【第四十七讲】 @Autowired 装配底层

【第四十七讲】 @Autowired 装配底层

• @Autowired 复杂之处
• 按类型装配时支持的类型
• 泛型的进一步认识

代码一

@Configuration
public class A47_1 {
    public static void main(String[] args) throws NoSuchFieldException, NoSuchMethodException {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(A47_1.class);
        DefaultListableBeanFactory beanFactory = context.getDefaultListableBeanFactory();
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");
        // 1. 根据成员变量的类型注入
        DependencyDescriptor dd1 = new DependencyDescriptor(Bean1.class.getDeclaredField("bean2"), false);
        System.out.println(beanFactory.doResolveDependency(dd1, "bean1", null, null));
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");
        // 2. 根据参数的类型注入
        Method setBean2 = Bean1.class.getDeclaredMethod("setBean2", Bean2.class);
        DependencyDescriptor dd2 = new DependencyDescriptor(new MethodParameter(setBean2, 0), false);
        System.out.println(beanFactory.doResolveDependency(dd2, "bean1", null, null));
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");
        // 3. 结果包装为 Optional<Bean2>
        DependencyDescriptor dd3 = new DependencyDescriptor(Bean1.class.getDeclaredField("bean3"), false);
        if (dd3.getDependencyType() == Optional.class) {
            dd3.increaseNestingLevel();
            Object result = beanFactory.doResolveDependency(dd3, "bean1", null, null);
            System.out.println(Optional.ofNullable(result));
        }
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");
        // 4. 结果包装为 ObjectProvider,ObjectFactory
        DependencyDescriptor dd4 = new DependencyDescriptor(Bean1.class.getDeclaredField("bean4"), false);
        if (dd4.getDependencyType() == ObjectFactory.class) {
            dd4.increaseNestingLevel();
            ObjectFactory objectFactory = new ObjectFactory() {
                @Override
                public Object getObject() throws BeansException {
                    return beanFactory.doResolveDependency(dd4, "bean1", null, null);
                }
            };
            System.out.println(objectFactory.getObject());
        }
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");
        // 5. 对 @Lazy 的处理
        DependencyDescriptor dd5 = new DependencyDescriptor(Bean1.class.getDeclaredField("bean2"), false);
        ContextAnnotationAutowireCandidateResolver resolver = new ContextAnnotationAutowireCandidateResolver();
        resolver.setBeanFactory(beanFactory);
        Object proxy = resolver.getLazyResolutionProxyIfNecessary(dd5, "bean1");
        System.out.println(proxy);
        System.out.println(proxy.getClass());
        /*
            学到了什么
                1. Optional 及 ObjectFactory 对于内嵌类型的处理, 源码参考 ResolvableType
                2. ObjectFactory 懒惰的思想
                3. @Lazy 懒惰的思想
            下一节, 继续学习 doResolveDependency 内部处理
         */
    }

    static class Bean1 {
        @Autowired @Lazy 
        private Bean2 bean2;

        @Autowired 
        public void setBean2(Bean2 bean2) {
            this.bean2 = bean2;
        }
        @Autowired 
        private Optional<Bean2> bean3;
        
        @Autowired 
        private ObjectFactory<Bean2> bean4;
    }

    @Component("bean2")
    static class Bean2 {
        /*@Override
        public String toString() {
            return super.toString();
        }*/
    }
}

代码二

@Configuration
public class A47_2 {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(A47_2.class);
        DefaultListableBeanFactory beanFactory = context.getDefaultListableBeanFactory();
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>> 1. 数组类型");
        testArray(beanFactory);
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>> 2. List 类型");
        testList(beanFactory);
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>> 3. applicationContext");
        testApplicationContext(beanFactory);
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>> 4. 泛型");
        testGeneric(beanFactory);
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>> 5. @Qualifier");
        testQualifier(beanFactory);
        /*
            学到了什么
                1. 如何获取数组元素类型
                2. Spring 如何获取泛型中的类型
                3. 特殊对象的处理, 如 ApplicationContext, 并注意 Map 取值时的类型匹配问题 (另见  TestMap)
                4. 谁来进行泛型匹配 (另见 TestGeneric)
                5. 谁来处理 @Qualifier
                6. 刚开始都只是按名字处理, 等候选者确定了, 才会创建实例
         */
    }

