@Autowired注解實現原理

在討論代碼細節之前，我們再來了解下基礎知識。Spring管理可用于整個應用程序的Java對象bean。他們所在的Spring容器，被稱為應用程序上下文。這意味著我們不需要處理他們的生命周期(初始化，銷毀)。該任務由此容器來完成。另外，該上下文具有入口點，在Web應用程序中，是dispatcher servlet。容器(也就是該上下文)會在它那里被啟動并且所有的bean都會被注入。

說的再清楚點，請看<context:annotation-config />的定義:

<xsd:element name="annotation-config">
<xsd:annotation>
<xsd:documentation><![CDATA[
Activates various annotations to be detected in bean classes: Spring's @Required and
@Autowired, as well as JSR 250's @PostConstruct, @PreDestroy and @Resource (if available),
JAX-WS's @WebServiceRef (if available), EJB 3's @EJB (if available), and JPA's
@PersistenceContext and @PersistenceUnit (if available). Alternatively, you may
choose to activate the individual BeanPostProcessors for those annotations.
Note: This tag does not activate processing of Spring's @Transactional or EJB 3's
@TransactionAttribute annotation. Consider the use of the <tx:annotation-driven>
tag for that purpose.
See javadoc for org.springframework.context.annotation.AnnotationConfigApplicationContext
for information on code-based alternatives to bootstrapping annotation-driven support.
]]></xsd:documentation>
</xsd:annotation>
</xsd:element>
可以看出 : 類內部的注解，如：@Autowired、@Value、@Required、@Resource以及EJB和WebSerivce相關的注解，是容器對Bean對象實例化和依賴注入時，通過容器中注冊的Bean后置處理器處理這些注解的。

所以配置了上面這個配置(<context:component-scan>假如有配置這個，那么就可以省略<context:annotation-config />)后，將隱式地向Spring容器注冊AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、PersistenceAnnotationBeanPostProcessor以及這4個專門用于處理注解的Bean后置處理器。

當 Spring 容器啟動時，AutowiredAnnotationBeanPostProcessor 將掃描 Spring 容器中所有 Bean，當發現 Bean 中擁有@Autowired 注解時就找到和其匹配（默認按類型匹配）的 Bean，并注入到對應的地方中去。源碼分析如下:

通過org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor可以實現依賴自動注入。通過這個類來處理@Autowired和@Value這倆Spring注解。它也可以管理JSR-303的@Inject注解(如果可用的話)。在AutowiredAnnotationBeanPostProcessor構造函數中定義要處理的注解:

public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
...
/**
* Create a new AutowiredAnnotationBeanPostProcessor
* for Spring's standard {@link Autowired} annotation.
* <p>Also supports JSR-330's {@link javax.inject.Inject} annotation, if available.
*/
@SuppressWarnings("unchecked")
public AutowiredAnnotationBeanPostProcessor() {
this.autowiredAnnotationTypes.add(Autowired.class);
this.autowiredAnnotationTypes.add(Value.class);
try {
this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
logger.info("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
}
catch (ClassNotFoundException ex) {
// JSR-330 API not available - simply skip.
}
}
...
}
之后，有幾種方法來對@Autowired注解進行處理。

第一個，private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz)解析等待自動注入類的所有屬性。它通過分析所有字段和方法并初始化org.springframework.beans.factory.annotation.InjectionMetadata類的實例來實現。

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<>();
Class<?> targetClass = clazz;
do {
final LinkedList<InjectionMetadata.InjectedElement> currElements = new LinkedList<>();
//分析所有字段
ReflectionUtils.doWithLocalFields(targetClass, field -> {
//findAutowiredAnnotation(field)此方法后面會解釋
AnnotationAttributes ann = findAutowiredAnnotation(field);
if (ann != null) {
if (Modifier.isStatic(field.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation is not supported on static fields: " + field);
}
return;
}
boolean required = determineRequiredStatus(ann);
currElements.add(new AutowiredFieldElement(field, required));
}
});
//分析所有方法
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}
AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
if (Modifier.isStatic(method.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation is not supported on static methods: " + method);
}
return;
}
if (method.getParameterCount() == 0) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation should only be used on methods with parameters: " +
method);
}
}
boolean required = determineRequiredStatus(ann);
PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
currElements.add(new AutowiredMethodElement(method, required, pd));
}
});
elements.addAll(0, currElements);
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class);
//返回一個InjectionMetadata初始化的對象實例
return new InjectionMetadata(clazz, elements);
}
...
/**
* 'Native' processing method for direct calls with an arbitrary target instance,
* resolving all of its fields and methods which are annotated with {@code @Autowired}.
* @param bean the target instance to process
* @throws BeanCreationException if autowiring failed
*/
public void processInjection(Object bean) throws BeanCreationException {
Class<?> clazz = bean.getClass();
InjectionMetadata metadata = findAutowiringMetadata(clazz.getName(), clazz, null);
try {
metadata.inject(bean, null, null);
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
"Injection of autowired dependencies failed for class [" + clazz + "]", ex);
}
}
InjectionMetadata類包含要注入的元素的列表。注入是通過Java的API Reflection (Field set(Object obj, Object value) 或Method invoke(Object obj，Object ... args)方法完成的。此過程直接在AutowiredAnnotationBeanPostProcessor的方法中調用public void processInjection(Object bean) throws BeanCreationException。它將所有可注入的bean檢索為InjectionMetadata實例，并調用它們的inject()方法。

public class InjectionMetadata {
...
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);
if (!elementsToIterate.isEmpty()) {
boolean debug = logger.isDebugEnabled();
for (InjectedElement element : elementsToIterate) {
if (debug) {
logger.debug("Processing injected element of bean '" + beanName + "': " + element);
}
//看下面靜態內部類的方法
element.inject(target, beanName, pvs);
}
}
}
...
public static abstract class InjectedElement {
protected final Member member;
protected final boolean isField;
...
/**
* Either this or {@link #getResourceToInject} needs to be overridden.
*/
protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)
throws Throwable {
if (this.isField) {
Field field = (Field) this.member;
ReflectionUtils.makeAccessible(field);
field.set(target, getResourceToInject(target, requestingBeanName));
}
else {
if (checkPropertySkipping(pvs)) {
return;
}
try {
//具體的注入看此處咯
Method method = (Method) this.member;
ReflectionUtils.makeAccessible(method);
method.invoke(target, getResourceToInject(target, requestingBeanName));
}
catch (InvocationTargetException ex) {
throw ex.getTargetException();
}
}
}
...
}
}
AutowiredAnnotationBeanPostProcessor類中的另一個重要方法是private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao)。它通過分析屬于一個字段或一個方法的所有注解來查找@Autowired注解。如果未找到@Autowired注解，則返回null，字段或方法也就視為不可注入。

@Nullable
private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao) {
if (ao.getAnnotations().length > 0) {
for (Class<? extends Annotation> type : this.autowiredAnnotationTypes) {
AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ao, type);
if (attributes != null) {
return attributes;
}
}
}
return null;
}
在上面的文章中，我們看到了Spring中自動注入過程。通過整篇文章可以看到，這種依賴注入是一種便捷易操作方式(可以在字段以及方法上完成)，也促使我們逐漸在拋棄XML配置文件。還增強了代碼的可讀性。