spring源码深入之BeanFactoryPostProcessor偷梁换柱

今天药忘吃喽~ 2022-12-08 15:38 188阅读 0赞

spring支持bean实例化之前进行操作  
**1.测试主类**

package java_config;
    import org.springframework.context.annotation.AnnotationConfigApplicationContext;
    
    public class Bootrap { 
        public static void main(String[] args) { 
            AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(StudentConfig.class);
            Teacher simpleBean =  context.getBean(Teacher.class);
            System.out.println(simpleBean.toString());
        }
    }

**2.Teacher类**

package java_config;
    
    
    /**
     * @author: zhaoxu
     * @date: 2020/9/20 16:00
     */
    
    public class Teacher { 
        private String name;
        private int age;
    
        public String getName() { 
            return name;
        }
        public void setName(String name) { 
            this.name = name;
        }
        public int getAge() { 
            return age;
        }
        public void setAge(int age) { 
            this.age = age;
        }
    
        public Teacher(String name, int age) { 
            this.age = age;
            this.name = name;
        }
    
        public Teacher() { 
            System.out.println("Teacher");
        }
    
        @Override
        public String toString() { 
            return "Teacher [name=" + name + ", age=" + age + "]";
        }
    }

**3.Student类**

package java_config;
    
    import org.springframework.stereotype.Component;
    
    @Component
    public class Student{ 
    
    	private String name;
    	private int age;
    	
    	public String getName() { 
    		return name;
    	}
    	public void setName(String name) { 
    		this.name = name;
    	}
    	public int getAge() { 
    		return age;
    	}
    	public void setAge(int age) { 
    		this.age = age;
    	}
    
        public Student(String name, int age) { 
            this.age = age;
            this.name = name;
        }
    
        public Student() { 
    		System.out.println("student");
        }
    
        @Override
    	public String toString() { 
    		return "Student [name=" + name + ", age=" + age + "]";
    	}
    	
    }

**4.StudentConfig类**

package java_config;
    
    import org.springframework.context.annotation.ComponentScan;
    
    @ComponentScan(value = "java_config")
    public class StudentConfig{ 
    
    }

Teacher类没有任何注解，但是student类中有一个@Component注解，spring可以获取到student类，通过实现`BeanFactoryPostProcessor`，能够在实例化bean之前，对bean进行为所欲为的操作

**5.postprocesser类实现BeanFactoryPostProcessor 接口**

package java_config;
    
    import org.springframework.beans.BeansException;
    import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
    import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
    import org.springframework.beans.factory.support.GenericBeanDefinition;
    import org.springframework.stereotype.Component;
    
    /**
     * @author: zhaoxu
     * @date: 2020/9/20 16:04
     */
    @Component
    public class postprocesser implements BeanFactoryPostProcessor { 
        @Override
        public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeansException { 
            GenericBeanDefinition student = (GenericBeanDefinition) configurableListableBeanFactory.getBeanDefinition("student");
            student.setBeanClass(Teacher.class);
        }
    }

这里将获取到的student BeanDefinition重新设置BeanClass为Teacher，最终spring能够获取到Teacher但是却找不到student  
![在这里插入图片描述][20200920163837374.png_pic_center]

[20200920163837374.png_pic_center]: /images/20221123/6283e70fd5ba493d989c967d1af95fe7.png