    private static void testQualifier(DefaultListableBeanFactory beanFactory) throws NoSuchFieldException {
        DependencyDescriptor dd5 = new DependencyDescriptor(Target.class.getDeclaredField("service"), true);
        Class<?> type = dd5.getDependencyType();
        ContextAnnotationAutowireCandidateResolver resolver = new ContextAnnotationAutowireCandidateResolver();
        resolver.setBeanFactory(beanFactory);
        for (String name : BeanFactoryUtils.beanNamesForTypeIncludingAncestors(beanFactory, type)) {
            BeanDefinition bd = beanFactory.getMergedBeanDefinition(name);
            //                                                             @Qualifier("service2")
            if (resolver.isAutowireCandidate(new BeanDefinitionHolder(bd,name), dd5)) {
                System.out.println(name);
                System.out.println(dd5.resolveCandidate(name, type, beanFactory));
            }
        }
    }
    private static void testGeneric(DefaultListableBeanFactory beanFactory) throws NoSuchFieldException {
        DependencyDescriptor dd4 = new DependencyDescriptor(Target.class.getDeclaredField("dao"), true);
        Class<?> type = dd4.getDependencyType();
        ContextAnnotationAutowireCandidateResolver resolver = new ContextAnnotationAutowireCandidateResolver();
        resolver.setBeanFactory(beanFactory);
        for (String name : BeanFactoryUtils.beanNamesForTypeIncludingAncestors(beanFactory, type)) {
            BeanDefinition bd = beanFactory.getMergedBeanDefinition(name);
            // 对比 BeanDefinition 与 DependencyDescriptor 的泛型是否匹配
            if (resolver.isAutowireCandidate(new BeanDefinitionHolder(bd,name), dd4)) {
                System.out.println(name);
                System.out.println(dd4.resolveCandidate(name, type, beanFactory));
            }
        }
    }
    private static void testApplicationContext(DefaultListableBeanFactory beanFactory) throws NoSuchFieldException, IllegalAccessException {
        DependencyDescriptor dd3 = new DependencyDescriptor(Target.class.getDeclaredField("applicationContext"), true);

        Field resolvableDependencies = DefaultListableBeanFactory.class.getDeclaredField("resolvableDependencies");
        resolvableDependencies.setAccessible(true);
        Map<Class<?>, Object> dependencies = (Map<Class<?>, Object>) resolvableDependencies.get(beanFactory);
//        dependencies.forEach((k, v) -> {
//            System.out.println("key:" + k + " value: " + v);
//        });
        for (Map.Entry<Class<?>, Object> entry : dependencies.entrySet()) {
            // 左边类型                      右边类型
            if (entry.getKey().isAssignableFrom(dd3.getDependencyType())) {
                System.out.println(entry.getValue());
                break;
            }
        }
    }
    private static void testList(DefaultListableBeanFactory beanFactory) throws NoSuchFieldException {
        DependencyDescriptor dd2 = new DependencyDescriptor(Target.class.getDeclaredField("serviceList"), true);
        if (dd2.getDependencyType() == List.class) {
            Class<?> resolve = dd2.getResolvableType().getGeneric().resolve();
            System.out.println(resolve);
            List<Object> list = new ArrayList<>();
            String[] names = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(beanFactory, resolve);
            for (String name : names) {
                Object bean = dd2.resolveCandidate(name, resolve, beanFactory);
                list.add(bean);
            }
            System.out.println(list);
        }
    }
    private static void testArray(DefaultListableBeanFactory beanFactory) throws NoSuchFieldException {
        DependencyDescriptor dd1 = new DependencyDescriptor(Target.class.getDeclaredField("serviceArray"), true);
        if (dd1.getDependencyType().isArray()) {
            Class<?> componentType = dd1.getDependencyType().getComponentType();
            System.out.println(componentType);
            String[] names = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(beanFactory, componentType);
            List<Object> beans = new ArrayList<>();
            for (String name : names) {
                System.out.println(name);
                Object bean = dd1.resolveCandidate(name, componentType, beanFactory);
                beans.add(bean);
            }
            Object array = beanFactory.getTypeConverter().convertIfNecessary(beans, dd1.getDependencyType());
            System.out.println(array);
        }
    }
    static class Target {
        @Autowired private Service[] serviceArray;
        @Autowired private List<Service> serviceList;
        @Autowired private ConfigurableApplicationContext applicationContext;
        @Autowired private Dao<Teacher> dao;
        @Autowired @Qualifier("service2") private Service service;
    }
    interface Dao<T> {

