IOC的工作过程

不念不忘少年蓝@ 2022-05-11 06:16 215阅读 0赞

## 1.如何创建BeanFactory工厂 ##

构建的入口在AbstractApplicationContext类的refresh方法中。

我们可以从一个例子来入手

public class test { private ApplicationContext context; @Test public void test1(){ context = new ClassPathXmlApplicationContext("applicationContext.xml"); SomeBean someBean = (SomeBean) context.getBean("someBean"); someBean.doSomething(); } }

这是一个简单的Spring入门的程序。非常简单，通过xml文件，创建一个容器context，然后从容器中获取一个bean。

我们可以使用IDE追踪源码([Spring的源码的下载链接][Spring]或者在[官网下载][Link 1])。

下面是ClassPathXmlApplicationContext类中的构造函数：

public ClassPathXmlApplicationContext(String configLocation) throws BeansException { this(new String[] { configLocation}, true, null); }

转向了另一个构造函数:

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)throws BeansException { super(parent); setConfigLocations(configLocations); //判断是否刷新 if (refresh) { refresh(); } }

我们可以进入这个refresh方法：

public void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { //1.准备刷新 prepareRefresh(); // 2.创建内部容器，该容器负责Bean的创建和管理 ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); prepareBeanFactory(beanFactory); try { postProcessBeanFactory(beanFactory); invokeBeanFactoryPostProcessors(beanFactory); registerBeanPostProcessors(beanFactory); initMessageSource(); initApplicationEventMulticaster(); onRefresh(); registerListeners(); //实例化Bean实例的入口(后面会详细讲解——如何创建Bean实例并构建Bean的关系网) finishBeanFactoryInitialization(beanFactory); finishRefresh(); } catch (BeansException ex) { destroyBeans(); cancelRefresh(ex); throw ex; } } }

我们发现此时我们进入了AbstractApplicationContext类的refresh方法，其实通过ClassPathXMLApplication的定义时，我们可以看到它继承了AbstractXmlApplicationContext类，一路追踪下去发现最后是继承了AbstractApplicationContext类。它们都没有对AbstractApplicationContext的refresh方法进行重写，所以会调用了AbstractApplicationContext的refresh方法。

**1.准备刷新**

初始化准备，即为Ioc容器的初始化过程做准备，该过程和 Spring的Environment有关。我们先不用关注。

**2.创建内部容器**

我们来关注一下AbstractApplicationContext的obtainFreshBeanFactory方法

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() { //1.刷新Spring容器中内部的BeanFactory refreshBeanFactory(); //2.得到内部的BeanFactory ConfigurableListableBeanFactory beanFactory = getBeanFactory(); if (logger.isDebugEnabled()) { logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory); } return beanFactory; }

**(1)刷新Spring容器中内部的BeanFactory**

因为AbstractApplicationContext只是定义了refreshBeanFactory的抽象方法，它的子类AbstractRefreshableApplicationContext才实现了refreshBeanFactory的方法。ClassPathXmlApplicationContext继承的也是AbstractRefreshableApplicationContext的子类，所以这里执行的refreshBeanFactory就是AbstractRefreshableApplicationContext类中的refreshBeanFactory方法。

我们来看看代码：

protected final void refreshBeanFactory() throws BeansException { //1.判断Spring容器中是否有BeanFactory，如果存在就销毁并关闭它。 if (hasBeanFactory()) { destroyBeans(); closeBeanFactory(); } try { //2.创建一个BeanFactory(默认创建一个DefaultListableFactory类作为内部BeanFactory) DefaultListableBeanFactory beanFactory = createBeanFactory(); //3.设置BeanFactory的id,和外部的Spring容器一样 beanFactory.setSerializationId(getId()); //4.定制BeanFactory初始化，暂不关注 customizeBeanFactory(beanFactory); //5.加载Bean实例 loadBeanDefinitions(beanFactory); //6.将BeanFactory设置为Spring容器的内部BeanFactory synchronized (this.beanFactoryMonitor) { this.beanFactory = beanFactory; } } catch (IOException ex) { throw new ApplicationContextException("I/O error " + getDisplayName(), ex); } }

**(2)得到内部的BeanFactory**

得到Spring内部的BeanFactory。

**总结**：我们可以通过一个流程图能清楚的看到创建BeanFactory的全过程  
![在这里插入图片描述][70]

## 2.如何创建Bean实例并构建Bean的关系网 ##

下面就是Bean的实例化代码(在上文中refresh方法的代码中我们有提到，创建完了BeanFactory)，是从AbstractApplicationContext类的finishBeanFactoryInitialization方法开始的。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) { //部分代码省略 // 不使用TempClassLoader beanFactory.setTempClassLoader(null); // 禁止修改当前Bean的配置信息 beanFactory.freezeConfiguration(); // 实例化non-lazy-int类的bean beanFactory.preInstantiateSingletons(); }

