首先來看一段代碼，看過上一節的朋友肯定對這段代碼並不陌生。這一段代碼詮釋了Spring加載bean的完整過程，包括讀取配置文件，掃描包，加載類，實例化bean，註入bean屬性依賴。

public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
 

        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);

        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);

            
 
// Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);

            // Initialize message source for this context.
 

            initMessageSource();

            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            onRefresh();

            // Check for listener beans and register them.
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            finishRefresh();
        }
    }
}

上一節介紹了Spring是如何加載class文件的，本節主要圍繞finishBeanFactoryInitialization(beanFactory)方法，聊聊Spring是如何實例化bean的，從上面代碼片段中的註解不難看出，此方法主要的任務就是實例化非懶加載的單例bean。閑話少敘，看代碼。

/**
 * Finish the initialization of this context‘s bean factory,
 * initializing all remaining singleton beans.
 */
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // Initialize conversion service for this context.
    if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
            beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
        beanFactory.setConversionService(
                beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    }

    // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
    String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }

    // Stop using the temporary ClassLoader for type matching.
    beanFactory.setTempClassLoader(null);

    // Allow for caching all bean definition metadata, not expecting further changes.
    beanFactory.freezeConfiguration();

    // Instantiate all remaining (non-lazy-init) singletons.
    beanFactory.preInstantiateSingletons();
}

上面代碼主要看最後一句beanFactory.preInstantiateSingletons()。

@Override
public void preInstantiateSingletons() throws BeansException {
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Pre-instantiating singletons in " + this);
    }

    // Iterate over a copy to allow for init methods which in turn register new bean definitions.
    // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
    List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);

    // Trigger initialization of all non-lazy singleton beans...
    for (String beanName : beanNames) {
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            if (isFactoryBean(beanName)) {
                final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
                boolean isEagerInit;
                if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                    isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                        @Override
                        public Boolean run() {
                            return ((SmartFactoryBean<?>) factory).isEagerInit();
                        }
                    }, getAccessControlContext());
                }
                else {
                    isEagerInit = (factory instanceof SmartFactoryBean &&
                            ((SmartFactoryBean<?>) factory).isEagerInit());
                }
                if (isEagerInit) {
                    getBean(beanName);
                }
            }
            else {
                getBean(beanName);
            }
        }
    }

    // Trigger post-initialization callback for all applicable beans...
    for (String beanName : beanNames) {
        Object singletonInstance = getSingleton(beanName);
        if (singletonInstance instanceof SmartInitializingSingleton) {
            final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged(new PrivilegedAction<Object>() {
                    @Override
                    public Object run() {
                        smartSingleton.afterSingletonsInstantiated();
                        return null;
                    }
                }, getAccessControlContext());
            }
            else {
                smartSingleton.afterSingletonsInstantiated();
            }
        }
    }
}

此方法首先將加載進來的beanDefinitionNames循環分析，如果是我們自己配置的bean就會走else中的getBean(beanName)，接著看。

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

doGetBean方法內容太多，一段一段看。

/**
 * 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 {

    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 dependsOnBean : dependsOn) {
                    if (isDependent(beanName, dependsOnBean)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "Circular depends-on relationship between ‘" + beanName + "‘ and ‘" + dependsOnBean + "‘");
                    }
                    registerDependentBean(dependsOnBean, beanName);
                    getBean(dependsOnBean);
                }
            }

            // 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;
}

這裏主要看Object sharedInstance = getSingleton(beanName)。

/**
 * Return the (raw) singleton object registered under the given name.
 * <p>Checks already instantiated singletons and also allows for an early
 * reference to a currently created singleton (resolving a circular reference).
 * @param beanName the name of the bean to look for
 * @param allowEarlyReference whether early references should be created or not
 * @return the registered singleton object, or {@code null} if none found
 */
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 != NULL_OBJECT ? singletonObject : null);
}

這裏能看到，Spring會把實例化好的bean存入singletonObjects，這是一個ConcurrentHashMap，

private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);

當然這裏我們bean並未實例化過，所以這裏應該也不能get出什麽東西來，也就是返回null了。if子句也就不會執行了。那麽接著看else子句的內容。

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);
        }
    }

這兩條驗證也都不會實現，接寫來就是重點了。

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 dependsOnBean : dependsOn) {
            if (isDependent(beanName, dependsOnBean)) {
                throw new BeanCreationException("Circular depends-on relationship between ‘" +
                        beanName + "‘ and ‘" + dependsOnBean + "‘");
            }
            registerDependentBean(dependsOnBean, beanName);
            getBean(dependsOnBean);
        }
    }

    // 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 ‘" + 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);
        }
    }
}

在這裏拿到RootBeanDefinition並check，並獲得bean的依賴，並循環叠代實例化bean。例如class A依賴於class B，就會先實例化B。下面的if ... else ...就是真正實例化bean的地方。其實真正實例化bean的方法是createBean(beanName, mbd, args)，只是區分了isSingleton或isPrototype，兩者的區別在於，單例的(Singleton)被緩存起來，而Prototype是不用緩存的。首先看一下createBean(beanName, mbd, args)。createBean方法中除了做了一些實例化bean前的檢查準備工作外，最核心的方法就是

