@Autowired是spring项目中经常使用到的注解，但大多数人都是简单的使用，并不知道原理，下面记录下个人在研究@Autowired时的收获和理解。
@Autowired的用法
Autowired能使用在什么地方，参数含义。
@Autowired在何处使用
@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
- 构造
- 方法
- 参数
- 字段
- 注解
@Autowired参数的意义
Autowired注解，只有一个required元素，默认是true，也是就是说这个值能改为false。true和false的意义不同。
require=ture 时，表示解析被标记的字段或方法，一定有对应的bean存在。
require=false 时，表示解析被标记的字段或方法，没有对应的bean存在不会报错。
public @interface Autowired {
/**
* Declares whether the annotated dependency is required.
* Defaults to {@code true}.
*/
boolean required() default true;
}
@Autowired的原理
首先通过查看@Autowired注解注释文档，可以了解到加载放在了AutowiredAnnotationBeanPostProcessor。
而AutowiredAnnotationBeanPostProcessor是何时来的又是怎么执行去加载@Autowired呢？
添加AutowiredAnnotationBeanPostProcessor
首先要知道再执行AbstractApplicationContext#refresh()方法时会执行obtainFreshBeanFactory()方法，而这个方法执行时，会在
DefaultListableBeanFactory#beanDefinitionNames数组中添加internalAutowiredAnnotationProcessor。而internalAutowiredAnnotationProcessor是和AutowiredAnnotationBeanPostProcessor被一起注册到registerPostProcessor中
if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
def.setSource(source);
beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
}
不知道可以参考BeanFactory的加载
项目启动会自动执行refresh方法中的registerBeanPostProcessors(beanFactory)方法
protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}
此方法使用委派模式进行委派。
public static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
...
registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);
...
}
getBeanNamesForType 是对前面加载的internalAutowiredAnnotationProcessor进行转换成AutowiredAnnotationBeanPostProcessor然后把返回值postProcessorNames转为priorityOrderedPostProcessors然后注册到registerBeanPostProcessors中
private static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, List postProcessors) {
for (BeanPostProcessor postProcessor : postProcessors) {
beanFactory.addBeanPostProcessor(postProcessor);
}
}
AbstractBeanFactory#beanPostProcessors
public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");
// Remove from old position, if any
this.beanPostProcessors.remove(beanPostProcessor);
// Track whether it is instantiation/destruction aware
if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {
this.hasInstantiationAwareBeanPostProcessors = true;
}
if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {
this.hasDestructionAwareBeanPostProcessors = true;
}
// Add to end of list
this.beanPostProcessors.add(beanPostProcessor);
}
到此知道了AutowiredAnnotationBeanPostProcessor的来龙去脉。下面就开下AutowiredAnnotationBeanPostProcessor如何去解析@Autowired。
AutowiredAnnotationBeanPostProcessor解析@Autowired
refresh()执行完registerBeanPostProcessors 方法后，继续执行finishBeanFactoryInitialization(beanFactory);
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
...
// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();
}
这里spring会创建所需要的bean，一般在controller层中引入的service也会在此时依赖加载和创建。
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
...
getBean(beanName);
...
}
...
当执行到我们自定义的controller层时，会在getBean中进行执行autowire解析。
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
protected T doGetBean(final String name, @Nullable final Class requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
return createBean(beanName, mbd, args);
}
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
...
doCreateBean(beanName, mbdToUse, args);
...
}
而doCreateBean是真正的创建bean实现，在创建的时候调用了applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
...
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
...
}
终于在applyMergedBeanDefinitionPostProcessors中看到了我们熟悉的身影BeanPostProcessor
protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class beanType, String beanName) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof MergedBeanDefinitionPostProcessor) {
MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
}
}
}
getBeanPostProcessors方法获取的就是上文中添加的AutowiredAnnotationBeanPostProcessor的集合beanPostProcessors。
则执行我们的想看到的AutowiredAnnotationBeanPostProcessor#postProcessMergedBeanDefinition。
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class beanType, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
findAutowiringMetadata查询这个beanName中，是否有
private InjectionMetadata findAutowiringMetadata(String beanName, Class clazz, @Nullable PropertyValues pvs) {
...
metadata = buildAutowiringMetadata(clazz);
...
}
private InjectionMetadata buildAutowiringMetadata(final Class clazz) {
List elements = new ArrayList<>();
Class targetClass = clazz;
AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
boolean required = determineRequiredStatus(ann);
PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
currElements.add(new AutowiredMethodElement(method, required, pd));
}
});
elements.addAll(0, currElements);
targetClass = targetClass.getSuperclass();
}
return new InjectionMetadata(clazz, elements);
}
findAutowiredAnnotation(bridgedMethod)找到这个类的autowire注解的类，添加到InjectionMetadata对象中。然后在checkConfigMembers方法中又注册到beanDefinition中。
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class beanType, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
public void checkConfigMembers(RootBeanDefinition beanDefinition) {
Set checkedElements = new LinkedHashSet<>(this.injectedElements.size());
for (InjectedElement element : this.injectedElements) {
Member member = element.getMember();
if (!beanDefinition.isExternallyManagedConfigMember(member)) {
beanDefinition.registerExternallyManagedConfigMember(member);
checkedElements.add(element);
if (logger.isTraceEnabled()) {
logger.trace("Registered injected element on class [" + this.targetClass.getName() + "]: " + element);
}
}
}
this.checkedElements = checkedElements;
}