//该标签可以用于构造函数、方法、参数、标签上
@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {

    /**
     * 
     * <p>Defaults to {@code true}.
     */
    boolean required() default true;
}

解析这个标签的类 AutowiredAnnotationBeanPostProcessor 这是个BeanPostProcessor类，参数注入主要是它的一个内部类AutowiredFieldElement来处理，而这个类的inject()方法被调用是在AutowiredAnnotationBeanPostProcessor的postProcessPropertyValues方法上

@Override
public PropertyValues postProcessPropertyValues(
        PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
    //这里获取注入元数据信息
    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;
}

AutowiredFieldElement.inject()

@Override
protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
    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<String> autowiredBeanNames = new LinkedHashSet<String>(1);
        TypeConverter typeConverter = beanFactory.getTypeConverter();
        try {
            //通过BeanFactory的resolveDependency()方法解决依赖的值。也就是这个参数需要注入的值
            value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
        }
        catch (BeansException ex) {
            throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
        }
        synchronized (this) {
            if (!this.cached) {
                if (value != null || this.required) {
                    this.cachedFieldValue = desc;
                    registerDependentBeans(beanName, autowiredBeanNames);
                    if (autowiredBeanNames.size() == 1) {
                        String autowiredBeanName = autowiredBeanNames.iterator().next();
                        if (beanFactory.containsBean(autowiredBeanName)) {
                            if (beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
                                this.cachedFieldValue = new ShortcutDependencyDescriptor(
                                        desc, autowiredBeanName, field.getType());
                            }
                        }
                    }
                }
                else {
                    this.cachedFieldValue = null;
                }
                this.cached = true;
            }
        }
    }
    if (value != null) {//这里就是通过反射设置参数可见性，然后把值设置到该参数上。
        ReflectionUtils.makeAccessible(field);
        field.set(bean, value);
    }
}