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springboot bean实例化和属性注入 基于springboot bean的实例化过程和属性注入过程

leileibest_437147623   2021-11-17 我要评论
想了解基于springboot bean的实例化过程和属性注入过程的相关内容吗leileibest_437147623在本文为您仔细讲解springboot bean实例化和属性注入的相关知识和一些Code实例欢迎阅读和指正我们先划重点:springboot,bean,bean实例化,bean属性注入下面大家一起来学习吧。

bean的实例化过程和属性注入过程

了解过springboot的加载流程的都知道springboot初始化bean都在refresh方法中。

这个方法代码如下:

// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.新建beanFactory
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
   // Allows post-processing of the bean factory in context subclasses.
   postProcessBeanFactory(beanFactory);
   // Invoke factory processors registered as beans in the context.
   // 加载实现beanFactoryPostProcessor的beanbean定义的时候执行
   invokeBeanFactoryPostProcessors(beanFactory);
   // Register bean processors that intercept bean creation.
   // 加载实现了beanPostProcessor在bean实例化前、后执行
   registerBeanPostProcessors(beanFactory);
   // Initialize message source for this context.
   initMessageSource();
   // Initialize event multicaster for this context.
   initApplicationEventMulticaster();
   // Initialize other special beans in specific context subclasses.
   onRefresh();
   // Check for listener beans and register them.
   registerListeners();
   // Instantiate all remaining (non-lazy-init) singletons.
   //实例化非懒加载的bean、bean封装、属性注入、注解注入(主要使用BeanPostProcessor或子类实现)等
   finishBeanFactoryInitialization(beanFactory);
   // Last step: publish corresponding event.
   finishRefresh();

这里我们主要看下finishBeanfactoryInitialization方法。此方法实现了bean的实例和属性注入等。进入此方法的最后一行。

// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();

注释写的很清楚实例化剩余非懒加载的bean。此方法的实现是核心类DefaultLisListableBeanFactory实现。这个方法中有个判断:bean是否是beanFactory的实现类。

如果是则获取bean的时候会从beanFactory实现类的getObject中获取我们重点看看getBean这个方法。

getBean是spring中最重要、最牛逼的方法之一具体的逻辑是通过doGetBean方法处理的。

我们看下doGetBean方法方法很长。我们分成几个部分去看。

1、先判断是否已经存在缓存中代码如下:

if (sharedInstance != null && args == null) {
   if (logger.isDebugEnabled()) {
      if (isSingletonCurrentlyInCreation(beanName)) {
         logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
               "' that is not fully initialized yet - a consequence of a circular reference");
      }
      else {
         logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
      }
   }
   bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);

2、从父beanfactory中获取

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 {
      // No args -> delegate to standard getBean method.
      return parentBeanFactory.getBean(nameToLookup, requiredType);
   }
}

3、直接创建RootBeanDefinition

//mark 给此bean 马克一下。防止重复创建
if (!typeCheckOnly) {
   markBeanAsCreated(beanName);
}
try {
   final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
   checkMergedBeanDefinition(mbd, beanName, args);
   // Guarantee initialization of beans that the current bean depends on.
   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 + "'");
         }
         registerDependentBean(dep, beanName);
         getBean(dep);
      }
   }

4、是singleton还是prototype类型的根据不同类型去实例化bean,代码只贴了单例的类型:

// Create bean instance.
if (mbd.isSingleton()) {
   sharedInstance = getSingleton(beanName, () -> {
      try {
         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);

5、检查bean的类型是否匹配

// 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.isDebugEnabled()) {
         logger.debug("Failed to convert bean '" + name + "' to required type '" +
               ClassUtils.getQualifiedName(requiredType) + "'", ex);
      }
      throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
   }
}

整个doGetBean方法改成这5个部分重点看下第4个部分中的createBean和getObjectForBeanInstance方法。

1、createBean方法里面主要是2个部分,bean直接是通过BeanPostProcessor的postProcessBeforeIntantiation方法获取的。

注释也是描述的很清楚:Give BeanPostProcessors a chance to return a proxy instead of the bean instance(英语渣渣不敢瞎翻译误导大家理解就好)

代码如下:

try {
   // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
   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);
}

第一个部分自己实现那么createBean方法第2个部分毋庸置疑肯定是spring去实例化bean代码如下:

try {
   Object beanInstance = doCreateBean(beanName, mbdToUse, args);
   if (logger.isDebugEnabled()) {
      logger.debug("Finished creating instance of bean '" + beanName + "'");
   }
   return beanInstance;
}

又是do开头的方法说明这个也是一个创建方法spring中一般do开头的都是用于创建某个对象。跟着代码走看下doCreateBean方法在查看此方法之前可能需要了解下BeanWarpper这个封装类。bean的封装、属性注入等都是用BeanWarpper去完成的。

