【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）