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首先来看一段代码，看过上一节的朋友肯定对这段代码并不陌生。这一段代码诠释了Spring加载bean的完整过程，包括读取配置文件，扫描包，加载类，实例化bean，注入bean属性依赖。

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();        }    }}

上一节介绍了Spring是如何加载class文件的，本节主要围绕finishBeanFactoryInitialization(beanFactory)方法，聊聊Spring是如何实例化bean的，从上面代码片段中的注解不难看出，此方法主要的任务就是实例化非懒加载的单例bean。闲话少叙，看代码。

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();}

上面代码主要看最后一句beanFactory.preInstantiateSingletons()。

public void preInstantiateSingletons() throws BeansException {    if (this.logger.isDebugEnabled()) {        this.logger.debug("Pre-instantiating singletons in " + this);    }    List
    
      beanNames;    synchronized (this.beanDefinitionMap) {        // 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.        beanNames = new ArrayList
     
      (this.beanDefinitionNames);    }    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
      
       () {                        @Override                        public Boolean run() {                            return ((SmartFactoryBean
       ) factory).isEagerInit();                        }                    }, getAccessControlContext());                }                else {                    isEagerInit = (factory instanceof SmartFactoryBean &&                            ((SmartFactoryBean
       ) factory).isEagerInit());                }                if (isEagerInit) {                    getBean(beanName);                }            }            else {                getBean(beanName);            }        }    }}
      
     
    

此方法首先将加载进来的beanDefinitionNames循环分析，如果是我们自己配置的bean就会走else中的getBean(beanName)，接着看。

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

doGetBean方法内容太多，一段一段看。

protected 
    
      T doGetBean(        final String name, final Class
     
       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);    }
     
    

这里主要看Object sharedInstance = getSingleton(beanName)。

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);}

这里能看到，Spring会把实例化好的bean存入singletonObjects，这是一个ConcurrentHashMap，

private final Map
    
      singletonObjects = new ConcurrentHashMap
     
      (64);
     
    

当然这里我们bean并未实例化过，所以这里应该也不能get出什么东西来，也就是返回null了。if子句也就不会执行了。那么接着看else子句的内容。

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);        }    }

这两条验证也都不会实现，接写来就是重点了。

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

在这里拿到RootBeanDefinition并check，并获得bean的依赖，并循环迭代实例化bean。例如class A依赖于class B，就会先实例化B。下面的if ... else ...就是真正实例化bean的地方。其实真正实例化bean的方法是createBean(beanName, mbd, args)，只是区分了isSingleton或isPrototype，两者的区别在于，单例的(Singleton)被缓存起来，而Prototype是不用缓存的。首先看一下createBean(beanName, mbd, args)。createBean方法中除了做了一些实例化bean前的检查准备工作外，最核心的方法就是

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

由于这个过程涉及到的代码都是一大坨，就不贴出所有代码了。

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);

首先就是创建一个bean的实例且封装到BeanWrapper中，在这里bean已经实例化了。具体的实现方法是在org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner)中。

@Overridepublic 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
    
     >() {                            @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);    }}
    

在这里不难看出实例化分两种情况，如果没有无参构造器是就生成CGLIB子类，否则就直接反射成实例。

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

既然已经有了实例对象了，那么，Spring是如何将bean的属性注入到bean的呢？返回到上面的doCreateBean方法中。往下看找到populateBean(beanName, mbd, instanceWrapper);，内幕就在这里。只贴部分代码：

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);    }}

这里是调用InstantiationAwareBeanPostProcessor的具体子类的ibp.postProcessPropertyValues方法注入属性。当我们使用@Resource注解的时候，具体的子类是CommonAnnotationBeanPostProcessor；如果使用的是@Autowired注解，则具体的子类是AutowiredAnnotationBeanPostProcessor。此方法内是委托InjectionMetadata对象来完成属性注入。

@Overridepublic 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注解的属性(Field)、方法(Method)及依赖的关系保存到checkedElements集合<Set>里，然后再执行自己的inject方法。

public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {    Collection
    
      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);        }    }}
    

真正干事的还是InjectedElement的inject方法。

@Overrideprotected 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
    
      autowiredBeanNames = new LinkedHashSet
     
      (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);        }    }}
     
    

其实别看代码这么多，最关键的部分就是：

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

在这里也就真相大白了，就是通过JDK反射特性，直接set值的。