【Spring源碼解析】—— 依賴注入結合SpringMVC Demo-xml配置理解

在IOC容器初始化的梳理之后，對依賴注入做一個總結，就是bean實例化的過程，bean的定義有兩種方式，一種是xml文件配置，一種是注解，這里是對xml配置文件的依賴注入的介紹，后續對bean與該部分的不同會再做介紹。

先對整個過程做一個整體說明：從refresh()的入口進入之后，因為是注入依賴實例化bean的過程，所以在這個過程中，會不斷判斷是否有可用的快捷方式，是什么類型的，是singleton的，還是protype類型的，進行getBean獲取bean的操作時，針對已經創建的可以直接返回的直接結束，若無則進行創建，創建中會要判斷是否有parentFactory，是否有依賴bean，set創建過程中可能需要的條件，例如method overrides的驗證和准備等，獲取可用的constructor，有可用的構造器，有參數的情況等，都要走自主選擇的邏輯，否則全部最后走兜底策略，就是默認構造器，注意，在對constructor選擇的過程中，可能會出現無可用，或者是Ambiguous的情況，會拋出異常；具體最終創建bean要用什么策略，提供兩種選擇

一、Demo部分的修改：

首先是xml配置相關，首先在cn.lx.controller包中新建一個TestController2.java，將之前配置的注解的形式全部注釋掉，在xml中將bean配置清楚，TestController2的類內容如下：

//@Controller
public class TestController2 { // @RequestMapping("/test.form")
    public void execute(){ return ; } }

修改applicationContext.xml文件，為如下：

<?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、依賴注入過程介紹：

依賴注入會發生兩種場景，一種是：IOC容器初始化到最后階段，會將none-Lazy-init的bean全部都實例化，其他的會在具體用到的時候實例化；先從第一種情況IOC容器初始化最后階段來看實例化的整個過程，實例化的過程后續階段都是相同的，只是入口不同而已

現在，正文開始，共分為三個步驟：入口-xml配置方式的實例化入口、獲取bean、new-bean（實例化bean）

入口部分-xml配置方式的實例化入口：

從AbstractApplicationContext類的refresh()方法中，之前在IOC初始化中主要從obtainFreshBeanFactory()方法調用后得到了ConfigurableListableBeanFactory類型的beanFactory，在beanFactry中已經對xml文件做了解析並且將需要的信息都存儲到了beanDefinitionMap中，之后將beanFactory作為參數傳遞給finishBeanFactoryInitialization方法，進入到其具體實現中：

//完成beanfactory的初始化，初始化所有保留的單例bean

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {

   // Initialize conversion service for this context.

   //類型轉換服務，用於提供線程安全的類型轉換

   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.

   //預實例化，對未設置lazy-init=true的進行預實例化處理

   beanFactory.preInstantiateSingletons();

}

之后，之后看最后一步，通過beanFactory.preInstantiateSingletons()進入到：DefaultListableBeanFactory類中，該方法會進行判斷，會判斷是否不是抽象且為singleton且是non lazy-init的，就會進入到是否為FactoryBean的判斷，最終發現不是FactoryBean,就進入了else的邏輯分支，即：getBean(beanName)了，從這個部分就進入到下一個關鍵階段：獲取Bean階段

@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...

   //觸發對於所有非懶加載的singleton beans的實例化

   //通過getBean()方法重點執行，其中getBean會找到doGetBean

   for (String beanName : beanNames) {

      RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);

      //判斷是否不為abstract、為singleton、lazyinit屬性為false，則進入到下一步

      if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {

         //判斷是否是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);

               }

            }

         }

         //判斷不是FactoryBean，直接進入getBean邏輯

         else {

            getBean(beanName);

         }

      }

   }

獲取Bean階段：

從亮色的：getBean方法進入到類：AbstractBeanFactory中，然后就看到do真正做事情的來啦：

@Override

public Object getBean(String name) throws BeansException {

   //實際執行

   return doGetBean(name, null, null, false);

}

興奮地點擊doGetBean，去看看它究竟干了啥？進入到類：AbstractBeanFactory類中，doGetBean所完成的內容主要包含：判斷是否已經有存在singleton的beanName，若有可以直接返回，若無則獲取此bean所依賴的beans-dependsOn，然后通過一個for循環判斷，是否這個中的所有dependsOn中的某個bean是否存在環形依賴，若存在的話，直接拋出異常，無異常則為給定的bean注冊依賴bean（依賴bean也會在給定bean銷毀之前被銷毀），最后走到createBean邏輯，至此進入到下一個階段：new-bean真正創建的過程了
doGetBean部分的重要代碼如下：

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
            @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

        //規范化beanName
        final String beanName = transformedBeanName(name);
        //類型:Object
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        //獲取已經創建的singleton的bean的實例對象，將之賦給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 + "'");
                }
            }

            //對於已經存在的直接返回
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        //沒有已經存在的singleton的bean的實例對象
        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.
                //確保被依賴的bean要初始化之后，其他的才能繼續初始化
                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 + "'");
                        }
                        //給給定bean注冊依賴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.
                //實例化

