通過前面的分析，我們結束了對XML配置文件的解析，接下來將進行bean加載的分析。對於加載bean的功能，在Spring中的調用方式為：

或者 MyTestBean tb = bf.getBean("myTestBean", MyTestBean.class);

一：整體分析

（1.1）AbstractBeanFactory 類

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {

    /**
 
 Parent bean factory, for bean inheritance support */
    private BeanFactory parentBeanFactory;

    /** ClassLoader to resolve bean class names with, if necessary */
    private ClassLoader beanClassLoader = ClassUtils.getDefaultClassLoader();

    /** ClassLoader to temporarily resolve bean class names with, if necessary 
 
*/
    private ClassLoader tempClassLoader;

    /** Whether to cache bean metadata or rather reobtain it for every access */
    private boolean cacheBeanMetadata = true;

    /** Resolution strategy for expressions in bean definition values */
    private BeanExpressionResolver beanExpressionResolver;

    
 
/** Spring ConversionService to use instead of PropertyEditors */
    private ConversionService conversionService;

    /** Custom PropertyEditorRegistrars to apply to the beans of this factory */
    private final Set<PropertyEditorRegistrar> propertyEditorRegistrars =
            new LinkedHashSet<PropertyEditorRegistrar>(4);

    /** Custom PropertyEditors to apply to the beans of this factory */
    private final Map<Class<?>, Class<? extends PropertyEditor>> customEditors =
            new HashMap<Class<?>, Class<? extends PropertyEditor>>(4);

    /** A custom TypeConverter to use, overriding the default PropertyEditor mechanism */
    private TypeConverter typeConverter;

    /** String resolvers to apply e.g. to annotation attribute values */
    private final List<StringValueResolver> embeddedValueResolvers = new LinkedList<StringValueResolver>();

    /** BeanPostProcessors to apply in createBean */
    private final List<BeanPostProcessor> beanPostProcessors = new ArrayList<BeanPostProcessor>();

    /** Indicates whether any InstantiationAwareBeanPostProcessors have been registered */
    private boolean hasInstantiationAwareBeanPostProcessors;

    /** Indicates whether any DestructionAwareBeanPostProcessors have been registered */
    private boolean hasDestructionAwareBeanPostProcessors;

    /** Map from scope identifier String to corresponding Scope */
    private final Map<String, Scope> scopes = new LinkedHashMap<String, Scope>(8);

    /** Security context used when running with a SecurityManager */
    private SecurityContextProvider securityContextProvider;

    /** Map from bean name to merged RootBeanDefinition */
    private final Map<String, RootBeanDefinition> mergedBeanDefinitions =
            new ConcurrentHashMap<String, RootBeanDefinition>(256);

    /** Names of beans that have already been created at least once */
    private final Set<String> alreadyCreated =
            Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(256));

    /** Names of beans that are currently in creation */
    private final ThreadLocal<Object> prototypesCurrentlyInCreation =
            new NamedThreadLocal<Object>("Prototype beans currently in creation");


    /**
     * Create a new AbstractBeanFactory.
     */
    public AbstractBeanFactory() {
    }

    /**
     * Create a new AbstractBeanFactory with the given parent.
     * @param parentBeanFactory parent bean factory, or {@code null} if none
     * @see #getBean
     */
    public AbstractBeanFactory(BeanFactory parentBeanFactory) {
        this.parentBeanFactory = parentBeanFactory;
    }


    //---------------------------------------------------------------------
    // Implementation of BeanFactory interface
    //---------------------------------------------------------------------

    @Override
    public Object getBean(String name) throws BeansException {
        return doGetBean(name, null, null, false);
    }

    @Override
    public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
        return doGetBean(name, requiredType, null, false);
    }

    @Override
    public Object getBean(String name, Object... args) throws BeansException {
        return doGetBean(name, null, args, false);
    }

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * @param name the name of the bean to retrieve
     * @param requiredType the required type of the bean to retrieve
     * @param args arguments to use when creating a bean instance using explicit arguments
     * (only applied when creating a new instance as opposed to retrieving an existing one)
     * @return an instance of the bean
     * @throws BeansException if the bean could not be created
     */
    public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
        return doGetBean(name, requiredType, args, false);
    }

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * @param name the name of the bean to retrieve
     * @param requiredType the required type of the bean to retrieve
     * @param args arguments to use when creating a bean instance using explicit arguments
     * (only applied when creating a new instance as opposed to retrieving an existing one)
     * @param typeCheckOnly whether the instance is obtained for a type check,
     * not for actual use
     * @return an instance of the bean
     * @throws BeansException if the bean could not be created
     */
    @SuppressWarnings("unchecked")
    protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
　　　　 //提取對應的beanName
        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 (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);
                }
            }

