1 @Autowired 干嘛的?
用来执行依赖注入.每当一个Spring管理的bean发现有该注解时,会直接注入相应的另一个Spring管理的bean.
1.1 不同地放置有不同作用
- 属性
Spring将通过扫描自定义的package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,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
1.2 bean的注入方式
- 名称
bean解析是通过bean名称 - 类型
解析基于bean的类型
1.3 @Qualifier 协作
如下相同类型的bean
<bean name="article1" class="com.sss.Article">
<property name="text" value="Content of the 1st Article" />
</bean>
<bean name="article2" class="com.sss.Article">
<property name="text" value="Content of the 2nd Article" />
</bean>
假如只是一个简单的@Autowired，Spring根本不知道你要注入哪个bean。这就需要@Qualifier(value =“beanName”)协作.
譬如,要从 com.javaedge.Article类型的bean中区分article1,article2:
@Qualifier(value="article1")
@Autowired
private Article firstArticle;
@Qualifier(value="article2")
@Autowired
private Article secondArticle;
2 优雅地使用@Autowired
启动自动注入
<context:annotation-config />
放在应用程序上下文配置。可以使在遇到@Autowired注解时启用依赖注入
- bean
// beans first
public class Comment {
private String content;
public void setContent(String content) {
this.content = content;
}
public String getContent() {
return this.content;
}
}
// sample controller
@Controller
public class TestController {
@Qualifier(value="comment1")
@Autowired
private Comment firstComment;
@Qualifier(value="comment2")
@Autowired
private Comment secondComment;
@RequestMapping(value = "/test", method = RequestMethod.GET)
public String test() {
System.out.println("1st comment text: "+firstComment.getText());
System.out.println("2nd comment text: "+secondComment.getText());
return "test";
}
}
// no-Spring managed class
public class TestNoSpring {
@Autowired
private Comment comment;
public void testComment(String content) {
if (comment == null) {
System.out.println("Comment's instance wasn't autowired because this class is not Spring-managed bean");
} else {
comment.setContent(content);
System.out.println("Comment's content: "+comment.getContent());
}
}
}
- 配置
<bean name="comment1" class="com.sss.exchanger.Comment">
<property name="content" value="Content of the 1st comment" />
</bean>
<bean name="comment2" class="com.sss.exchanger.Comment">
<property name="content" value="Content of the 2nd comment" />
</bean>
打开http://localhost:8080/test来运行TestController
TestController的注解字段正确地自动注入，而TestNoSpring的注解字段并没有注入进去
1st comment text: Content of the 1st comment
2nd comment text: Content of the 2nd comment
Comment's instance wasn't autowired because this class is not Spring-managed bean
哪里不对 ？TestNoSpring类不由Spring所管理
这就是为什么Spring不能注入Comment实例的依赖
3 隐藏在@Autowired注解背后的秘密
Spring管理可用于整个应用程序的Java对象bean,我们不需要处理他们的生命周期(初始化，销毁)。该任务由此容器来完成。
该上下文具有入口点，在Web应用程序中，是dispatcherservlet。
容器(也就是该上下文)会在它那里被启动并且所有的bean都会被注入
看<context:annotation-config />的定义
<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>
类内部的注解，如@Autowired、@Value、@Required、@Resource以及Web Serivce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后置处理器处理这些注解的
所以配置了上面这个配置(<context:component-scan>假如有配置这个，那么就可以省略<context:annotation-config />)后，将隐式地向Spring容器注册AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、PersistenceAnnotationBeanPostProcessor以及这4个专门用于处理注解的Bean后置处理器。
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。
4 源码分析
通过org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor可以实现依赖自动注入
通过这个类来处理@Autowired @Value Spring
它也可以管理JSR-303的@Inject
- 在
AutowiredAnnotationBeanPostProcessor构造函数中定义要处理的注解
之后,有几种方法对@Autowired处理
第一个，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<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<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();
}
}
}
...
}
}
findAutowiredAnnotation(AccessibleObject ao)
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。
转载：https://blog.csdn.net/qq_33589510/article/details/105071416
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