Spring 高级 @Autowired bean 后处理器运行分析

上篇:https://www.cnblogs.com/mangoubiubiu/p/16591665.html 知道了

AutowiredAnnotationBeanPostProcessor 执行依赖注入的时候，实际上是先调用了postProcessProperties方法，而postProcessProperties方法内部分为2步去进行依赖注入，第一步是用findAutowiringMetadata方法找到某个类型中标注了@Autowired 方法成员变量收集起来，信息叫做InjectionMetadata，找到信息后 InjectionMetadata 执行 inject 才完成反射调用进行依赖注入。

那么inject 内部是如何进行依赖注入的呢？

package com.mangoubiubiu.show.a04;


import org.springframework.beans.PropertyValues;
import org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor;
import org.springframework.beans.factory.annotation.InjectionMetadata;
import org.springframework.beans.factory.config.DependencyDescriptor;
import org.springframework.beans.factory.support.DefaultListableBeanFactory;
import org.springframework.context.annotation.ContextAnnotationAutowireCandidateResolver;
import org.springframework.core.MethodParameter;
import org.springframework.core.env.StandardEnvironment;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class DigInAutowired {

    public static void main(String[] args) throws Throwable {

        DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
        beanFactory.registerSingleton("bean2",new Bean2());//创建过程，依赖注入，初始化
        beanFactory.registerSingleton("bean3",new Bean3());

        beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());//value
        beanFactory.addEmbeddedValueResolver(new StandardEnvironment()::resolvePlaceholders);
        //1、查找哪些属性、方法加了@Autowired,这称之为InjectionMetadata
        AutowiredAnnotationBeanPostProcessor processor = new AutowiredAnnotationBeanPostProcessor();
        //设置bean 工厂 让bean工厂去找Bean
        processor.setBeanFactory(beanFactory);

        //自己创建一个bean1 自己创建的并没有依赖注入
        Bean1 bean1 = new Bean1();

        System.out.println(bean1);

//        processor.postProcessProperties(null,bean1,"bean1");
        Method findAutowiringMetadata =  AutowiredAnnotationBeanPostProcessor.class.getDeclaredMethod("findAutowiringMetadata", String.class, Class.class, PropertyValues.class);
        findAutowiringMetadata.setAccessible(true);
        //获取 Bean1 上加了 @Value @Autowired 的成员变量 , 方法参数
        InjectionMetadata metadata = (InjectionMetadata)findAutowiringMetadata.invoke(processor, "bean1", Bean1.class, null);
        //进行依赖注入
        metadata.inject(bean1,"bean1",null);
        System.out.println(metadata);
        System.out.println(bean1);


        //成员变量的依赖注入
        Field bean3 = Bean1.class.getDeclaredField("bean3");
        //第二个成员变量  true  找不到就报错   false 找不到就找不到
        DependencyDescriptor dd1 = new DependencyDescriptor(bean3,false);
        //根据成员变量信息找成员变量类型，然后再在容器中找符合该类型的bean
        Object o = beanFactory.doResolveDependency(dd1, null, null, null);
        System.out.println(o);


        //方法上的依赖注入
        Method setBean2 = Bean1.class.getDeclaredMethod("setBean2", Bean2.class);

        DependencyDescriptor dd2 = new DependencyDescriptor(new MethodParameter(setBean2,0),false);

        Object os = beanFactory.doResolveDependency(dd2, null, null, null);
        System.out.println(os);


        //方法上的值注入
        Method setHomeAutowired = Bean1.class.getDeclaredMethod("setHomeAutowired", String.class);

        DependencyDescriptor dd3 = new DependencyDescriptor(new MethodParameter(setHomeAutowired,0),false);

        Object oss = beanFactory.doResolveDependency(dd3, null, null, null);
        System.out.println(oss);
    }
}

总结：

很简单分三步，先找到 Bean3的成员变量封装成DependencyDescriptor
然后在调用beanFactory的方法在容器中找符合该类型的bean
完成依赖注入

注意：如果容器内没有该类型的bean 封装DependencyDescriptor时

boolean required = true 找不到是会直接报错 boolean required = false 找不到返回null

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com.mangoubiubiu.show.a04.DigInAutowired
Bean1{bean2=null, bean3=null, home='null'}
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'bean1': Unsatisfied dependency expressed through field 'bean3'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.mangoubiubiu.show.a04.Bean3' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:659)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:639)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119)
    at com.mangoubiubiu.show.a04.DigInAutowired.main(DigInAutowired.java:43)
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.mangoubiubiu.show.a04.Bean3' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1799)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1355)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1309)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:656)
    ... 3 more

Process finished with exit code 1