使用过@Autowired的都应该知道，这个注解是手动注入依赖对象的，并且先通过bean类型去查找然后再根据属性名去查找
准备4个类：
1 Dao 接口
2 IndexDao 测试类
3 IndexDao2 实现了Dao接口
4 IndexDao3类 实现了Dao接口
public class IndexDao {
@Autowire
Dao dao；
}这样spring启动时肯定会报错：
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.luban.dao.Dao' available: expected single matching bean but found 2: indexDao2,indexDao3
为什么会出现这个问题呢？ 因为Dao有两个实现类，但是在IndexDao中注入时并没有指定使用哪个类，于是先根据类型去查找，查找到了两个，但是这样肯定是不行的，于是查找beanName 为dao的bean,但是还是没有找到，于是就出这个错了，那么怎么证明这个问题呢？下面我们就看看源码来揭开面纱
贴上spring 容器启动源码 ：
public abstract class AbstractApplicationContext extends DefaultResourceLoader
implements ConfigurableApplicationContext {
//只贴出部分代码，
//刷新容器： 也就是spring 容器初始化，将bean实例化
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
准备工作包括设置启动时间，是否激活标识位，
// 初始化属性源(property source)配置
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//返回一个factory 为什么需要返回一个工厂
//1 因为要对工厂进行初始化
//zjh
//返回的是注册bean时的 DefaultListableBeanFactory ,在register中已经将bean(App.class)放入了factory中
//这个工厂已经有一些值，在new AnnotationConfigApplicationContext()时已经创建
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
//准备工厂 配置其标准的特征(通俗来讲是就为beanFactory填充数据)
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
//这个方法在当前版本的spring是没用任何代码的
//可能spring期待在后面的版本中去扩展吧
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
//在spring的环境中去执行已经被注册的 factory processors(在获取bd读取器时注册的)
//设置执行自定义的(通过用户手动add的)BeanFactoryPostProcessors 和spring内部自己定义的(ConfigurationClassPostProcessor)
//通过CompentScan获取到所有用户自定定义的bean并且存入到map中
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
//注册beanPostProcessor 和上一步执行的是不同的bean,这个注册的是可以修改对象中属性的后置处理器
//注册到DefaultListableBeanFactory(它的父类)中存放后置处理器中的list中
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
//初始化应用事件广播器
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.
//开始实例化bean： 这个地方就是我们要讲的地方
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
}2 finishBeanFactoryInitialization(beanFactory); 这个方法就是我们开始讲的地方: 初始化bean
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
......................
..................
// Instantiate all remaining (non-lazy-init) singletons.
//实例化所有的单例对象
/* 此处是重点*/
beanFactory.preInstantiateSingletons();
}3 调用getBean(String bean)方法获取bean实例
4 经过一系列方法到达了AbstractAutowireCapableBeanFactory.doCreateBean()方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
省略代码
// Initialize the bean instance.