看下代码:

// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
   instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
   instanceWrapper = createBeanInstance(beanName, mbd, args);
}

进入createBeanInstance方法中就是调用构造器去实例化bean返回beanWrapper。通过构造器获取实例代码如下:

// Need to determine the constructor...
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
      mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
      mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
   return autowireConstructor(beanName, mbd, ctors, args);
}
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);

这里实例化我一开始以为就是简单的反射后面我点进去时候发现里面一直提策略实例化我目前了解的是可以解决Override的问题等。如果有兴趣的可以自行查看。到这里为止。我们终于实例化bean了。下面看下第二步bean的属性注入等。

代码如下:

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);
   }
}

populateBean方法名称就暴露他是干啥的:填充bean。我放一段比较重要的部分:

if (hasInstAwareBpps || needsDepCheck) {
   if (pvs == null) {
      pvs = mbd.getPropertyValues();
   }
   PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
   if (hasInstAwareBpps) {
      for (BeanPostProcessor bp : getBeanPostProcessors()) {
         if (bp instanceof InstantiationAwareBeanPostProcessor) {
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
            pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
            if (pvs == null) {
               return;
            }
         }
      }
   }
   if (needsDepCheck) {
      checkDependencies(beanName, mbd, filteredPds, pvs);
   }
}

这段代码貌似没有什么注入、填充什么的我们看下InstantiationAwareBeanPostProcessor这个接口发现这个接口有个很熟悉的实现类是:AutowiredAnnotationBeanPostProcessor。这个类的方法中我们看到终于看到了jnject方法。但是在inject之前需要调用下findAutowiringMatedata方法获取一下元数据:

// Fall back to class name as cache key, for backwards compatibility with custom callers.
String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
// Quick check on the concurrent map first, with minimal locking.
InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
   synchronized (this.injectionMetadataCache) {
      metadata = this.injectionMetadataCache.get(cacheKey);
      if (InjectionMetadata.needsRefresh(metadata, clazz)) {
         if (metadata != null) {
            metadata.clear(pvs);
         }
         metadata = buildAutowiringMetadata(clazz);
         this.injectionMetadataCache.put(cacheKey, metadata);
      }
   }
} 
return metadata;

那么问题又来了这个injectionMetadataCache集合值是从那里来的呢?AutowiredAnnotationBeanPostProcessor实现了MergeBeandefinitionPostProcessor,那么就好了肯定是在bean实例的时候调用了postProcessMergedBeanDefintion这个方法。

果然在doCreateBean方法中有这么一段:

// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
   if (!mbd.postProcessed) {
      try {
         //通过 BeanPostProcessor将需要注解的元数据 放到Map中
         applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
      }
      catch (Throwable ex) {
         throw new BeanCreationException(mbd.getResourceDescription(), beanName,
               "Post-processing of merged bean definition failed", ex);
      }
      mbd.postProcessed = true;
   }
}

哎spring缜密啊。元数据有了下面我们看下element.inject()方法是如何操作的:

if (this.isField) {
   Field field = (Field) this.member;
   ReflectionUtils.makeAccessible(field);
   field.set(target, getResourceToInject(target, requestingBeanName));
}
else {
   if (checkPropertySkipping(pvs)) {
      return;
   }
   try {
      Method method = (Method) this.member;
      ReflectionUtils.makeAccessible(method);
      method.invoke(target, getResourceToInject(target, requestingBeanName));
   }
   catch (InvocationTargetException ex) {
      throw ex.getTargetException();
   }
}

方法简单明了直接使用反射将值set到属性中至此 bean的实例、属性注入基本完成下面我们回头来看doGetBean的另一个方法getObjectForBeanInstance。

2、getObjectForBeanInstance方法。对于是FactoryBean类型的 bean通过getObject获取到bean的代理实例跟着方法一直走下去会到getObejct()方法中。

if (System.getSecurityManager() != null) {
   AccessControlContext acc = getAccessControlContext();
   try {
      object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
   }
   catch (PrivilegedActionException pae) {
      throw pae.getException();
   }
}
else {
   //从FactoryBean实现bean中getObejct获取到bean
   object = factory.getObject();
}

到此 finishBeanfactoryInitialization方法执行结束!

bean实例化流程说明

bean实例化流程流程是在onRefresh方法的finishBeanFactoryInitialization中。进入该方法

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        // Initialize conversion service for this context.
        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.
        beanFactory.preInstantiateSingletons();
    }

1、beanFactory.freezeConfiguration(); 标记正在实例化

当前有183个bean正在实例化中。

2、调用beanFactory.preInstantiateSingletons(); 实例化bean

以上为个人经验希望能给大家一个参考也希望大家多多支持。


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