                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, ()-> {
                        try {
                            //實例化
                            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過程：

進入到AbstractAutowireCapableBeanFactory類的createBean方法中，該方法主要做了三件事情：首先將RootBeanDefinition（注：一個RootBeanDefinition類型的mbdToUse，RootBeanDefinition就是根bean定義，表示合並的bean定義，在運行時支持Spring Factory中定義的特定的bean）類的一個實例對其設置beanClass，並驗證和准備method override，其次在真正進行create之前先判斷是會否有short-cut快捷方式可以直接返回，最后，進入到doCreateBean()中，其中doCreateBean()方法的參數有三個，一個是：一個是beanName，一個是mbdToUse，一個是參數args；關鍵部分的代碼如下：

//依賴注入，實例化一個bean
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException {

   if (logger.isTraceEnabled()) {
      logger.trace("Creating instance of bean '" + beanName + "'");
   }
   //一個RootBeanDefinition類型的mbdToUse
   //RootBeanDefinition就是根bean定義，表示合並的bean定義，在運行時支持Spring Factory中定義的特定的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：根據特定的beanDefinition解析bean class，並得到一個class的引用；將已解析的類存儲到bean定義中以供后續使用
   Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
   if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
      //將解析后得到的resolvedCalss設置給RootBeanDefinition 根bean定義的 mbdToUse
      mbdToUse = new RootBeanDefinition(mbd);
      mbdToUse.setBeanClass(resolvedClass);
   }

   //在RootBeanDefinition中可以去准備Bean所需要的各種屬性，例如：beanClass解析后的類信息的set，例如method override
   //這些信息在后續去進行實際的創建bean實例化的過程都是需要用到的

   // Prepare method overrides.
   try {
      //要回頭去看解析bean的存入到BeanDefinition中的時候，就已經設置過overrides的相關屬性了
      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.
      //在實例化之前判斷，是否有short-cut快捷方式可以應用
      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是真正做實例化操作的部分
      //這里的三個參數，一個是beanName，一個是mbdToUse，一個是參數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);
   }
}

繼續：doCreateBean，會做三件事情：首先，判斷是否是單例，如果單例的話，需要將其FactoryBeanInstanceCache中的該beanName取出，若為null，則需要繼續創建，否則直接用cache中取出的接口；后面會重點說一下：創建createBeanInstance的過程，之后initializeBean，最后registerDisposableBeanIfNecessary，將bean注冊為一次性的

主要部分代碼代碼見下方：

//方法說明：真正執行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;
   //判斷是單例，則從factoryBeanInstanceCache中將其取出，若存在，則基於當前已經緩存的進行后續處理
   //否則需要繼續創建
   if (mbd.isSingleton()) {
      instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
   }
   if (instanceWrapper == null) {
      //創建一個特定bean的new instance新的實例
      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.
   //填充過程
   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.
   //將bean注冊為一次性的
   try {
      registerDisposableBeanIfNecessary(beanName, bean, mbd);
   }
   catch (BeanDefinitionValidationException ex) {
      throw new BeanCreationException(
            mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
   }

   return exposedObject;
}

 

下面重點說明：createBeanInstance過程，會根據有參和無參，分別進入到：autowireConstructor和instantiateBean中，重點代碼見：

//創建一個特定bean的new instance新的實例
//會使用恰當的實例化策略：工廠方法、構造函數自動裝配或簡單實例化
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) {
         //有參數的情況走到的
         return autowireConstructor(beanName, mbd, null, null);
      }
      else {
         //無參的情況走到的
         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.
   //調用無參構造器
   return instantiateBean(beanName, mbd);
}

如果是有參所用到的autowireConstructor,主要是：解析參數、獲取constructorToUse，如果一直沒有找到可用的，直接拋出異常，有可用的繼續bw.setBeanInstance(instantiate(beanName, mbd, constructorToUse, argsToUse))；進入instantiate中，該方法是在：ContructorReSolver類中，該方法中可以看到：strategy的字眼，這里針對不同的情況進行了不同的策略選擇，具體見下方：

/類內部的私有方法
//實例化，通過對應的Constructor和參數進行某個類的對象的實例化
//autowire的走到了這里
private Object instantiate(
      String beanName, RootBeanDefinition mbd, Constructor constructorToUse, Object[] argsToUse) {

   try {
      //獲取初始化策略
      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);
   }
}

那么具體所給出的不同策略的支持是什么呢？進入到了：通過jdk反射和cglib來進行具體的實例化的選擇

SimpleInstantiationStrategy類中，

//最終的很多實現的調到的都是：Simplexxx、或者是Defalutxxx之類的
//這是有參數的contructor的走的路徑
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
      final Constructor<?> ctor, Object... args) {

   //沒有method是override的情況，就是說不能通過繼承來實現的，那么必須不能用cglib了，
   //否則可以用cglib動態代理
   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、依賴注入過程時序圖：（備注：原圖若出現看不清的情況，可下載后查看，請保留版權）