Spring容器注入bean的五种方法逐个解析
作者:尚少
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 { @Bean public 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 { @Bean public 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 { @Override public 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: // 返回配置类的全路径,配置类的@Bean方法返回的对象也能注入到容器中 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 { @Override public 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 { @Override public Object getObject() throws Exception { return new A(); } @Override public 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 { @Override public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException { BeanDefinition definition = new RootBeanDefinition(A.class); registry.registerBeanDefinition("a", definition); } @Override public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException { } }
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