Spring之BeanFactory：解析getBean()方法

初探getBean()方法

在使用Spring的时候可以通过如下方式调用getBean方法来获取某个Bean：

User user =  context.getBean(User.class);

AbstractApplicationContext中定义了一系列getBean方法，代码如下：

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

    @Override
    public Object getBean(String name) throws BeansException {
        assertBeanFactoryActive();
        return getBeanFactory().getBean(name);
    }

    @Override
    public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
        assertBeanFactoryActive();
        return getBeanFactory().getBean(name, requiredType);
    }

    @Override
    public Object getBean(String name, Object... args) throws BeansException {
        assertBeanFactoryActive();
        return getBeanFactory().getBean(name, args);
    }

    @Override
    public <T> T getBean(Class<T> requiredType) throws BeansException {
        assertBeanFactoryActive();
        return getBeanFactory().getBean(requiredType);
    }

    @Override
    public <T> T getBean(Class<T> requiredType, Object... args) throws BeansException {
        assertBeanFactoryActive();
        return getBeanFactory().getBean(requiredType, args);
    }

解读：

上述getBean方法本质上是通过BeanFactory的getBean方法来实现的。 

Note：

AbstractApplicationContext中getBeanFactory为抽象方法，代码如下：

    /**
     * Subclasses must return their internal bean factory here. They should implement the
     * lookup efficiently, so that it can be called repeatedly without a performance penalty.
     * <p>Note: Subclasses should check whether the context is still active before
     * returning the internal bean factory. The internal factory should generally be
     * considered unavailable once the context has been closed.
     * @return this application context's internal bean factory (never {@code null})
     * @throws IllegalStateException if the context does not hold an internal bean factory yet
     * (usually if {@link #refresh()} has never been called) or if the context has been
     * closed already
     * @see #refreshBeanFactory()
     * @see #closeBeanFactory()
     */
    @Override
    public abstract ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;

解读：

此方法的具体逻辑由子类实现。

Note：

此方法的返回值类型是ConfigurableListableBeanFactory。

类图

下图为BeanFactory相关类图：

解读：

• HierarhicalBeanFactory

该接口允许子容器访问父容器，其中方法如下图所示：

• ListableBeanFactory

该接口定义了访问容器中 Bean 基本信息的若干方法，如：查看 Bean 的个数，获取某一类型 Bean 的配置名，其中方法如下图所示：

• ConfigurableBeanFactory

该接口增强了 IoC 容器的可定制性，它定义了设置类装载器、属性编辑器、容器初始化后置处理器等方法，下图仅列出了其中部分方法：

 

• AutowireCapableBeanFactory

该接口定义了将容器中的 Bean 按某种规则（如：按名称匹配、按类型匹配）进行自动装配的方法，其中方法如下图所示：

Note：

从上述类图可以发现ConfigurableListableBeanFactory的唯一实现类是DefaultListableBeanFactory。

深入getBean()方法

BeanFactory接口中的方法如下图所示：

解读：

这几个getBean方法由AbstractBeanFactory、DefaultListableBeanFactory实现

（1）org.springframework.beans.factory.support.AbstractBeanFactory中getBean()方法的定义如下：

    //---------------------------------------------------------------------
    // 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);
    }

（2）org.springframework.beans.factory.support.DefaultListableBeanFactory中getBean()方法的定义如下：

    //---------------------------------------------------------------------
    // Implementation of remaining BeanFactory methods
    //---------------------------------------------------------------------

    @Override
    public <T> T getBean(Class<T> requiredType) throws BeansException {
        return getBean(requiredType, (Object[]) null);
    }

    @SuppressWarnings("unchecked")
    @Override
    public <T> T getBean(Class<T> requiredType, @Nullable Object... args) throws BeansException {
        Assert.notNull(requiredType, "Required type must not be null");
        Object resolved = resolveBean(ResolvableType.forRawClass(requiredType), args, false);
        if (resolved == null) {
            throw new NoSuchBeanDefinitionException(requiredType);
        }
        return (T) resolved;
    }

AbstractBeanFactory

AbstractBeanFactory中的getBean方法均调用了doGetBean(...)方法，该方法的定义如下：

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

        String beanName = transformedBeanName(name);
        Object beanInstance;

        // Eagerly check singleton cache for manually registered singletons.
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
            if (logger.isTraceEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            beanInstance = 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 if (requiredType != null) {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
                else {
                    return (T) parentBeanFactory.getBean(nameToLookup);
                }
            }

            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
                    .tag("beanName", name);
            try {
                if (requiredType != null) {
                    beanCreation.tag("beanType", requiredType::toString);
                }
                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);
                        try {
                            getBean(dep);
                        }
                        catch (NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }

                // 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;
                        }
                    });
                    beanInstance = 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);
                    }
                    beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                }

                else {
                    String scopeName = mbd.getScope();
                    if (!StringUtils.hasLength(scopeName)) {
                        throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
                    }
                    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);
                            }
                        });
                        beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                    }
                    catch (IllegalStateException ex) {
                        throw new ScopeNotActiveException(beanName, scopeName, ex);
                    }
                }
            }
            catch (BeansException ex) {
                beanCreation.tag("exception", ex.getClass().toString());
                beanCreation.tag("message", String.valueOf(ex.getMessage()));
                cleanupAfterBeanCreationFailure(beanName);
                throw ex;
            }
            finally {
                beanCreation.end();
            }
        }

        return adaptBeanInstance(name, beanInstance, requiredType);
    }

解读：

上述方法调用了createBean方法，对应的参数分别是beanName、mbd、args（建议重点关注一下mbd，RootBeanDefinition类型）。

Note：

Spring 配置文件中每一个 bean 节点元素在 Spring 容器里都由一个BeanDefinition 对象表示，它包含了某个 Bean 的具体信息。

BeanDefinition相关类图如下：

createBean方法的定义如下：

    /**
     * Create a bean instance for the given merged bean definition (and arguments).
     * The bean definition will already have been merged with the parent definition
     * in case of a child definition.
     * <p>All bean retrieval methods delegate to this method for actual bean creation.
     * @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
     */
    protected abstract Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
            throws BeanCreationException;

解读：

上述createBean方法为抽象方法，由子类实现。

---

AbstractAutowireCapableBeanFactory —— createBean方法

AbstractAutowireCapableBeanFactory中CreateBean方法

/**
 * 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, Object[] args) throws BeanCreationException {
    if (logger.isDebugEnabled()) {
        logger.debug("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.
    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.
        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);
    }

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

解读：

一般情况下，某个类型的Bean是由doCreateBean方法实现的，对应代码如下：

/**
 * Actually create the specified bean. Pre-creation processing has already happened
 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
 * <p>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(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
    Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

    // Allow post-processors to modify the merged bean definition.
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            mbd.postProcessed = true;
        }
    }

    // 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.isDebugEnabled()) {
            logger.debug("Eagerly caching bean '" + beanName +
                    "' to allow for resolving potential circular references");
        }
        addSingletonFactory(beanName, new ObjectFactory<Object>() {
            @Override
            public Object getObject() throws BeansException {
                return getEarlyBeanReference(beanName, mbd, bean);
            }
        });
    }

    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        populateBean(beanName, mbd, instanceWrapper);
        if (exposedObject != null) {
            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)) {
                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.");
                }
            }
        }
    }

    // Register bean as disposable.
    try {
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

解读：

上述方法涵盖了Bean生命周期（即：实例化 -> 属性赋值 -> 初始化 -> 销毁）中的前三个阶段