Object exposedObject = bean;
try {
//设置属性，非常重要 @Autowired和@Resource 也是在这里面注入属性
//这个地方会调用两次后置处理器的方法 第五次和第六次
//分别是InstantiationAwareBeanPostProcessor这个类中的 这两个
//postProcessAfterInstantiation()和postProcessPropertyValues
populateBean(beanName, mbd, instanceWrapper);
//执行后置处理器，aop 就是在这里完成的处理
//第七次和第八次在这里面完成 执行的是 BeanPostProcessor中的
//1 postProcessBeforeInitialization
//2 postProcessAfterInitialization
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<>(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;
}5 通过populateBean(beanName, mbd, instanceWrapper);来填充@Autowired和@Resouece注入的对象
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
boolean continueWithPropertyPopulation = true;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//第五次调用后置处理器的方法
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
//自动装配模式： byType好byName
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
if (hasInstAwareBpps || needsDepCheck) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
//这里会解析通过@Autowired和@Resource来注入属性
//通过AutowireAnnotationBeanPostProcess 解析 @Autowired
//Comm.......AnnotationBeanPostProcess 解析 @Resource
//上面两个后置处理器都是spring 注册时放进去的(7个)
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
//第六次调用后置处理器方法 填充属性，注入
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
6 上面ibp.postProcessPropertyValues（。。。。）实际上调用的是AutowireAnnotationBeanPostProcess .postProcessPropertyValues（）方法，用来专门解析加有@Autowired注解的属性
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
//获取需要注入的值: 这个地方开始-=======
metadata.inject(bean, beanName, pvs);
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}7 获取当前bean 所有依赖的对象,并且一个一个的注入
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection<InjectedElement> checkedElements = this.checkedElements;
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
//判断是否有需要依赖的值
if (!elementsToIterate.isEmpty()) {
//循环遍历所有需要被依赖的对象
for (InjectedElement element : elementsToIterate) {
if (logger.isDebugEnabled()) {
logger.debug("Processing injected element of bean '" + beanName + "': " + element);
}
//注入对象
element.inject(target, beanName, pvs);
}
}
}8 AutowiredFieldElement.inject(target,beanBname,pvs), element包含了被注入属性的信息：类，属性名
@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
try {
//获取需要被注入的对象
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
}9 解析依赖关系
@Override
@Nullable
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
if (Optional.class == descriptor.getDependencyType()) {
return createOptionalDependency(descriptor, requestingBeanName);
}
else if (ObjectFactory.class == descriptor.getDependencyType() ||
ObjectProvider.class == descriptor.getDependencyType()) {
return new DependencyObjectProvider(descriptor, requestingBeanName);
}
else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
return new Jsr330ProviderFactory().createDependencyProvider(descriptor, requestingBeanName);
}
else {
//这是重点
//一般返回的是空
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
//解析依赖关系： 将被依赖的bean注入： 先根据type然后根据名称
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}10 doResolveDependency();真正去解析依赖的方法
参数1：descriptor 注入对象的类描述信息： 类全名
参数2 ：当前bean的名称
参数3 ：autowiredBeanNames 传出参数： 注入的bean的搜索符合条件的beanName
参数4：typeConverter: 类型转换器
@Nullable
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
Object shortcut = descriptor.resolveShortcut(this);
if (shortcut != null) {
return shortcut;
}
//注入属性的类型
Class<?> type = descriptor.getDependencyType();
/** 用于检查一个类定义是否有自动注入请求的解析器： 为空 特殊请款除外*/
Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
if (value != null) {
if (value instanceof String) {
String strVal = resolveEmbeddedValue((String) value);
BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
value = evaluateBeanDefinitionString(strVal, bd);
}
TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
return (descriptor.getField() != null ?
converter.convertIfNecessary(value, type, descriptor.getField()) :
converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
}
//一些逻辑判断无影响
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
//根据注入对象的类型查询匹配到的 bean 可能有多个 比如接口的实现类
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
//根据类型找不到bean, 报错
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
//根据bean的类型如果找到了不止一个，那么就根据属性名称来确定注入的bean
if (matchingBeans.size() > 1) {
//根据beanName从多个bean匹配
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
//找不到，报错
if (autowiredBeanName == null) {
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
return descriptor.resolveNotUnique(type, matchingBeans);
}
else {
return null;
}
}
//获得bean
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();
}
if (autowiredBeanNames != null) {
autowiredBeanNames.add(autowiredBeanName);
}
if (instanceCandidate instanceof Class) {
//如果当前bean依赖的对象并没有创建，那么就会通过下面的方法创建
instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;
if (result instanceof NullBean) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
result = null;
}
if (!ClassUtils.isAssignableValue(type, result)) {
throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
}
return result;
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}根据最后一步分析可知，1 beanType去查询bean,如果存在多个则使用属性名称去匹配，
如果根据beanType获取不到bean 则直接报错
//根据类型找不到bean, 报错
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}所有由上面的分析可知，加有@Autowired注解的属性是先通过属性的类型去查找bean,然后再通过属性名去查找
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