Spring-createBean部分源码

createBean源码：

/**
 * Central method of this class: creates a bean instance,
 * populates the bean instance, applies post-processors, etc.
 * @see #doCreateBean
 */
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
        throws BeanCreationException {
​
    if (logger.isTraceEnabled()) {
        logger.trace("Creating instance of bean '" + beanName + "'");
    }
    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.
    // 马上就要实例化Bean了，确保beanClass被加载了
    Class resolvedClass = resolveBeanClass(mbd, beanName);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }
​
    // Prepare method overrides.
    try {
        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.
        // 实例化前
        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 {
        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);
    }
}
​
/**
 * Resolve the bean class for the specified bean definition,
 * resolving a bean class name into a Class reference (if necessary)
 * and storing the resolved Class in the bean definition for further use.
 * @param mbd the merged bean definition to determine the class for
 * @param beanName the name of the bean (for error handling purposes)
 * @param typesToMatch the types to match in case of internal type matching purposes
 * (also signals that the returned {@code Class} will never be exposed to application code)
 * @return the resolved bean class (or {@code null} if none)
 * @throws CannotLoadBeanClassException if we failed to load the class
 */
@Nullable
protected Class resolveBeanClass(RootBeanDefinition mbd, String beanName, Class... typesToMatch)
        throws CannotLoadBeanClassException {
​
    try {
        // 如果beanClass被加载了，return (this.beanClass instanceof Class);
        if (mbd.hasBeanClass()) {
            return mbd.getBeanClass();
        }
​
        // 如果beanClass没有被加载
        if (System.getSecurityManager() != null) {
            return AccessController.doPrivileged((PrivilegedExceptionAction>)
                    () -> doResolveBeanClass(mbd, typesToMatch), getAccessControlContext());
        }
        else {
            return doResolveBeanClass(mbd, typesToMatch);
        }
    }
    catch (PrivilegedActionException pae) {
        ClassNotFoundException ex = (ClassNotFoundException) pae.getException();
        throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), ex);
    }
    catch (ClassNotFoundException ex) {
        throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), ex);
    }
    catch (LinkageError err) {
        throw new CannotLoadBeanClassException(mbd.getResourceDescription(), beanName, mbd.getBeanClassName(), err);
    }
}
​
@Nullable
private Class doResolveBeanClass(RootBeanDefinition mbd, Class... typesToMatch)
        throws ClassNotFoundException {
​
    ClassLoader beanClassLoader = getBeanClassLoader();
    ClassLoader dynamicLoader = beanClassLoader;
    boolean freshResolve = false;
​
    if (!ObjectUtils.isEmpty(typesToMatch)) {
        // When just doing type checks (i.e. not creating an actual instance yet),
        // use the specified temporary class loader (e.g. in a weaving scenario).
        ClassLoader tempClassLoader = getTempClassLoader();
        if (tempClassLoader != null) {
            dynamicLoader = tempClassLoader;
            freshResolve = true;
            if (tempClassLoader instanceof DecoratingClassLoader) {
                DecoratingClassLoader dcl = (DecoratingClassLoader) tempClassLoader;
                for (Class typeToMatch : typesToMatch) {
                    dcl.excludeClass(typeToMatch.getName());
                }
            }
        }
    }
​
    String className = mbd.getBeanClassName();
    if (className != null) {
        // 解析Spring表达式，有可能直接返回了一个Class对象
        Object evaluated = evaluateBeanDefinitionString(className, mbd);
        if (!className.equals(evaluated)) {
            // A dynamically resolved expression, supported as of 4.2...
            if (evaluated instanceof Class) {
                return (Class) evaluated;
            }
            else if (evaluated instanceof String) {
                className = (String) evaluated;
                freshResolve = true;
            }
            else {
                throw new IllegalStateException("Invalid class name expression result: " + evaluated);
            }
        }
        if (freshResolve) {
            // When resolving against a temporary class loader, exit early in order
            // to avoid storing the resolved Class in the bean definition.
            if (dynamicLoader != null) {
                try {
                    return dynamicLoader.loadClass(className);
                }
                catch (ClassNotFoundException ex) {
                    if (logger.isTraceEnabled()) {
                        logger.trace("Could not load class [" + className + "] from " + dynamicLoader + ": " + ex);
                    }
                }
            }
            return ClassUtils.forName(className, dynamicLoader);
        }
    }
​
    // Resolve regularly, caching the result in the BeanDefinition...
    return mbd.resolveBeanClass(beanClassLoader);
}
​
/**
 * 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.
        // synthetic表示合成，如果某些Bean式合成的，那么则不会经过BeanPostProcessor的处理
        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;
}
​
/**
 * Apply InstantiationAwareBeanPostProcessors to the specified bean definition
 * (by class and name), invoking their {@code postProcessBeforeInstantiation} methods.
 * 
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) {
            // 注意：返回的类型没判断，可以和参数beanClass类型不一样
            return result; // 有结果就返回，没有结果循环。实例化前取第一个
        }
    }
    return null;
}
​
@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源码：

/**
 * Actually create the specified bean. Pre-creation processing has already happened
 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
 * 
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 {
​
    // 实例化bean
    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        // 有可能在本Bean创建之前，就有其他Bean把当前Bean给创建出来了（比如依赖注入过程中）
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        // 创建Bean实例
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    Object bean = instanceWrapper.getWrappedInstance();
    Class beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }
​
    // 后置处理合并后的BeanDefinition
    // 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;
        }
    }
​
    // 为了解决循环依赖提前缓存单例创建工厂
    // 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");
        }
        // 循环依赖-添加到三级缓存
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
​
    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        // 属性填充
        populateBean(beanName, mbd, instanceWrapper);
​
        // 初始化
        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)) {
                // beanName被哪些bean依赖了，现在发现beanName所对应的bean对象发生了改变，那么则会报错
                String[] dependentBeans = getDependentBeans(beanName);
                Set 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;
}

总结BeanPostProcessor

1、InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation()

2、实例化

3、MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition()

4、InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation()

5、自动注入

6、InstantiationAwareBeanPostProcessor.postProcessProperties()

7、Aware对象

8、BeanPostProcessor.postProcessBeforeInitialization()

9、初始化

10、BeanPostProcessor.postProcessAfterInitialization()