spring源碼解析 -- 構造bean

源碼分析基於spring 4.3.x

前面文章已經分析了spring如何加載xml配置中的bean元數據，如今來分析一下spring構造bean的過程。
關於閱讀源碼的思路，可參考 -- 如何閱讀java源碼

BeanFactory xmlBeanFactory = new XmlBeanFactory(new ClassPathResource("application.xml"));
Blog bean = (Blog)xmlBeanFactory.getBean("blog");

XmlBeanFactory的父類AbstractBeanFactory是spring中一個很重要的類，他爲BeanFactory提供基礎服務。構造bean就是由該類實現的。
AbstractBeanFactory#doGetBean

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;

    Object sharedInstance = getSingleton(beanName);    // #1
    if (sharedInstance != null && args == null) {
        ...
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);    // #2
    }

    else {
        if (isPrototypeCurrentlyInCreation(beanName)) {    // #3
            throw new BeanCurrentlyInCreationException(beanName);
        }

        BeanFactory parentBeanFactory = getParentBeanFactory();
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {    // #4
            ...
        }

        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);    // #5
        }

        try {
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);    // #6
            checkMergedBeanDefinition(mbd, beanName, args);

            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);    // #7
                    }
                    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, () -> {        // #8
                    try {
                        return createBean(beanName, mbd, args);    // #9
                    }
                    catch (BeansException ex) {
                        destroySingleton(beanName);
                        throw ex;
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            ... 
        }
        catch (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }

    // Check if required type matches the type of the actual bean instance.
    if (requiredType != null && !requiredType.isInstance(bean)) {    // #10
        try {
            T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
            if (convertedBean == null) {
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
            return convertedBean;
        }
        catch (TypeMismatchException ex) {
            if (logger.isTraceEnabled()) {
                logger.trace("Failed to convert bean '" + name + "' to required type '" +
                        ClassUtils.getQualifiedName(requiredType) + "'", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    return (T) bean;
}

#1 單例的bean，先從緩衝中查詢
getSingleton方法會查詢
singletonObject，earlySingletonObjects，singletonFactories等緩存，它們都是簡單的map，緩存了單例的bean，正在建立的bean和ObjectFactory對象。

#2 若是構造的bean是FactoryBean，進行對應的處理
getObjectForBeanInstance方法會根據參數beanInstance進行處理，若是beanInstance是FactoryBean，會調用其getObject()方法建立bean，若是不是，返回直接返回該beanInstance參數。
FactoryBean是spring提供的一個擴展接口，用戶實現該接口能夠自定義bean的建立。

#3 若是如今正在建立這個bean，則直接報錯，這時極可能陷入循環引用了
#4 當前BeanFactory不存在對應的BeanDefinition，嘗試經過父BeanFactory構造bean
#5 標記這個bean正在構造中
#6 獲取BeanDefinition
前面解析spring加載bean數據的文章中說過，spring會將bean元數據轉化爲BeanDefinition，存入DefaultListableBeanFactory#beanDefinitionMap屬性中。
getMergedLocalBeanDefinition方法會獲取對應BeanDefinition，若是BeanDefinition存在ParentName，會獲取父BeanDefinition，再合併元數據。

#7 先構造依賴的bean
#8 使用ObjectFactory構造並註冊一個bean，getSingleton方法也要完成構造bean的準備和藹後工做。
#9 匿名的ObjectFactory，調用AbstractAutowireCapableBeanFactory#createBean進行實際的構造bean工做
#10 bean類型轉換

spring中bean有singleton，prototype等範圍，這裏只關注singleton類型的bean的構造過程。

#8步驟，DefaultSingletonBeanRegistry#getSingleton（注意getSingleton有重載方法）

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
        Assert.notNull(beanName, "Bean name must not be null");
        synchronized (this.singletonObjects) {
            Object singletonObject = this.singletonObjects.get(beanName);
            if (singletonObject == null) {
                ...
                beforeSingletonCreation(beanName);    // #1
                boolean newSingleton = false;
                boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = new LinkedHashSet<>();
                }
                try {
                    singletonObject = singletonFactory.getObject();    // #2
                    newSingleton = true;
                }
                ...
                finally {
                    if (recordSuppressedExceptions) {
                        this.suppressedExceptions = null;
                    }
                    afterSingletonCreation(beanName);
                }
                if (newSingleton) {
                    addSingleton(beanName, singletonObject);    // #3
                }
            }
            return singletonObject;
        }
    }

#1 僅作錯誤檢查,也是提供給子類的擴展方法
#2 真正構造bean的方法，調用AbstractBeanFactory#doGetBean方法#9步驟的匿名類處理，實際調用AbstractAutowireCapableBeanFactory#createBean
#3 單例的bean，加入緩衝中

真正的構造bean方法
AbstractAutowireCapableBeanFactory#createBean

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;

        Class<?> resolvedClass = resolveBeanClass(mbd, beanName);    // #1
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

        try {
            mbdToUse.prepareMethodOverrides();    // #2
        }
        ...

