Spring5源码解析-@Autowired
@Autowired所具有的功能
@Autowired是一个用来执行依赖注入的注解。每当一个Spring管理的bean发现有这个注解时候，它会直接注入相应的另一个Spring管理的bean。
该注解可以在不同的层次上应用:
- 类字段:Spring将通过扫描自定义的packages(例如在我们所注解的controllers)或通过在配置文件中直接查找bean。
- 方法:使用@Autowired注解的每个方法都要用到依赖注入。但要注意的是，方法签名中呈现的所有对象都必须是Spring所管理的bean。如果你有一个方法，比如setTest(Article article, NoSpringArticle noSpringArt) ，其中只有一个参数 (Article article)是由Spring管理的，那么就将抛出一个org.springframework.beans.factory.BeanCreationException异常。这是由于Spring容器里并没有指定的一个或多个参数所指向的bean，所以也就无法解析它们。完整的异常跟踪如下:
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'testController': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire method: public void org.krams.tutorial.controller.TestController.ix(com.mysite.controller.IndexController,com.mysite.nospring.data.Article); nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.mysite.nospring.data.Article] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {}
-
构造函数:@Autowired的工作方式和方法相同。
对象注入需要遵循一些规则。一个bean可以按照下面的方式注入: -
名称:bean解析是通过bean名称(看后面的例子)。
I * 类型:解析过程基于bean的类型。
原理剖析
启动注解的自动注入
首先看看
tag for that purpose.
See javadoc for org.springframework.context.annotation.AnnotationConfigApplicationContext
for information on code-based alternatives to bootstrapping annotation-driven support.
]]>
可以看出：类内部的注解，如@Autowired、@Value、@Required、@Resource以及EJB和WebService相关的注解，是容器对Bean对象实例化和依赖注入时，通过容器中注册的Bean后置处理器AnnotationConfigApplicationContext处理这些注解的。
所以配置了上面这个配置(context:component-scan(开启扫描包)假如有配置这个，那么就可以省略
当 Spring 容器启动时，AutowiredAnnotationBeanPostProcessor 将扫描 Spring 容器中所有 Bean，当发现 Bean 中拥有@Autowired 注解时就找到和其匹配（默认按类型匹配）的 Bean，并注入到对应的地方中去。
通过org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor可以实现依赖自动注入
在AutowiredAnnotationBeanPostProcessor构造函数中定义要处理的注解:
public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
...
/**
* Create a new AutowiredAnnotationBeanPostProcessor
* for Spring's standard {@link Autowired} annotation.
* 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)
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，有几种方法来对@Autowired注解进行处理。
- private InjectionMetadata buildAutowiringMetadata(final Class clazz)解析等待自动注入类的所有属性。它通过分析所有字段和方法并初始化org.springframework.beans.factory.annotation.InjectionMetadata类的实例来实现。
private InjectionMetadata buildAutowiringMetadata(final Class clazz) {
LinkedList elements = new LinkedList<>();
Class targetClass = clazz;
do {
final LinkedList currElements = new LinkedList<>();
//分析所有字段
ReflectionUtils.doWithLocalFields(targetClass, field -> {
//findAutowiredAnnotation(field)此方法后面会解释
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);
//返回一个InjectionMetadata初始化的对象实例
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类包含要注入的元素的列表。注入是通过Java的API Reflection (Field set(Object obj, Object value) 或Method invoke(Object obj，Object … args)方法完成的。此过程直接在AutowiredAnnotationBeanPostProcessor的方法中调用public void processInjection(Object bean) throws BeanCreationException。它将所有可注入的bean检索为InjectionMetadata实例，并调用它们的inject()方法。
public class InjectionMetadata {
...
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection checkedElements = this.checkedElements;
Collection 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);
}
//看下面静态内部类的方法
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 {
//具体的注入看此处咯
Method method = (Method) this.member;
ReflectionUtils.makeAccessible(method);
method.invoke(target, getResourceToInject(target, requestingBeanName));
}
catch (InvocationTargetException ex) {
throw ex.getTargetException();
}
}
}
...
}
}
AutowiredAnnotationBeanPostProcessor类中的另一个重要方法是private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao)。它通过分析属于一个字段或一个方法的所有注解来查找@Autowired注解。如果未找到@Autowired注解，则返回null，字段或方法也就视为不可注入。
@Nullable
private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao) {
if (ao.getAnnotations().length > 0) {
for (Class type : this.autowiredAnnotationTypes) {
AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ao, type);
if (attributes != null) {
return attributes;
}
}
}
return null;
}
在上面的文章中，我们看到了Spring中自动注入过程。通过整篇文章可以看到，这种依赖注入是一种便捷易操作方式(可以在字段以及方法上完成)，也促使我们逐渐在抛弃XML配置文件。还增强了代码的可读性。
转自：
Spring5源码解析-@Autowired