Spring source code learning ③ AbstractBeanFactory createBean execution process

Spring source code learning ③ AbstractBeanFactory createBean execution process

The Spring source series articles will follow the principle of going from shallow to deep, from easy to difficult, and from macro to micro. The goal is to reduce the difficulty of learning as much as possible, instead of getting lost in the source code. It is very purposeful to explore and study Spring's implementation principle and source code for this scenario. What will this series of articles give you? Become a Spring expert from a Spring user. This article will be synchronized on the WeChat public account [DevXJava], which is convenient for reading on the WeChat client.

The content of this article is AbstractBeanFactory createBean an exploration of the method, which will involve a lot of source code research. If you have read it and Spring 源码学习 ② BeanFactory doGetBean 执行流程 understand the context, it will be helpful to understand the content of this article. If you haven’t read it, don’t worry, because the content of this article focuses on AbstractBeanFactory To explain createBean the method, our focus is only on createBean the method, and createBean the method is used as an entry point to explain the creation process of spring beans.

Scenes

In order to reduce interference, we still choose to start from the simplest scene.

public class BeanFactoryGetBeanExp {

    public static void main(String[] args) throws Exception {

        DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();

        // 在 beanFactory 中注册 bean1
        BeanDefinition bdf = BeanDefinitionBuilder.genericBeanDefinition(Bean1.class).getBeanDefinition();
        beanFactory.registerBeanDefinition("bean1" , bdf);
        
        for (String name : beanFactory.getBeanDefinitionNames()) {
            System.out.println("bean ----------> " + name);
        }

        // 调用 getBean , debug 进入观察获取 bean1 过程
        Bean1 bean1 = beanFactory.getBean(Bean1.class);
        System.out.println(bean1);

    }

    static class Bean1 {
        public Bean1() {
            System.out.println("Bean1 默认构造函数执行 >>>>>>>>>>>>>>>>>>>>>>");
        }
    }
}

The execution flow of createBean

resolveBeforeInstantiation source code

applyBeanPostProcessorsBeforeInstantiation The method can return a bean instance. If the returned bean instance is not null, it will be createBean returned directly to the caller by the method. applyBeanPostProcessorsAfterInitialization Executes all BeanPostProcessor post-processing procedures on the instance.

/**
 * Apply before-instantiation post-processors, resolving whether there is a
 * before-instantiation shortcut for the specified bean.
 * @param beanName the name of the bean
 * @param mbd the bean definition for the bean
 * @return the shortcut-determined bean instance, or {@code null} if none
 */
@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
   Object bean = null;
   if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
      // Make sure bean class is actually resolved at this point.
      if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
         Class<?> targetType = determineTargetType(beanName, mbd);
         if (targetType != null) {
            bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
            if (bean != null) {
               bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
            }
         }
      }
      mbd.beforeInstantiationResolved = (bean != null);
   }
   return bean;
}

applyBeanPostProcessorsBeforeInstantiation source code

InstantiationAwareBeanPostProcessor The postProcessBeforeInstantiation can return the bean instance directly, and after one of them InstantiationAwareBeanPostProcessor returns the bean instance, the following processors will not be called again.

/**
 * Apply InstantiationAwareBeanPostProcessors to the specified bean definition
 * (by class and name), invoking their {@code postProcessBeforeInstantiation} methods.
 * <p>Any returned object will be used as the bean instead of actually instantiating
 * the target bean. A {@code null} return value from the post-processor will
 * result in the target bean being instantiated.
 * @param beanClass the class of the bean to be instantiated
 * @param beanName the name of the bean
 * @return the bean object to use instead of a default instance of the target bean, or {@code null}
 * @see InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation
 */
@Nullable
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
   for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
      Object result = bp.postProcessBeforeInstantiation(beanClass, beanName);
      if (result != null) {
         return result;
      }
   }
   return null;
}

applyBeanPostProcessorsAfterInitialization source code

@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
      throws BeansException {