从上面的代码我们可以发现Bean的实例化是在BeanFactory中的preInstantiateSingletons方法中实现的。我们又进入到DefaultListableBeanFactory类中preInstantiateSingletons方法(关于为什么是DefaultListableBeanFactory类，上文创建BeanFactory已经讲过了)。我们来看代码(其中省略了部分不重要的代码):

public void preInstantiateSingletons() throws BeansException { //1.获取XML配置文件解析时，解析到的所有beanname List<String> beanNames; synchronized (this.beanDefinitionMap) { beanNames = new ArrayList<String>(this.beanDefinitionNames); } //2.遍历所有没有标注过lazy-init的singleton的beanname,创建bean //遍历beanNames保存的beanname for (String beanName : beanNames) { //利用beanname获取BeanDefinition //(XML解析时会生成BeanDefinition对象 //将XML中的各属性添加到BeanDefinition的相关标志位中，比如abstractFlag，scope等) RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); //非abstract，非lazy-init的singleton bean才能需要在容器初始化阶段创建 if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) { //判断是否是FactoryBean if (isFactoryBean(beanName)) { //如果是，则会获取FactoryBean实例，FactoryBean前面会加一个&符号 final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName); boolean isEagerInit; if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) { isEagerInit = AccessController.doPrivileged( new PrivilegedAction<Boolean>() { @Override public Boolean run() { return ((SmartFactoryBean<?>) factory).isEagerInit(); } }, getAccessControlContext()); } else { isEagerInit = (factory instanceof SmartFactoryBean && ((SmartFactoryBean<?>) factory).isEagerInit()); } if (isEagerInit) { getBean(beanName); } } //非Factorybean，直接调用getBean方法。 else { getBean(beanName); } } } // Trigger post-initialization callback for all applicable beans... for (String beanName : beanNames) { Object singletonInstance = getSingleton(beanName); if (singletonInstance instanceof SmartInitializingSingleton) { final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance; if (System.getSecurityManager() != null) { AccessController.doPrivileged(new PrivilegedAction<Object>() { @Override public Object run() { smartSingleton.afterSingletonsInstantiated(); return null; } }, getAccessControlContext()); } else { smartSingleton.afterSingletonsInstantiated(); } } } }

我们可以发现这段代码又是调用了父类AbstractBeanFactory类中的 getBean完成了bean的创建和初始化。

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

紧接着又调用了父类AbstractBeanFactory类中doGetBean方法。

protected <T> T doGetBean( final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)throws BeansException { //beanname转换，去掉FactoryBean的&前缀。 final String beanName = transformedBeanName(name); Object bean; Object sharedInstance = getSingleton(beanName); //判断singleton bean是否创建好了，创建好了直接从内存中取出 if (sharedInstance != null && args == null) { bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } //还没创建好 else { //正在创建，则直接异常返回 if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } //检查是否有beanname对应的BeanDefinition //得到父工厂 BeanFactory parentBeanFactory = getParentBeanFactory(); //检查没有找到的话，就看看父工厂中有没有BeanDefinition if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { //获取原始的name，包含了FactoryBeanName String nameToLookup = originalBeanName(name); if (args != null) { // Delegation to parent with explicit args. return (T) parentBeanFactory.getBean(nameToLookup, args); } else { // No args -> delegate to standard getBean method. return parentBeanFactory.getBean(nameToLookup, requiredType); } } if (!typeCheckOnly) { markBeanAsCreated(beanName); } try { //找到了beanName对应的BeanDefinition，合并parent的BeanDefinition(XML中的parent属性) final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); checkMergedBeanDefinition(mbd, beanName, args); // 处理dependsOn属性 String[] dependsOn = mbd.getDependsOn(); if (dependsOn != null) { //遍历所有的dependOn bean，先要注册和创建依赖项的bean for (String dependsOnBean : dependsOn) { //check两个bean是不是循环依赖，spring不能出现bean的循环依赖 if (isDependent(beanName, dependsOnBean)) { throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'"); } //注册并创建依赖的bean registerDependentBean(dependsOnBean, beanName); getBean(dependsOnBean); } } // 处理scope属性 if (mbd.isSingleton()) { //singleton，必须保证线程安全的情况下创建bean，保证单例 sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { public Object getObject() throws BeansException { try { //反射创建bean实例，这个过程很复杂，稍后分析 return createBean(beanName, mbd, args); } catch (BeansException ex) { //异常处理，清除bean destroySingleton(beanName); throw ex; } } }); //获取bean实例 bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } //多例的话，创建一个全新的实例 else if (mbd.isPrototype()) { Object prototypeInstance = null; try { //创建前的回调 beforePrototypeCreation(beanName); //反射创建bean实例，稍后分析 prototypeInstance = createBean(beanName, mbd, args); } finally { //创建后的回调，清除inCreation的标志 afterPrototypeCreation(beanName); } bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd); } //其他scope的 值 else { String scopeName = mbd.getScope(); final Scope scope = this.scopes.get(scopeName); //scope属性不能接收空值 if (scope == null) { throw new IllegalStateException("No Scope registered for scope '" + 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; } } //check创建的bean是否是requiredType指明的类型。如果不是，先转换，转成不成就只能类型不匹配抛出异常了 if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) { try { return getTypeConverter().convertIfNecessary(bean, requiredType); } catch (TypeMismatchException ex) { throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } return (T) bean; }

