@Autowried注解由注解处理器AutowiredAnnotationBeanPostProcessor进行解析，AutowiredAnnotationBeanPostProcessor 同时还会处理@Value和JSR-330规范中的@Inject，如图
image.png
下面用示例代码来演示
public class DependencyResolutionDemo {
@Autowired
private User user;
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
context.register(DependencyResolutionDemo.class);
context.refresh();
context.close();
}
@Bean
public User user() {
User user = new User();
user.setId(2L);
return user;
}
}
Spring容器在启动时，会进行一系列的操作，在解析@Autowried注解时，会调用AutowiredAnnotationBeanPostProcessor#postProcessProperties 这个方法
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
// 因为要对 DependencyResolutionDemo这个对象进行解析，所以
// 此时的bean就是 DependencyResolutionDemo 这个对象，beanName为dependencyResolutionDemo
// 这个方法是为了获取DependencyResolutionDemo对象内部有哪些需要注入的对象
// 当前类需要注入一个User对象，所以metadata 内部存储的是User类型
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
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;
}
跟随metadata.inject()方法，会跳到InjectionMetadata#inject()方法中
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
// target 为DependencyResolutionDemo对象，beanName为dependencyResolutionDemo
Collection checkedElements = this.checkedElements;
Collection elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
// 遍历DependencyResolutionDemo中需要注入的对象，按顺序注入
for (InjectedElement element : elementsToIterate) {
if (logger.isTraceEnabled()) {
logger.trace("Processing injected element of bean '" + beanName + "': " + element);
}
// 此element对象是 AutowiredAnnotationBeanPostProcessor中的一个内部类，
element.inject(target, beanName, pvs);
}
}
}
AutowiredAnnotationBeanPostProcessor的内部类
AutowiredFieldElement#inject
private class AutowiredFieldElement extends InjectionMetadata.InjectedElement {
private final boolean required;
private volatile boolean cached = false;
@Nullable
private volatile Object cachedFieldValue;
public AutowiredFieldElement(Field field, boolean required) {
super(field, null);
this.required = required;
}
@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
// 获取当前要注入的字段，User
Field field = (Field) this.member;
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);
Assert.state(beanFactory != null, "No BeanFactory available");
TypeConverter typeConverter = beanFactory.getTypeConverter();
try {
// 通过resolveDependency来获取User对象，然后赋值给 value，在代码最后会通过反射将
// 对象赋值到DependencyResolutionDemo的成员变量user中
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
}
// 省略其他代码...
}
if (value != null) {
// 通过反射将解析到的对象注入到target中
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
}
beanFactory.resolveDependency实际调用的是 DefaultListableBeanFactory#resolveDependency
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
// 省略其他代码...
// 判断要注入的Bean是否为懒加载，如果是则返回一个代码对象
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
// 如果不是则获取真实对象
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
Object shortcut = descriptor.resolveShortcut(this);
if (shortcut != null) {
return shortcut;
}
Class type = descriptor.getDependencyType();
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());
try {
return converter.convertIfNecessary(value, type, descriptor.getTypeDescriptor());
}
catch (UnsupportedOperationException ex) {
// A custom TypeConverter which does not support TypeDescriptor resolution...
return (descriptor.getField() != null ?
converter.convertIfNecessary(value, type, descriptor.getField()) :
converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
}
}
// 解析要注入的Bean是否是容器类型，如数组，Collection，Map，如果是的话会将符合条件的
// Bean加入容器中，直接返回。如果要注入的成员是，List userlist这种，就会走这个方法
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
// 返回符合条件的BeanMap， key是Bean名称，value是Bean实例
Map matchingBeans = findAutowireCandidates(beanName, type, descriptor);
// 如果没有找到符合条件的Bean，并且@Autowored中的require = true会抛出
// NoSuchBeanDefinitionException 异常，
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
// 如果找到的Bean数量大于1，则会去查找标注了@Primary的那个Bean名称进行返回
if (matchingBeans.size() > 1) {
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
// 如果Bean没有被@Primay标记，则会抛出，NoUniqueBeanDefinitionException
if (autowiredBeanName == null) {
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
}
else {
// In case of an optional Collection/Map, silently ignore a non-unique case:
// possibly it was meant to be an empty collection of multiple regular beans
// (before 4.3 in particular when we didn't even look for collection beans).
return null;
}
}
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();
}
if (autowiredBeanNames != null) {
autowiredBeanNames.add(autowiredBeanName);
}
if (instanceCandidate instanceof Class) {
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);
}
}
// 解析容器Bean，如果DependencyResolutionDemo 需要注入一个List user, 则此方法会返回所有符
//合条件的Bean
private Object resolveMultipleBeans(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set autowiredBeanNames, @Nullable TypeConverter typeConverter) {
final Class type = descriptor.getDependencyType();
// 如果是Stream类型，则走此种方法
if (descriptor instanceof StreamDependencyDescriptor) {
Map matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (autowiredBeanNames != null) {
autowiredBeanNames.addAll(matchingBeans.keySet());
}
Stream