Spring IOC 容器源码分析 - 创建单例 bean 的过程

public T doGetBean(...) {
    // 省略不相关代码
    if (mbd.isSingleton()) {
        sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
            @Override
            public Object getObject() throws BeansException {
                try {
                    return createBean(beanName, mbd, args);
                }
                catch (BeansException ex) {
                    destroySingleton(beanName);
                    throw ex;
                }
            }
        });
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
    }
    // 省略不相关代码
}

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "'beanName' must not be null");
    synchronized (this.singletonObjects) {
        // 从缓存中获取单例 bean，若不为空，则直接返回，不用再初始化
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                        "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                        "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
            }
            /* 
             * 将 beanName 添加到 singletonsCurrentlyInCreation 集合中，
             * 用于表明 beanName 对应的 bean 正在创建中
             */
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<Exception>();
            }
            try {
                // 通过 getObject 方法调用 createBean 方法创建 bean 实例
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            }
            catch (IllegalStateException ex) {
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    throw ex;
                }
            }
            catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            }
            finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                // 将 beanName 从 singletonsCurrentlyInCreation 移除
                afterSingletonCreation(beanName);
            }
            if (newSingleton) {
                /* 
                 * 将 <beanName, singletonObject> 键值对添加到 singletonObjects 集合中，
                 * 并从其他集合（比如 earlySingletonObjects）中移除 singletonObject 记录
                 */
                addSingleton(beanName, singletonObject);
            }
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }
}

protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
    if (logger.isDebugEnabled()) {
        logger.debug("Creating instance of bean '" + beanName + "'");
    }
    RootBeanDefinition mbdToUse = mbd;

    // 解析 bean 的类型
    Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }

    try {
        // 处理 lookup-method 和 replace-method 配置，Spring 将这两个配置统称为 override method
        mbdToUse.prepareMethodOverrides();
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                beanName, "Validation of method overrides failed", ex);
    }

    try {
        // 在 bean 初始化前应用后置处理，如果后置处理返回的 bean 不为空，则直接返回
        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);
    }

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

public void prepareMethodOverrides() throws BeanDefinitionValidationException {
    MethodOverrides methodOverrides = getMethodOverrides();
    if (!methodOverrides.isEmpty()) {
        Set<MethodOverride> overrides = methodOverrides.getOverrides();
        synchronized (overrides) {
            // 循环处理每个 MethodOverride 对象
            for (MethodOverride mo : overrides) {
                prepareMethodOverride(mo);
            }
        }
    }
}

protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
    // 获取方法名为 mo.getMethodName() 的方法数量，当方法重载时，count 的值就会大于1
    int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
    // count = 0，表明根据方法名未找到相应的方法，此时抛出异常
    if (count == 0) {
        throw new BeanDefinitionValidationException(
                "Invalid method override: no method with name '" + mo.getMethodName() +
                "' on class [" + getBeanClassName() + "]");
    }
    // 若 count = 1，表明仅存在已方法名为 mo.getMethodName()，这意味着方法不存在重载
    else if (count == 1) {
        // 方法不存在重载，则将 overloaded 成员变量设为 false
        mo.setOverloaded(false);
    }
}

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
    Object bean = null;
    // 检测是否解析过，mbd.beforeInstantiationResolved 的值在下面的代码中会被设置
    if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            Class<?> targetType = determineTargetType(beanName, mbd);
            if (targetType != null) {
                // 应用前置处理
                bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                if (bean != null) {
                    // 应用后置处理
                    bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                }
            }
        }
        // 设置 mbd.beforeInstantiationResolved
        mbd.beforeInstantiationResolved = (bean != null);
    }
    return bean;
}

protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
    for (BeanPostProcessor bp : getBeanPostProcessors()) {
        // InstantiationAwareBeanPostProcessor 一般在 Spring 框架内部使用，不建议用户直接使用
        if (bp instanceof InstantiationAwareBeanPostProcessor) {
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
            // bean 初始化前置处理
            Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
            if (result != null) {
                return result;
            }
        }
    }
    return null;
}