   Object result = existingBean;
   for (BeanPostProcessor processor : getBeanPostProcessors()) {
      Object current = processor.postProcessAfterInitialization(result, beanName);
      if (current == null) {
         return result;
      }
      result = current;
   }
   return result;
}

doCreateBean execution process

In the process of initializing beans in spring, many core operations are doCreateBean completed in the process. Next we will carefully analyze doCreateBean the implementation details in the method.
doCreateBean The key methods are: createBeanInstance (creating bean instances), populateBean (filling beans or giving spring features to beans, such as dependency injection), initializeBean (also giving spring features to beans)

addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)) In order to solve the circular reference problem, the bean is ObjectFactory exposed to the cache as an anonymous inner class singletonFactories , and the actual call points to the getEarlyBeanReference .

/**
 * Actually create the specified bean. Pre-creation processing has already happened
 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
 * <p>Differentiates between default bean instantiation, use of a
 * factory method, and autowiring a constructor.
 * @param beanName the name of the bean
 * @param mbd the merged bean definition for the bean
 * @param args explicit arguments to use for constructor or factory method invocation
 * @return a new instance of the bean
 * @throws BeanCreationException if the bean could not be created
 * @see #instantiateBean
 * @see #instantiateUsingFactoryMethod
 * @see #autowireConstructor
 */
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException {

   // Instantiate the bean.
   BeanWrapper instanceWrapper = null;
   if (mbd.isSingleton()) {
      instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
   }
   if (instanceWrapper == null) {
      instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   Object bean = instanceWrapper.getWrappedInstance();
   Class<?> beanType = instanceWrapper.getWrappedClass();
   if (beanType != NullBean.class) {
      mbd.resolvedTargetType = beanType;
   }

   // Allow post-processors to modify the merged bean definition.
   synchronized (mbd.postProcessingLock) {
      if (!mbd.postProcessed) {
         try {
            applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
         }
         catch (Throwable ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                  "Post-processing of merged bean definition failed", ex);
         }
         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.isTraceEnabled()) {
         logger.trace("Eagerly caching bean '" + beanName +
               "' to allow for resolving potential circular references");
      }
      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
   }

   // Initialize the bean instance.
   Object exposedObject = bean;
   try {
      populateBean(beanName, mbd, instanceWrapper);
      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);
      }
   }

   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<>(dependentBeans.length);
            for (String dependentBean : dependentBeans) {
               if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                  actualDependentBeans.add(dependentBean);
               }
            }
            if (!actualDependentBeans.isEmpty()) {
               throw new BeanCurrentlyInCreationException(beanName,
                     "Bean with name '" + beanName + "' has been injected into other beans [" +
                     StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                     "] in its raw version as part of a circular reference, but has eventually been " +
                     "wrapped. This means that said other beans do not use the final version of the " +
                     "bean. This is often the result of over-eager type matching - consider using " +
                     "'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
            }
         }
      }
   }

   // Register bean as disposable.
   try {
      registerDisposableBeanIfNecessary(beanName, bean, mbd);
   }
   catch (BeanDefinitionValidationException ex) {
      throw new BeanCreationException(
            mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
   }

   return exposedObject;
}

createBeanInstance execution process

createBeanInstance actually completes the creation of bean instances. What needs to be focused on is and autowireConstructor handles the scenario of constructor injection and the scenario of instantiation using the default constructor. instantiateBean autowireConstructor instantiateBean

autowireConstructor method

/**
 * "autowire constructor" (with constructor arguments by type) behavior.
 * Also applied if explicit constructor argument values are specified,
 * matching all remaining arguments with beans from the bean factory.
 * <p>This corresponds to constructor injection: In this mode, a Spring
 * bean factory is able to host components that expect constructor-based
 * dependency resolution.
 * @param beanName the name of the bean
 * @param mbd the bean definition for the bean
 * @param ctors the chosen candidate constructors
 * @param explicitArgs argument values passed in programmatically via the getBean method,
 * or {@code null} if none (implying the use of constructor argument values from bean definition)
 * @return a BeanWrapper for the new instance
 */
protected BeanWrapper autowireConstructor(
                String beanName, RootBeanDefinition mbd, @Nullable Constructor<?>[] ctors, @Nullable Object[] explicitArgs) {