        try {
            Object bean = resolveBeforeInstantiation(beanName, mbdToUse);    // #3
            if (bean != null) {
                return bean;
            }
        }
        ...

        try {
            Object beanInstance = doCreateBean(beanName, mbdToUse, args);    // #4
            if (logger.isTraceEnabled()) {
                logger.trace("Finished creating instance of bean '" + beanName + "'");
            }
            return beanInstance;
        }
        ...
    }

#1 確保jdk已加載bean的class
#2 對bean的lookup-method和replace-method作檢查工做
#3 spring的擴展機制，調用BeanPostProcessor#postProcessBeforeInstantiation方法，
注意，若是resolveBeforeInstantiation返回非null對象，這裏將直接返回該對象做爲bean，spring再也不構造該bean。
#4 繼續構造bean

AbstractAutowireCapableBeanFactory#doCreateBean

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

    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);    // #1
    }
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);    // #2
    }
    final Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    // Allow post-processors to modify the merged bean definition.
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);    // #3
            }
            catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    ...

    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        populateBean(beanName, mbd, instanceWrapper);    // #4
        exposedObject = initializeBean(beanName, exposedObject, mbd);    // #5
    }
    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);
        }
    }

    ...

    // Register bean as disposable.
    try {
        registerDisposableBeanIfNecessary(beanName, bean, mbd);        // #6
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

#1 查詢BeanWrapper緩衝
#2 構造一個空的（屬性未注入）bean，生成BeanWrapper
#3 spring擴展機制，調用MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition
#4 populateBean負責注入屬性到bean中，後面有對應文章解析該步驟
#5 spring擴展機制，調用Aware方法和init方法，並調用BeanPostProcessor#postProcessAfterInitialization方法。
#6 若是bean存在Destroy方法，或存在對應的DestructionAwareBeanPostProcessor，註冊該bean爲disposable。（該bean銷燬時要調用對應的銷燬機制）

這裏涉及到循環引用的處理，比較繁瑣，因此省略了不少代碼，只保留bean建立相關的代碼。

BeanWrapper是對bean的包裝類，提供了對bean的class，property進行操做的方法。

#2步驟，AbstractAutowireCapableBeanFactory#createBeanInstance

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
        Class<?> beanClass = resolveBeanClass(mbd, beanName);    // #1

        if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                    "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
        }

        if (mbd.getFactoryMethodName() != null)  {    // #2
            return instantiateUsingFactoryMethod(beanName, mbd, args);
        }

        boolean resolved = false;
        boolean autowireNecessary = false;
        if (args == null) {
            synchronized (mbd.constructorArgumentLock) {
                if (mbd.resolvedConstructorOrFactoryMethod != null) {    // #3
                    resolved = true;
                    autowireNecessary = mbd.constructorArgumentsResolved;
                }
            }
        }
        if (resolved) {    
            if (autowireNecessary) {
                return autowireConstructor(beanName, mbd, null, null);    // #4
            }
            else {
                return instantiateBean(beanName, mbd);    // #5
            }
        }

        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);    // #6
        if (ctors != null ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
            return autowireConstructor(beanName, mbd, ctors, args);
        }

        // No special handling: simply use no-arg constructor.
        return instantiateBean(beanName, mbd);
    }

#1 獲取bean的class
#2 若是存在factoryMethod，經過factoryMethod構造bean
#3 判斷該class已經構造過bean了
#4 使用以前選擇好的構造函數構造bean
#5 使用無參構造函數構造bean
#6 spring根據構造函數的參數，自行選擇構造函數，邏輯較複雜。

下面看看使用無參構造方法構造bean，AbstractAutowireCapableBeanFactory#instantiateBean -> SimpleInstantiationStrategy#instantiate

public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
        if (bd.getMethodOverrides().isEmpty()) {
            Constructor<?> constructorToUse;
            synchronized (bd.constructorArgumentLock) {
                constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
                if (constructorToUse == null) {
                    final Class<?> clazz = bd.getBeanClass();
                    if (clazz.isInterface()) {
                        throw new BeanInstantiationException(clazz, "Specified class is an interface");
                    }
                    try {
                        if (System.getSecurityManager() != null) {
                            ...
                        }
                        else {
                            constructorToUse =    clazz.getDeclaredConstructor((Class[]) null);    // #1
                        }
                        bd.resolvedConstructorOrFactoryMethod = constructorToUse;    // #2
                    }
                    catch (Throwable ex) {
                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                    }
                }
            }
            return BeanUtils.instantiateClass(constructorToUse);    // #3
        }
        else {
            return instantiateWithMethodInjection(bd, beanName, owner);        // #4
        }
    }

#1 獲取無參構造函數
#2 添加無參構造函數加入到BeanDefinition緩存中
#2 BeanUtils#instantiateClass經過構造函數構造bean
#3 bean中存在lookup-method或replace-method，必須使用CGLIB構造bean

到這裏，bean已經構造完成，下一步就是注入屬性了。

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