    }
    @Component("dao1") static class Dao1 implements Dao<Student> {
    }
    @Component("dao2") static class Dao2 implements Dao<Teacher> {
    }

    static class Student {

    }

    static class Teacher {

    }

    interface Service {

    }

    @Component("service1")
    static class Service1 implements Service {

    }

    @Component("service2")
    static class Service2 implements Service {

    }

    @Component("service3")
    static class Service3 implements Service {

    }
}

代码三

@Configuration
public class A47_3 {
    public static void main(String[] args) throws NoSuchFieldException {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(A47_3.class);
        DefaultListableBeanFactory beanFactory = context.getDefaultListableBeanFactory();
        testPrimary(beanFactory);
        testDefault(beanFactory);

        /*
            学到了什么
                1. @Primary 的处理, 其中 @Primary 会在 @Bean 解析或组件扫描时被解析 (另见 TestPrimary)
                2. 最后的防线, 通过属性或参数名匹配
         */
    }

    private static void testDefault(DefaultListableBeanFactory beanFactory) throws NoSuchFieldException {
        DependencyDescriptor dd = new DependencyDescriptor(Target2.class.getDeclaredField("service3"), false);
        Class<?> type = dd.getDependencyType();
        for (String name : BeanFactoryUtils.beanNamesForTypeIncludingAncestors(beanFactory, type)) {
            if(name.equals(dd.getDependencyName())) {
                System.out.println(name);
            }
        }
    }

    private static void testPrimary(DefaultListableBeanFactory beanFactory) throws NoSuchFieldException {
        DependencyDescriptor dd = new DependencyDescriptor(Target1.class.getDeclaredField("service"), false);
        Class<?> type = dd.getDependencyType();
        for (String name : BeanFactoryUtils.beanNamesForTypeIncludingAncestors(beanFactory, type)) {
            if (beanFactory.getMergedBeanDefinition(name).isPrimary()) {
                System.out.println(name);
            }
        }
    }

    static class Target1 {
        @Autowired private Service service;
    }

    static class Target2 {
        @Autowired private Service service3;
    }

    interface Service {

    }
    @Component("service1") static class Service1 implements Service {

    }
    @Component("service2") static class Service2 implements Service {

    }
    @Component("service3") static class Service3 implements Service {

    }
}

总结

1. @Autowired 本质上是根据成员变量或方法参数的类型进行装配
2. 如果待装配类型是 Optional，需要根据 Optional 泛型找到 bean，再封装为 Optional 对象装配
3. 如果待装配的类型是 ObjectFactory，需要根据 ObjectFactory 泛型创建 ObjectFactory 对象装配
   • 此方法可以延迟 bean 的获取
4. 如果待装配的成员变量或方法参数上用 @Lazy 标注，会创建代理对象装配
   • 此方法可以延迟真实 bean 的获取
   • 被装配的代理不作为 bean
5. 如果待装配类型是数组，需要获取数组元素类型，根据此类型找到多个 bean 进行装配
6. 如果待装配类型是 Collection 或其子接口，需要获取 Collection 泛型，根据此类型找到多个 bean
7. 如果待装配类型是 ApplicationContext 等特殊类型
   • 会在 BeanFactory 的 resolvableDependencies 成员按类型查找装配
   • resolvableDependencies 是 map 集合，key 是特殊类型，value 是其对应对象
   • 不能直接根据 key 进行查找，而是用 isAssignableFrom 逐一尝试右边类型是否可以被赋值给左边的 key 类型
8. 如果待装配类型有泛型参数
   • 需要利用 ContextAnnotationAutowireCandidateResolver 按泛型参数类型筛选
9. 如果待装配类型有 @Qualifier
   • 需要利用 ContextAnnotationAutowireCandidateResolver 按注解提供的 bean 名称筛选
10. 有 @Primary 标注的 @Component 或 @Bean 的处理
11. 与成员变量名或方法参数名同名 bean 的处理