            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, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                            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, new ObjectFactory<Object>() {
                            @Override
                            public Object getObject() throws BeansException {
                                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.
        if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
            try {
                return getTypeConverter().convertIfNecessary(bean, requiredType);
            }
            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());
            }
        }
        return (T) bean;
    }


}

僅從代碼量上就能看出bean的加載經歷了一個相當復雜的過程，其中涉及各種各樣的考慮。通過上面的代碼，可以粗略了解整個Spring加載bean的過程。對應加載過程中所涉及的步驟大致如下：

（1）轉換對應beanName

（2）嘗試從緩存中加載單例。

（3）bean的實例化

（4）原型模式的依賴檢查。

（5）檢測parentBeanFactory

（6）將存儲XML配置文件的GernericBeanDefinition轉換為RootBeanDefinition

（7）剩下的步驟

經過上面的步驟，bean的加載就結束了，這個時候就可以返回我們所需要的bean了。其中最重要的就是步驟（8），針對不同的scope進行bean的創建，你會看到各種常用的Spring特性在這裏的實現。

二：FactoryBean的使用

public interface FactoryBean<T> {

    /**
     * Return an instance (possibly shared or independent) of the object
     * managed by this factory.
     * <p>As with a {@link BeanFactory}, this allows support for both the
     * Singleton and Prototype design pattern.
     * <p>If this FactoryBean is not fully initialized yet at the time of
     * the call (for example because it is involved in a circular reference),
     * throw a corresponding {@link FactoryBeanNotInitializedException}.
     * <p>As of Spring 2.0, FactoryBeans are allowed to return {@code null}
     * objects. The factory will consider this as normal value to be used; it
     * will not throw a FactoryBeanNotInitializedException in this case anymore.
     * FactoryBean implementations are encouraged to throw
     * FactoryBeanNotInitializedException themselves now, as appropriate.
     * @return an instance of the bean (can be {@code null})
     * @throws Exception in case of creation errors
     * @see FactoryBeanNotInitializedException
     */
    T getObject() throws Exception;

    /**
     * Return the type of object that this FactoryBean creates,
     * or {@code null} if not known in advance.
     * <p>This allows one to check for specific types of beans without
     * instantiating objects, for example on autowiring.
     * <p>In the case of implementations that are creating a singleton object,
     * this method should try to avoid singleton creation as far as possible;
     * it should rather estimate the type in advance.
     * For prototypes, returning a meaningful type here is advisable too.
     * <p>This method can be called <i>before</i> this FactoryBean has
     * been fully initialized. It must not rely on state created during
     * initialization; of course, it can still use such state if available.
     * <p><b>NOTE:</b> Autowiring will simply ignore FactoryBeans that return
     * {@code null} here. Therefore it is highly recommended to implement
     * this method properly, using the current state of the FactoryBean.
     * @return the type of object that this FactoryBean creates,
     * or {@code null} if not known at the time of the call
     * @see ListableBeanFactory#getBeansOfType
     */
    Class<?> getObjectType();

    /**
     * Is the object managed by this factory a singleton? That is,
     * will {@link #getObject()} always return the same object
     * (a reference that can be cached)?
     * <p><b>NOTE:</b> If a FactoryBean indicates to hold a singleton object,
     * the object returned from {@code getObject()} might get cached
     * by the owning BeanFactory. Hence, do not return {@code true}
     * unless the FactoryBean always exposes the same reference.
     * <p>The singleton status of the FactoryBean itself will generally
     * be provided by the owning BeanFactory; usually, it has to be
     * defined as singleton there.
     * <p><b>NOTE:</b> This method returning {@code false} does not
     * necessarily indicate that returned objects are independent instances.
     * An implementation of the extended {@link SmartFactoryBean} interface
     * may explicitly indicate independent instances through its
     * {@link SmartFactoryBean#isPrototype()} method. Plain {@link FactoryBean}
     * implementations which do not implement this extended interface are
     * simply assumed to always return independent instances if the
     * {@code isSingleton()} implementation returns {@code false}.
     * @return whether the exposed object is a singleton
     * @see #getObject()
     * @see SmartFactoryBean#isPrototype()
     */
    boolean isSingleton();

}

一般情況下，Spring通過反射機制利用bean的class屬性指定實現類來實例化bean。在某些情況下，實例化bean過程比較復雜，如果按照傳統的方式，則需要在<bean>中提供大量的配置信息，配置方式的靈活性是受限的，這時采用編碼的方式可能會得到一個簡單的方案。Spring為此提供了一個org.springframework.beans.factory.FactoryBean的工廠類接口，用戶可以通過實現該接口定制實例化bean的邏輯。

Spring 源碼分析（六）--bean的加載整體分析