将bean注入Spring容器的五种方式

前言

  我们在项目开发中都用到Spring，知道对象是交由Spring去管理。那么将一个对象加入到Spring容器中，有几种方法呢，我们来总结一下。

@ComponentScan + @Component

  @ComponentScan可以放在启动类上，指定要扫描的包路径；该包路径下被@Component修饰的类，都会被注入到Spring容器中。

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;@ComponentScan(basePackages = "com.gs.beanRegister")
public class BootStrap {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BootStrap.class);A bean = context.getBean(A.class);bean.say();}}

  com.gs.beanRegister包下：

import org.springframework.stereotype.Component;@Component
public class A {public void say() {System.out.println("这是a");}}

  注：在SpringBoot中，由于其自动装配的特性，所以@ComponentScan可以不加，只要@Component修饰的类和启动类在同一包下或者在启动类所在包的子包下。

@Configuration + @Bean

  @Configuration用来声明一个配置类，如果它的方法被@Bean修饰，那么该方法返回的对象也会被注入到Spring容器中。
  代码方面，BootStrap 类不动，A类的@Component去掉，com.gs.beanRegister包下建个配置类：

import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Bean;@Configuration
public class MyConfiguration {@Beanpublic A a() {return new A();}}

通过@Import注解

  这个注解可能平时大家接触得不多，它有好几种使用方式。

      1.直接导入类的class

import org.springframework.context.annotation.Import;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;@Import(A.class)
public class BootStrap {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BootStrap.class);A bean = context.getBean(A.class);//B bean = context.getBean(B.class);bean.say();}}

  A类不用添加任何注解：

public class A {public void say() {System.out.println("这是a");}}

      2.导入配置类

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Import;@Import(MyConfiguration.class)
public class BootStrap {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BootStrap.class);A bean = context.getBean(A.class);bean.say();}}

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;// 使用@Import导入配置类时，@Configuration可以不加
//@Configuration
public class MyConfiguration {@Beanpublic A a() {return new A();}}

      3.导入ImportSelector的实现类

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Import;@Import(MyImportSelector.class)
public class BootStrap {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BootStrap.class);A bean = context.getBean(A.class);bean.say();}}

import org.springframework.context.annotation.ImportSelector;
import org.springframework.core.type.AnnotationMetadata;public class MyImportSelector implements ImportSelector {@Overridepublic String[] selectImports(AnnotationMetadata metadata) {// 返回要注入的bean的全路径，A类不用任何注解修饰// SpringBoot的自动装配，就用到了这种方式return new String[] { A.class.getName() };}}

      4.导入ImportBeanDefinitionRegistrar的实现类

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Import;@Import(MyImportBeanDefinitionRegistrar.class)
public class BootStrap {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BootStrap.class);A bean = context.getBean(A.class);bean.say();}}

import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.RootBeanDefinition;
import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
import org.springframework.core.type.AnnotationMetadata;public class MyImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {@Overridepublic void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {// 构建bean的元数据，A类不用任何注解修饰// spring-mybatis扫描mapper接口，生成代理类，就是用的这种方式BeanDefinition definition = new RootBeanDefinition(A.class);registry.registerBeanDefinition("a", definition);}}

借助FactoryBean接口

  实现FactoryBean接口的类，除了本身会被注入外，getObject方法返回的对象也会被注入到Spring容器中。

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Import;@Import(MyFactoryBean.class)
public class BootStrap {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(BootStrap.class);A bean = context.getBean(A.class);bean.say();}}

import org.springframework.beans.factory.FactoryBean;public class MyFactoryBean implements FactoryBean {@Overridepublic Object getObject() throws Exception {return new A();}@Overridepublic Class<?> getObjectType() {return A.class;}}

借助BeanDefinitionRegistryPostProcessor接口

  在Spring容器启动时，会调用该接口的postProcessBeanDefinitionRegistry方法，大概意思是等BeanDefinition(上面提到的bean的元数据)加载完成后，再对它进行后置处理。所以可以在此调整BeanDefinition，从而把对应的bean注入。

import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;public class BootStrap {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();BeanDefinitionRegistryPostProcessor postProcessor = new MyBeanDefinitionRegistryPostProcessor();context.addBeanFactoryPostProcessor(postProcessor);context.refresh();A a = context.getBean(A.class);a.say();}}

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;
import org.springframework.beans.factory.support.RootBeanDefinition;public class MyBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {@Overridepublic void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {BeanDefinition definition = new RootBeanDefinition(A.class);registry.registerBeanDefinition("a", definition);}@Overridepublic void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {}
}