调用了AbstractAutowireCapableBeanFactory类的createBean通过反射创建bean实例

protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) throws BeanCreationException { resolveBeanClass(mbd, beanName); //处理bean中定义的覆盖方法，主要是xml:lookup-method或者replace-method。 //标记override的方法为已经加载过的，避免不必要的参数检查开销 try { mbd.prepareMethodOverrides(); } catch (BeanDefinitionValidationException ex) { throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName, "Validation of method overrides failed", ex); } //调用BeanPostProcessors bean后处理器， //使得bean后处理器可以返回一个proxy bean，从而代替我们要创建的bean。 //回调后处理器的postProcessBeforeInstantiation方法， //如果这个方法中返回了一个bean，也就是使用了proxy，则再回调postProcessAfterInitialization()方法。 //之后返回这个Proxy bean即可。 try { Object bean = resolveBeforeInstantiation(beanName, mbd); if (bean != null) { return bean; } } catch (Throwable ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "BeanPostProcessor before instantiation of bean failed", ex); } //doCreateBean创建bean实例，后面详细分析 Object beanInstance = doCreateBean(beanName, mbd, args); return beanInstance; }

我们重点分析AbstractAutowireCapableBeanFactory类中的doCreateBean方法。

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) { //创建bean实例，如果是singleton，先尝试从缓存中取，取不到则创建 BeanWrapper instanceWrapper = null; if (mbd.isSingleton()) { instanceWrapper = this.factoryBeanInstanceCache.remove(beanName); } if (instanceWrapper == null) { //反射创建bean实例 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) { 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<Object>() { @Override public Object getObject() throws BeansException { return getEarlyBeanReference(beanName, mbd, bean); } }); } // Initialize the bean instance. Object exposedObject = bean; try { 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); } } if (!actualDependentBeans.isEmpty()) { //抛出异常 } } } } // 注册bean为可销毁的bean，bean销毁时，会回调destroy-method registerDisposableBeanIfNecessary(beanName, bean, mbd); return exposedObject; }

使用AbstractAutowireCapableBeanFactory类中createBeanInstance 反射创建bean实例

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) { // 先创建class对象，反射的套路。利用bean的class属性进行反射，所以class属性一定要是bean的实现类 Class<?> beanClass = resolveBeanClass(mbd, beanName); //class如果不是public的，则抛出异常。因为没法进行实例化 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()); } //使用FactoryBean的factory-method来创建，支持静态工厂和实例工厂 if (mbd.getFactoryMethodName() != null) { return instantiateUsingFactoryMethod(beanName, mbd, args); } // 无参数情况时，创建bean。调用无参构造方法。 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) { //autowire创建 自动装配 return autowireConstructor(beanName, mbd, null, null); } else { //普通创建 return instantiateBean(beanName, mbd); } } // 有参数的情况时，创建bean。先利用参数的个数，类型等，确定最精确匹配的构造方法。 Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName); if (ctors != null || mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR || mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) { return autowireConstructor(beanName, mbd, ctors, args); } // 有参数时。又没有获取到构造方法，则只能调用无参数构造方法创建实例了 return instantiateBean(beanName, mbd); }

AbstractAutowireCapableBeanFactory类中instantiateBean，使用无参构造方法，反射创建bean实例代码如下：

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>() { @Override public Object run() { return getInstantiationStrategy().instantiate(mbd, beanName, parent); } }, getAccessControlContext()); } else { //创建实例，关键点 beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent); } BeanWrapper bw = new BeanWrapperImpl(beanInstance); initBeanWrapper(bw); return bw; } catch (Throwable ex) { //抛出异常 } }

关键点在SimpleInstantiationStrategy类中的instantiate方法

public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) { if (beanDefinition.getMethodOverrides().isEmpty()) { Constructor<?> constructorToUse; //保证线程安全的情况下，获取Constructor synchronized (beanDefinition.constructorArgumentLock) { //获取构造方法或者factory-method constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod; //BeanDefinition中如果没有Constructor或者factory-method，则直接使用默认无参数的构造方法 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<?>>() { @Override public Constructor<?> run() throws Exception { return clazz.getDeclaredConstructor((Class[]) null); } }); } //获取默认无参数的构造方法 else { constructorToUse = clazz.getDeclaredConstructor((Class[]) null); } beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse; } catch (Exception ex) { throw new BeanInstantiationException(clazz, "No default constructor found", ex); } } } //使用上一步得到的Constructor，反射获取bean实例 return BeanUtils.instantiateClass(constructorToUse); } else { // Must generate CGLIB subclass. return instantiateWithMethodInjection(beanDefinition, beanName, owner); } }

instantiate方法主要做两件事，确定Constructor或者factory-method 利用Constructor，反射创建bean实例。

[Spring]: https://download.csdn.net/download/jdfk423/10679115
[Link 1]: https://repo.spring.io/webapp/#/artifacts/browse/tree/General/libs-release-local/org/springframework
[70]: /images/20220511/161c0db40dd84a10931ab11b7b2b8158.png