Object beanInstance = doCreateBean(beanName, mbd, args);

由於這個過程涉及到的代碼都是一大坨，就不貼出所有代碼了。

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);

首先就是創建一個bean的實例且封裝到BeanWrapper中，在這裏bean已經實例化了。具體的實現方法是在org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner)中。

@Override
public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
    // Don‘t override the class with CGLIB if no overrides.
    if (beanDefinition.getMethodOverrides().isEmpty()) {
        Constructor<?> constructorToUse;
        synchronized (beanDefinition.constructorArgumentLock) {
            constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod;
            if (constructorToUse == null) {
                final Class<?> clazz = beanDefinition.getBeanClass();
                if (clazz.isInterface()) {
                    throw new BeanInstantiationException(clazz, "Specified class is an interface");
                }
                try {
                    if (System.getSecurityManager() != null) {
                        constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {
                            @Override
                            public Constructor<?> run() throws Exception {
                                return clazz.getDeclaredConstructor((Class[]) null);
                            }
                        });
                    }
                    else {
                        constructorToUse =  clazz.getDeclaredConstructor((Class[]) null);
                    }
                    beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
                }
                catch (Exception ex) {
                    throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                }
            }
        }
        return BeanUtils.instantiateClass(constructorToUse);
    }
    else {
        // Must generate CGLIB subclass.
        return instantiateWithMethodInjection(beanDefinition, beanName, owner);
    }
}

這裏不難看出實例化分兩種情況，如果沒有無參構造器是就生成CGLIB子類，否則就直接反射成實例。

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
    Assert.notNull(ctor, "Constructor must not be null");
    try {
        ReflectionUtils.makeAccessible(ctor);
        return ctor.newInstance(args);
    }

既然已經有了實例對象了，那麽，Spring是如何將bean的屬性註入到bean的呢？返回到上面的doCreateBean方法中。往下看找到populateBean(beanName, mbd, instanceWrapper);，內幕就在這裏。只貼部分代碼：

boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);

if (hasInstAwareBpps || needsDepCheck) {
    PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
    if (hasInstAwareBpps) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                if (pvs == null) {
                    return;
                }
            }
        }
    }
    if (needsDepCheck) {
        checkDependencies(beanName, mbd, filteredPds, pvs);
    }
}

這裏是調用InstantiationAwareBeanPostProcessor的具體子類的ibp.postProcessPropertyValues方法註入屬性。當我們使用@Resource註解的時候，具體的子類是CommonAnnotationBeanPostProcessor；如果使用的是@Autowired註解，則具體的子類是AutowiredAnnotationBeanPostProcessor。此方法內是委托InjectionMetadata對象來完成屬性註入。

@Override
public PropertyValues postProcessPropertyValues(
        PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {

    InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass());
    try {
        metadata.inject(bean, beanName, pvs);
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
    }
    return pvs;
}

findAutowiringMetadata方法能拿到使用了特定註解的屬性(Field)、方法(Method)及依賴的關系保存到checkedElements集合<Set>裏，然後再執行自己的inject方法。

public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {
    Collection<InjectedElement> elementsToIterate =
            (this.checkedElements != null ? this.checkedElements : this.injectedElements);
    if (!elementsToIterate.isEmpty()) {
        boolean debug = logger.isDebugEnabled();
        for (InjectedElement element : elementsToIterate) {
            if (debug) {
                logger.debug("Processing injected method of bean ‘" + beanName + "‘: " + element);
            }
            element.inject(target, beanName, pvs);
        }
    }
}

真正幹事的還是InjectedElement的inject方法。

@Override
protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
    Field field = (Field) this.member;
    try {
        Object value;
        if (this.cached) {
            value = resolvedCachedArgument(beanName, this.cachedFieldValue);
        }
        else {
            DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
            desc.setContainingClass(bean.getClass());
            Set<String> autowiredBeanNames = new LinkedHashSet<String>(1);
            TypeConverter typeConverter = beanFactory.getTypeConverter();
            value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
            synchronized (this) {
                if (!this.cached) {
                    if (value != null || this.required) {
                        this.cachedFieldValue = desc;
                        registerDependentBeans(beanName, autowiredBeanNames);
                        if (autowiredBeanNames.size() == 1) {
                            String autowiredBeanName = autowiredBeanNames.iterator().next();
                            if (beanFactory.containsBean(autowiredBeanName)) {
                                if (beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
                                    this.cachedFieldValue = new RuntimeBeanReference(autowiredBeanName);
                                }
                            }
                        }
                    }
                    else {
                        this.cachedFieldValue = null;
                    }
                    this.cached = true;
                }
            }
        }
        if (value != null) {
            ReflectionUtils.makeAccessible(field);
            field.set(bean, value);
        }
    }
    catch (Throwable ex) {
        throw new BeanCreationException("Could not autowire field: " + field, ex);
        }
    }
}

其實別看代碼這麽多，最關鍵的部分就是：

if (value != null) {
    ReflectionUtils.makeAccessible(field);
    field.set(bean, value);
}

在這裏也就真相大白了，就是通過JDK反射特性，直接set值的。

原文鏈接：http://geeekr.com/read-spring-source-two-beans-initialization/

看看Spring的源碼(二)——bean實例化