    return new ConstructorResolver(this).autowireConstructor(beanName, mbd, ctors, explicitArgs);
}

instantiateBean method

/**
 * Instantiate the given bean using its default constructor.
 * @param beanName the name of the bean
 * @param mbd the bean definition for the bean
 * @return a BeanWrapper for the new instance
 */
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
    try {
            Object beanInstance;
            if (System.getSecurityManager() != null) {
                    beanInstance = AccessController.doPrivileged(
                                    (PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this),
                                    getAccessControlContext());
            }
            else {
                    beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
            }
            BeanWrapper bw = new BeanWrapperImpl(beanInstance);
            initBeanWrapper(bw);
            return bw;
    }
    catch (Throwable ex) {
            throw new BeanCreationException(
                            mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
    }
}

SimpleInstantiationStrategy#instantiate

In fact, it obtains the default no-argument constructor of the target Class and makes a reflective call to instantiate the object.

@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
   // Don't override the class with CGLIB if no overrides.
   if (!bd.hasMethodOverrides()) {
      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(
                        (PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
               }
               else {
                  constructorToUse = clazz.getDeclaredConstructor();
               }
               bd.resolvedConstructorOrFactoryMethod = constructorToUse;
            }
            catch (Throwable ex) {
               throw new BeanInstantiationException(clazz, "No default constructor found", ex);
            }
         }
      }
      return BeanUtils.instantiateClass(constructorToUse);
   }
   else {
      // Must generate CGLIB subclass.
      return instantiateWithMethodInjection(bd, beanName, owner);
   }
}

populateBean execution process

populateBean Two core operations are performed in the method, the execution InstantiationAwareBeanPostProcessor postProcessAfterInstantiation (post-processing of the bean) method and postProcessProperties the (dependency injection) method.
In order not to diverge, this article will not explain what post-processing spring does to beans by default, and how to complete dependency injection, which will be explained in detail in subsequent articles.

/**
 * Populate the bean instance in the given BeanWrapper with the property values
 * from the bean definition.
 * @param beanName the name of the bean
 * @param mbd the bean definition for the bean
 * @param bw the BeanWrapper with bean instance
 */
@SuppressWarnings("deprecation")  // for postProcessPropertyValues
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
   if (bw == null) {
      if (mbd.hasPropertyValues()) {
         throw new BeanCreationException(
               mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
      }
      else {
         // Skip property population phase for null instance.
         return;
      }
   }

   // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
   // state of the bean before properties are set. This can be used, for example,
   // to support styles of field injection.
   if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
         if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
            return;
         }
      }
   }

   PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

   int resolvedAutowireMode = mbd.getResolvedAutowireMode();
   if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
      MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
      // Add property values based on autowire by name if applicable.
      if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
         autowireByName(beanName, mbd, bw, newPvs);
      }
      // Add property values based on autowire by type if applicable.
      if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
         autowireByType(beanName, mbd, bw, newPvs);
      }
      pvs = newPvs;
   }

   boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
   boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

   PropertyDescriptor[] filteredPds = null;
   if (hasInstAwareBpps) {
      if (pvs == null) {
         pvs = mbd.getPropertyValues();
      }
      for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
         PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
         if (pvsToUse == null) {
            if (filteredPds == null) {
               filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            }
            pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
            if (pvsToUse == null) {
               return;
            }
         }
         pvs = pvsToUse;
      }
   }
   if (needsDepCheck) {
      if (filteredPds == null) {
         filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
      }
      checkDependencies(beanName, mbd, filteredPds, pvs);
   }

