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 {

    @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 {
    }
}