spring容器会在启动的过程中创建bean的实例,在创建实例的过程中会把@Autowired注解的属性都去找到相应类注入。
现在来分析一下bean的初始化和依赖注入
refresh()中的finishbeanfactoryInitialization()方法就是初始化整个beanfactory注册的bean
finishbeanfactoryInitialization()中preInstantiateSingletons()部分代码

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);
                }
            }
        }

循环beanDefinitionNames中的beanName调用getBean()

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) {
                //如果缓存中存在
                        ···
        }

        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)) {
                //如果存在父级容器，则去父级容器中获取
            }

            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(),"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
                        }
                        registerDependentBean(dependsOnBean,beanName);
                        getBean(dependsOnBean);
                    }
                }

                // Create bean instance.
                // 如果是单例
                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,mbd);
                }
                // 如果是原型
                else if (mbd.isPrototype()) {
                    // It's a prototype -> create a new instance.
                    Object prototypeInstance = null;
                    try {
                        beforePrototypeCreation(beanName);
                        prototypeInstance = createBean(beanName,args);
                    }
                    finally {
                        afterPrototypeCreation(beanName);
                    }
                    bean = getObjectForBeanInstance(prototypeInstance,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,args);
                                }
                                finally {
                                    afterPrototypeCreation(beanName);
                                }
                            }
                        });
                        bean = getObjectForBeanInstance(scopedInstance,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;
            }
        }

如果没有在缓存中获取，都会调用到createBean()中的doCreateBean()
其中最重要的两句是

//创建bean的实例
instanceWrapper = createBeanInstance(beanName,args);
//注入bean中的属性
populateBean(beanName,instanceWrapper);

bean实例创建是通过反射调用Constructor.newInstance()来创建实例的
注入是在创建实例后,populateBean()方法调用BeanPostProcessor的实现类AutowiredAnnotationBeanPostProcessor

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;
                        }
                    }
                }
            }

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

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

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 element of bean '" + beanName + "': " + element);
                }
                element.inject(target,pvs);
            }
        }
    }

inject()方法中解决依赖

beanfactory.resolveDependency(desc,autowiredBeanNames,typeConverter);

//如果是数组
if (type.isArray()) { ···· Map<String,Object> matchingBeans = findAutowireCandidates(beanName,componentType,targetDesc); ··· //如果是集合 }else if (Collection.class.isAssignableFrom(type) && type.isInterface()) { ··· Map<String,elementType,targetDesc); ··· //如果是Map }else if (Map.class.isAssignableFrom(type) && type.isInterface()) { ··· Map<String,valueType,targetDesc); ··· }else { ··· Map<String,type,descriptor); ··· }

findAutowireCandidates()方法中会去调用getBean获取

for (String candidateName : candidateNames) {
            if (!isSelfReference(beanName,candidateName) && isAutowireCandidate(candidateName,descriptor)) {
                result.put(candidateName,getBean(candidateName));
            }
        }

如果有@Autowired注解会去获取该属性的getBean方法，如此递归解决依赖。