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# 常见 Bean 后处理器

# 常见 BeanPostProcessor 使用

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public class A04Application {
    public static void main(String[] args) {
        //GenericApplicationContext是一个干净的容器, 同时有比DefaultListableApplicationContext方便
        // 添加并使用beanFactoryPostProcessor与BeanPostProcessor都有镶银的方法
        GenericApplicationContext context = new GenericApplicationContext();

        //用原始方法注册三个Bean
        context.registerBean("bean1", Bean1.class);
        context.registerBean("bean2", Bean2.class);
        context.registerBean("bean3", Bean3.class);
        //注册Bean4, 探究@ConfiurationProperties注解
        context.registerBean("bean4", Bean4.class);

        //添加配合@Autowired实现@Value注入的解析器
        context.getDefaultListableBeanFactory().setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
        //添加处理Autowired注解的后处理器
        context.registerBean(AutowiredAnnotationBeanPostProcessor.class);
        //添加处理@Resource @PostConstruct @PreDestroy注解的后处理器
        context.registerBean(CommonAnnotationBeanPostProcessor.class);

        //添加处理@ConfiurationProperties注解的相关bean
        ConfigurationPropertiesBindingPostProcessor.register(context.getDefaultListableBeanFactory());

        //执行beanFactory后处理器, 添加bean后处理器, 初始化所有单例
        context.refresh();

        //获取bean4
        System.out.println(context.getBean(Bean4.class));

        context.close();
    }
}

# @Autowired bean 后处理器执行分析

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public class DigInAutowired {
    public static void main(String[] args) throws Throwable {
        DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();

        //图方便可以直接使用registerSingleton创建单例, 不用先创建BeanDefinition, 然后再添加BeanDefinition了
        //但是该方法参数为对象(成品bean), 就不会再走依赖注入初始化那些步骤了
        beanFactory.registerSingleton("bean2", new Bean2());
        beanFactory.registerSingleton("bean3", new Bean3());

        //添加@Value支持
        beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
        //添加${}解析器
        beanFactory.addEmbeddedValueResolver(new StandardEnvironment()::resolvePlaceholders);

        //创建Autowired后处理器对象, 并尝试直接自己调用
        AutowiredAnnotationBeanPostProcessor processor = new AutowiredAnnotationBeanPostProcessor();
        processor.setBeanFactory(beanFactory);

        Bean1 bean1 = new Bean1();
        System.out.println(bean1);

        //执行依赖注入
        processor.postProcessProperties(null, bean1, "bean1");

        //对postProcessProperties方法进行分解, 通过反射一步一步来进行postProcessProperties中的方法
        Method findAutowiringMetadata = AutowiredAnnotationBeanPostProcessor.class.getDeclaredMethod("findAutowiringMetadata", String.class, Class.class, PropertyValues.class);
        findAutowiringMetadata.setAccessible(true);
        //获取Bean1上添加了@Value, @Autowired的成员变量和方法参数
        InjectionMetadata metadata = (InjectionMetadata) findAutowiringMetadata.invoke(processor, "bean1", Bean1.class, null);
        System.out.println(metadata);

        //调用metadata的inject()方法实现依赖注入
        metadata.inject(bean1, "bean1", null);

        System.out.println(bean1);

        //按类型查找值
        //@Autowired属性
        Field bean3 = Bean1.class.getDeclaredField("bean3");
        DependencyDescriptor descriptor = new DependencyDescriptor(bean3, false);
        Object o = beanFactory.doResolveDependency(descriptor, null, null, null);
        System.out.println(o);

        //@Autowired方法
        Method setBean2 = Bean1.class.getDeclaredMethod("setBean2", Bean2.class);
        DependencyDescriptor descriptor1 = new DependencyDescriptor(
                new MethodParameter(setBean2, 0),
                false
        );
        Object o1 = beanFactory.doResolveDependency(descriptor1, null, null, null);
        System.out.println(o1);

        //@Autowired @Value
        Method setHome = Bean1.class.getDeclaredMethod("setHome", String.class);
        DependencyDescriptor descriptor2 = new DependencyDescriptor(
                new MethodParameter(setHome, 0),
                false
        );
        Object o2 = beanFactory.doResolveDependency(descriptor2, null, null, null);
        System.out.println(o2);
    }
}