通过继承来实现的，那么必须不能用cglib了， //否则可以用cglib动态代理 if (!bd.hasMethodOverrides()) { if (System.getSecurityManager() != null) { // use own privileged to change accessibility (when security is on) AccessController.doPrivileged((PrivilegedAction<Object>) () -> { ReflectionUtils.makeAccessible(ctor); return null; }); } return BeanUtils.instantiateClass(ctor, args); } else { return instantiateWithMethodInjection(bd, beanName, owner, ctor, args); } }
三、依赖注入过程时序图：（备注：原图若出现看不清的情况，可下载后查看，请保留版权）
