今天主要和大家分享一些在工作中可能会用到的Spring依赖注入，依赖查找方面的技术点整理。
Spring依赖查找专题
单一类型查找
常见用法如下所示：
Object getBean(String name) throws BeansException; <T> T getBean(Class<T> requiredType) throws BeansException; <T> T getBean(String name, Class<T> requiredType) throws BeansException 复制代码
ObjectProvider
随着Spring版本的升高，也开始出现了延迟查找的功能。当我们实际需要用到某个bean的时候才将其从容器中进行初始化并且提取出来。
<T> ObjectProvider<T> getBeanProvider(Class<T> requiredType); <T> ObjectProvider<T> getBeanProvider(ResolvableType requiredType); 复制代码
Spring会返回一个ObjectProvider，当查询的时候才会触发bean的创建。延迟查找的好处在于，如果一个bean需要注入到spring容器中，但是不希望太过早地去进行初始化，那么可以思考使用ObjectProvider的方式来进行初始化。
集合类型查找
Bean的名称查询
String[] getBeanNamesForType(@Nullable Class<?> type); String[] getBeanNamesForType(@Nullable Class<?> type, boolean includeNonSingletons, boolean allowEagerInit); 复制代码
获取同类型Bean实例列表
getBeansOfType(Class) 复制代码
按照注解去查询
String[] getBeanNamesForAnnotation(Class<? extends Annotation> annotationType); Map<String, Object> getBeansWithAnnotation(Class<? extends Annotation> annotationType) throws BeansException; <A extends Annotation> A findAnnotationOnBean(String beanName, Class<A> annotationType) throws NoSuchBeanDefinitionException; 复制代码
使用依赖查找时候的一些小心得：
对于判断一个bean是否存在，可以采用判断其beandefinition是否存在，一般这样不会触发其中bean的初始化操作，例如：getBeanNamesForType。反观getBeansOfType可能回触发bean的初始化
层次性的bean查找
可能大多数人在实际使用Spring容器的时候对于层次性的bean做计算并没有太多的实战尝试，这里我举个例子：
例如说A容器中包含了Bean A，如果B容器继承了A容器，那么按道理来说也应该能够获得Bean A资源，这种设计可以减少Bean的额外存储。
如果你理解了我上边所说的这个案例之后，再来看看下边的这张图可能就会有更加深入的理解了。
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关于层次性的bean获取，我这里给出一个小的demo供大家学习：
package org.idea.spring.look.up.factory; import org.springframework.beans.factory.BeanFactory; import org.springframework.beans.factory.HierarchicalBeanFactory; import org.springframework.beans.factory.config.ConfigurableListableBeanFactory; import org.springframework.context.ApplicationContext; import org.springframework.context.annotation.AnnotationConfigApplicationContext; /** * 层次性的依赖查找 {@link org.springframework.beans.factory.HierarchicalBeanFactory} * * @Author idea * @Date created in 10:55 下午 2021/4/10 */ public class SpringHierarchicalLookUpDemo { public static void main(String[] args) { AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(); applicationContext.register(SpringHierarchicalLookUpDemo.class); applicationContext.refresh(); ParentIocContainer parentIocContainer = new ParentIocContainer(); ApplicationContext parentApplicationContext = parentIocContainer.getAndStartApplicationContext(); // ConfigurableListableBeanFactory -> ConfigurableBeanFactory -> HierarchicalBeanFactory ConfigurableListableBeanFactory configurableListableBeanFactory = applicationContext.getBeanFactory(); System.out.println("此时的父类BeanFactory为：" + configurableListableBeanFactory.getParentBeanFactory()); configurableListableBeanFactory.setParentBeanFactory(parentApplicationContext); System.out.println("此时的父类BeanFactory为：" + configurableListableBeanFactory.getParentBeanFactory()); ParentIocContainer.ParentBean parentBean = (ParentIocContainer.ParentBean) configurableListableBeanFactory.getBean("parentBean"); System.out.println(parentBean); isContainedBean(configurableListableBeanFactory, "parentBean"); displayContainsBean(configurableListableBeanFactory, "parentBean"); } /** * 这里是子类可以获取自己和父类层次内部的bean，如果是使用containsLocalBean方法的话就只能判断当前所在层次的容器上下文 * * @param beanFactory * @param beanName */ public static void isContainedBean(HierarchicalBeanFactory beanFactory, String beanName) { System.out.println("getBean is " + beanFactory.getBean(beanName)); System.out.printf("contained is [%s] ,beanFactory is [%s],beanName is [%s]\n", beanFactory.containsLocalBean(beanName), beanFactory, beanName); } /** * 查找关于父类容器内部的bean * * @param beanFactory * @param beanName */ private static void displayContainsBean(HierarchicalBeanFactory beanFactory, String beanName) { System.out.printf("contained is [%s] ,beanFactory is [%s],beanName is [%s]\n", isContainedBeanInHoldApplication(beanFactory, beanName), beanFactory, beanName); } /** * 使用递归判断 -- 自上到下判断父类容器是否含有bean * * @param hierarchicalBeanFactory * @param beanName * @return */ public static boolean isContainedBeanInHoldApplication(HierarchicalBeanFactory hierarchicalBeanFactory, String beanName) { BeanFactory parentBeanFactory = hierarchicalBeanFactory.getParentBeanFactory(); if (parentBeanFactory instanceof HierarchicalBeanFactory) { HierarchicalBeanFactory parentHierarchicalBeanFactory = HierarchicalBeanFactory.class.cast(parentBeanFactory); if (isContainedBeanInHoldApplication(parentHierarchicalBeanFactory, beanName)) { return true; } } return hierarchicalBeanFactory.containsBean(beanName); } } 复制代码
对应的父类容器案例：
package org.idea.spring.look.up.factory; import org.springframework.context.ApplicationContext; import org.springframework.context.annotation.AnnotationConfigApplicationContext; /** * 父类ioc容器 这里面的ioc容器只包含有ParentBean这个类 * * @Author idea * @Date created in 8:46 上午 2021/4/11 */ public class ParentIocContainer { public static AnnotationConfigApplicationContext applicationContext = null; class ParentBean { int id; public ParentBean(){ System.out.println("this is no arg init"); } @Override public String toString() { return "ParentBean{" + "id=" + id + '}'; } } public ApplicationContext getAndStartApplicationContext(){ applicationContext = new AnnotationConfigApplicationContext(); applicationContext.register(ParentIocContainer.class); //需要支持无参构造函数 applicationContext.registerBean("parentBean",ParentBean.class); applicationContext.refresh(); return applicationContext; } public static void main(String[] args) { ParentIocContainer parentIocContainer = new ParentIocContainer(); ApplicationContext applicationContext = parentIocContainer.getAndStartApplicationContext(); String[] str = applicationContext.getBeanNamesForType(ParentBean.class); for (String beanName : str) { System.out.println(beanName); } } } 复制代码
从这段代码中可以看出，HierarchicalBeanFactory是一种常见的层次类BeanFactory，并且当我们需要判断一个bean是否存在某个容器上下文中的时候，不妨可以试试使用BeanFacoty自带的这个方法：
org.springframework.beans.factory.HierarchicalBeanFactory#containsLocalBean /** * Return whether the local bean factory contains a bean of the given name, * ignoring beans defined in ancestor contexts. * <p>This is an alternative to {@code containsBean}, ignoring a bean * of the given name from an ancestor bean factory. * @param name the name of the bean to query * @return whether a bean with the given name is defined in the local factory * @see BeanFactory#containsBean */ boolean containsLocalBean(String name); 复制代码
