spring源碼學習（五）Bean的加載（上）

這篇文章主要介紹spring的bean的加載。

這是doGetBean的方法，也是加載bean的方法。

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

        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        Object sharedInstance = getSingleton(beanName);
        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);
        }

        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 {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }
            //如果不僅做類型檢查，這裡要進行記錄
            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);
                    }
                }

                // 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);
                }

                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.
        // Note that the following return declarations are technically violating the
        // non-null policy for the getBean methods: However, these will only result
        // in null under very specific circumstances: such as a user-declared factory
        // method returning null or a user-provided FactoryBean.getObject() returning
        // null, without any custom post-processing of such null values. We will pass
        // them on as null to corresponding injection points in that exceptional case
        // but do not expect user-level getBean callers to deal with such null values.
        // In the end, regular getBean callers should be able to assign the outcome
        // to non-null variables/arguments without being compromised by rather esoteric
        // corner cases, in particular in functional configuration and Kotlin scenarios.
        // A future Spring generation might eventually forbid null values completely
        // and throw IllegalStateExceptions instead of leniently passing them through.
        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());
            }
        }
        // For the nullability warning, see the elaboration in the comment above;
        // in short: This is never going to be null unless user-declared code enforces null.
        return (T) bean;
    }

           

這段代碼非常長，bean對象加載的過程就是上面這個代碼段。先把轉換beanName，然後看看有沒有緩存，若有緩存則執行個體化bean，若沒有則看看是非是原型模式依賴的情況，檢查parentBeanFactory,把gernericBeanDefinition轉換成RootBeanFactory，尋找依賴，針對不同的scope建立bean,轉換類型。

getSingleton函數體

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            synchronized (this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return singletonObject;
    }
           

檢查緩存中是否存在執行個體，如果為空，則從earlySingletonObjects裡面擷取，如果還擷取不到，再嘗試從SingletonFactories裡面擷取beanName對應的ObjectFactory，調用這個ObjectFactory來建立bean，然後放到earlySingletonObjects裡面去，然後從SingletonFactories把這個beanName去掉。

getObjectForBeanInstance函數

protected Object getObjectForBeanInstance(
            Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

        // Don't let calling code try to dereference the factory if the bean isn't a factory.
        if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
            throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
        }

        // Now we have the bean instance, which may be a normal bean or a FactoryBean.
        // If it's a FactoryBean, we use it to create a bean instance, unless the
        // caller actually wants a reference to the factory.
        if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
            return beanInstance;
        }

        Object object = null;
        if (mbd == null) {
            object = getCachedObjectForFactoryBean(beanName);
        }
        if (object == null) {
            // Return bean instance from factory.
            FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
            // Caches object obtained from FactoryBean if it is a singleton.
            if (mbd == null && containsBeanDefinition(beanName)) {
                mbd = getMergedLocalBeanDefinition(beanName);
            }
            boolean synthetic = (mbd != null && mbd.isSynthetic());
            object = getObjectFromFactoryBean(factory, beanName, !synthetic);
        }
        return object;
    }
           

對factoryBean進行驗證，對bean進行轉換，将factory解析bean的工作交給getObjectFromFactoryBean完成。

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
        if (factory.isSingleton() && containsSingleton(beanName)) {
            synchronized (getSingletonMutex()) {
                Object object = this.factoryBeanObjectCache.get(beanName);
                if (object == null) {
                    object = doGetObjectFromFactoryBean(factory, beanName);
                    // Only post-process and store if not put there already during getObject() call above
                    // (e.g. because of circular reference processing triggered by custom getBean calls)
                    Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
                    if (alreadyThere != null) {
                        object = alreadyThere;
                    }
                    else {
                        if (shouldPostProcess) {
                            try {
                                object = postProcessObjectFromFactoryBean(object, beanName);
                            }
                            catch (Throwable ex) {
                                throw new BeanCreationException(beanName,
                                        "Post-processing of FactoryBean's singleton object failed", ex);
                            }
                        }
                        this.factoryBeanObjectCache.put(beanName, object);
                    }
                }
                return object;
            }
        }
        else {
            Object object = doGetObjectFromFactoryBean(factory, beanName);
            if (shouldPostProcess) {
                try {
                    object = postProcessObjectFromFactoryBean(object, beanName);
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
                }
            }
            return object;
        }
    }
           

在這個方法體裡主要保證傳回的bean如果是單例就保證全局唯一性，不用重複建立單例，可以使用緩存來提高性能，以便下擷取。

doGetObjectFromFactoryBean方法

private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
            throws BeanCreationException {

        Object object;
        try {
            if (System.getSecurityManager() != null) {
                AccessControlContext acc = getAccessControlContext();
                try {
                    object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () ->
                            factory.getObject(), acc);
                }
                catch (PrivilegedActionException pae) {
                    throw pae.getException();
                }
            }
            else {
                object = factory.getObject();
            }
        }
        catch (FactoryBeanNotInitializedException ex) {
            throw new BeanCurrentlyInCreationException(beanName, ex.toString());
        }
        catch (Throwable ex) {
            throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
        }

        // Do not accept a null value for a FactoryBean that's not fully
        // initialized yet: Many FactoryBeans just return null then.
        if (object == null) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(
                        beanName, "FactoryBean which is currently in creation returned null from getObject");
            }
            object = new NullBean();
        }
        return object;
    }
           

當bean是FactoryBean類型時，提取Bean是得到FactoryBean中的getObject方法中放回的bean。

我們拿到factory.getObject()，然後程式回到getObjectFromFactoryBean方法中，沒有直接傳回，而是執行了postProcessObjectFromFactoryBean(object, beanName);方法。

protected Object postProcessObjectFromFactoryBean(Object object, String beanName) throws BeansException {
        return object;
    }
           

沒有做任何處理，應該是為了讓繼承的類重寫方法，然後可以對擷取到的object來進行處理。