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

    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
       // bean 初始化后置处理
        result = beanProcessor.postProcessAfterInitialization(result, beanName);
        if (result == null) {
            return result;
        }
    }
    return result;
}

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
        throws BeanCreationException {

    /* 
     * BeanWrapper 是一个基础接口，由接口名可看出这个接口的实现类用于包裹 bean 实例。
     * 通过 BeanWrapper 的实现类可以方便的设置/获取 bean 实例的属性
     */
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        // 从缓存中获取 BeanWrapper，并清理相关记录
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        /* 
         * 创建 bean 实例，并将实例包裹在 BeanWrapper 实现类对象中返回。createBeanInstance 
         * 中包含三种创建 bean 实例的方式：
         *   1. 通过工厂方法创建 bean 实例
         *   2. 通过构造方法自动注入（autowire by constructor）的方式创建 bean 实例
         *   3. 通过无参构造方法方法创建 bean 实例
          *
         * 若 bean 的配置信息中配置了 lookup-method 和 replace-method，则会使用 CGLIB 
         * 增强 bean 实例。关于这个方法，后面会专门写一篇文章介绍，这里先说这么多。
         */
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    // 此处的 bean 可以认为是一个原始的 bean 实例，暂未填充属性
    final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
    Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
    mbd.resolvedTargetType = beanType;

    // 这里又遇到后置处理了，此处的后置处理是用于处理已“合并的 BeanDefinition”。关于这种后置处理器具体的实现细节就不深入理解了，大家有兴趣可以自己去看
    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;
        }
    }

    /*
     * earlySingletonExposure 是一个重要的变量，这里要说明一下。该变量用于表示是否提前暴露
     * 单例 bean，用于解决循环依赖。earlySingletonExposure 由三个条件综合而成，如下：
     *   条件1：mbd.isSingleton() - 表示 bean 是否是单例类型
     *   条件2：allowCircularReferences - 是否允许循环依赖
     *   条件3：isSingletonCurrentlyInCreation(beanName) - 当前 bean 是否处于创建的状态中
     * 
     * earlySingletonExposure = 条件1 && 条件2 && 条件3 
     *                        = 单例 && 是否允许循环依赖 && 是否存于创建状态中。
     */
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
            isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        if (logger.isDebugEnabled()) {
            logger.debug("Eagerly caching bean '" + beanName +
                    "' to allow for resolving potential circular references");
        }
        // 添加工厂对象到 singletonFactories 缓存中
        addSingletonFactory(beanName, new ObjectFactory<Object>() {
            @Override
            public Object getObject() throws BeansException {
                // 获取早期 bean 的引用，如果 bean 中的方法被 AOP 切点所匹配到，此时 AOP 相关逻辑会介入
                return getEarlyBeanReference(beanName, mbd, bean);
            }
        });
    }

    Object exposedObject = bean;
    try {
        // 向 bean 实例中填充属性，populateBean 方法也是一个很重要的方法，后面会专门写文章分析
        populateBean(beanName, mbd, instanceWrapper);
        if (exposedObject != null) {
            /*
             * 进行余下的初始化工作，详细如下：
             * 1. 判断 bean 是否实现了 BeanNameAware、BeanFactoryAware、
             *    BeanClassLoaderAware 等接口，并执行接口方法
             * 2. 应用 bean 初始化前置操作
             * 3. 如果 bean 实现了 InitializingBean 接口，则执行 afterPropertiesSet 
             *    方法。如果用户配置了 init-method，则调用相关方法执行自定义初始化逻辑
             * 4. 应用 bean 初始化后置操作
             * 
             * 另外，AOP 相关逻辑也会在该方法中织入切面逻辑，此时的 exposedObject 就变成了
             * 一个代理对象了
             */
            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) {
            // 若 initializeBean 方法未改变 exposedObject 的引用，则此处的条件为 true。
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            }
            // 下面的逻辑我也没完全搞懂，就不分析了。见谅。
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<String>(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 " +
                            "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
                }
            }
        }
    }

    try {
        // 注册销毁逻辑
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}