   if (pvs != null) {
      applyPropertyValues(beanName, mbd, bw, pvs);
   }
}

initializeBean execution flow

/**
 * Initialize the given bean instance, applying factory callbacks
 * as well as init methods and bean post processors.
 * <p>Called from {@link #createBean} for traditionally defined beans,
 * and from {@link #initializeBean} for existing bean instances.
 * @param beanName the bean name in the factory (for debugging purposes)
 * @param bean the new bean instance we may need to initialize
 * @param mbd the bean definition that the bean was created with
 * (can also be {@code null}, if given an existing bean instance)
 * @return the initialized bean instance (potentially wrapped)
 * @see BeanNameAware
 * @see BeanClassLoaderAware
 * @see BeanFactoryAware
 * @see #applyBeanPostProcessorsBeforeInitialization
 * @see #invokeInitMethods
 * @see #applyBeanPostProcessorsAfterInitialization
 */
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
   if (System.getSecurityManager() != null) {
      AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
         invokeAwareMethods(beanName, bean);
         return null;
      }, getAccessControlContext());
   }
   else {
      invokeAwareMethods(beanName, bean);
   }

   Object wrappedBean = bean;
   if (mbd == null || !mbd.isSynthetic()) {
      wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
   }

   try {
      invokeInitMethods(beanName, wrappedBean, mbd);
   }
   catch (Throwable ex) {
      throw new BeanCreationException(
            (mbd != null ? mbd.getResourceDescription() : null),
            beanName, "Invocation of init method failed", ex);
   }
   if (mbd == null || !mbd.isSynthetic()) {
      wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
   }

   return wrappedBean;
}

registerDisposableBeanIfNecessary

registerDisposableBeanIfNecessary It depends on whether the bean implements DisposableBean the interface, and implements the interface when the instance needs to release some resources, and the DisposableBean destroy method will be executed as the spring container is closed. This is limited to Singleton beans and other Scope beans, and does not include Prototype beans of type .

/**
 * Add the given bean to the list of disposable beans in this factory,
 * registering its DisposableBean interface and/or the given destroy method
 * to be called on factory shutdown (if applicable). Only applies to singletons.
 * @param beanName the name of the bean
 * @param bean the bean instance
 * @param mbd the bean definition for the bean
 * @see RootBeanDefinition#isSingleton
 * @see RootBeanDefinition#getDependsOn
 * @see #registerDisposableBean
 * @see #registerDependentBean
 */
protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) {
   AccessControlContext acc = (System.getSecurityManager() != null ? getAccessControlContext() : null);
   if (!mbd.isPrototype() && requiresDestruction(bean, mbd)) {
      if (mbd.isSingleton()) {
         // Register a DisposableBean implementation that performs all destruction
         // work for the given bean: DestructionAwareBeanPostProcessors,
         // DisposableBean interface, custom destroy method.
         registerDisposableBean(beanName, new DisposableBeanAdapter(
               bean, beanName, mbd, getBeanPostProcessorCache().destructionAware, acc));
      }
      else {
         // A bean with a custom scope...
         Scope scope = this.scopes.get(mbd.getScope());
         if (scope == null) {
            throw new IllegalStateException("No Scope registered for scope name '" + mbd.getScope() + "'");
         }
         scope.registerDestructionCallback(beanName, new DisposableBeanAdapter(
               bean, beanName, mbd, getBeanPostProcessorCache().destructionAware, acc));
      }
   }
}

At this point BeanFactory , the creation of a bean instance is all completed. Next, we will summarize the bean creation process, explaining those processes that the bean creation has gone through, and what impact each process will have on the bean creation.

Summary of the bean creation process

Overall, the bean instance creation process has gone through the main steps in the figure below. It can be seen that spring has reserved a lot of extension points for us during the bean creation process, allowing us to rewrite the bean and control the creation process.

The role and purpose of this article is to serve as a guide. Readers who want to learn more or have a better grasp still need to spare time to explore the source code and try different scenarios. More detailed details and source code will be provided in the follow-up content of the series of articles. This article will be synchronized on the WeChat public account [DevXJava], which is convenient for reading on the WeChat client.

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