Java Spring Bean实例化的基本流程

Spring容器在进行初始化时，会将xml配置的<bean>的信息封装成一个BeanDefinition对象，所有的BeanDefinition存储到一个名为beanDefinitionMap的Map集合中去，Spring框架在对该Map进行遍历，使用反射创建Bean实例对象，创建好的Bean对象存储在一个名为singletonObjects的Map集合中，当调用getBean方法时则最终从该Map集合中取出Bean实例对象返回。

流程：

• 加载xml配置文件，解析获取配置中的每个<bean>的信息，封装成一个个的BeanDefinition对象
• 将BeanDefinition存储在一个名为beanDefinitionMap的Map<String,BeanDefinition>中;
• ApplicationContext底层遍历beanDefinitionMap，创建Bean实例对象
  创建好的Bean实例对象，被存储到一个名为singletonObjects的Map<String,Object>中;
• 当执行applicationContext.getBean(beanName)时，从singletonObjects去匹配Bean实例返回

beans.xml

    <bean class="org.example.UserDaoImpl" id="userDao"/>
    <bean class="org.example.UserServiceImpl" id="userService"/>

主函数断点调试运行

    public static void main(String[] args) {
        ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
    }

Bean信息定义对象-BeanDefinition

BeanDefinition.class

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.springframework.beans.factory.config;

import org.springframework.beans.BeanMetadataElement;
import org.springframework.beans.MutablePropertyValues;
import org.springframework.core.AttributeAccessor;
import org.springframework.core.ResolvableType;
import org.springframework.lang.Nullable;

public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
    String SCOPE_SINGLETON = "singleton";
    String SCOPE_PROTOTYPE = "prototype";
    int ROLE_APPLICATION = 0;
    int ROLE_SUPPORT = 1;
    int ROLE_INFRASTRUCTURE = 2;

    void setParentName(@Nullable String var1);

    @Nullable
    String getParentName();

    void setBeanClassName(@Nullable String var1);

    @Nullable
    String getBeanClassName();

    void setScope(@Nullable String var1);

    @Nullable
    String getScope();

    void setLazyInit(boolean var1);

    boolean isLazyInit();

    void setDependsOn(@Nullable String... var1);

    @Nullable
    String[] getDependsOn();

    void setAutowireCandidate(boolean var1);

    boolean isAutowireCandidate();

    void setPrimary(boolean var1);

    boolean isPrimary();

    void setFactoryBeanName(@Nullable String var1);

    @Nullable
    String getFactoryBeanName();

    void setFactoryMethodName(@Nullable String var1);

    @Nullable
    String getFactoryMethodName();

    ConstructorArgumentValues getConstructorArgumentValues();

    default boolean hasConstructorArgumentValues() {
        return !this.getConstructorArgumentValues().isEmpty();
    }

    MutablePropertyValues getPropertyValues();

    default boolean hasPropertyValues() {
        return !this.getPropertyValues().isEmpty();
    }

    void setInitMethodName(@Nullable String var1);

    @Nullable
    String getInitMethodName();

    void setDestroyMethodName(@Nullable String var1);

    @Nullable
    String getDestroyMethodName();

    void setRole(int var1);

    int getRole();

    void setDescription(@Nullable String var1);

    @Nullable
    String getDescription();

    ResolvableType getResolvableType();

    boolean isSingleton();

    boolean isPrototype();

    boolean isAbstract();

    @Nullable
    String getResourceDescription();

    @Nullable
    BeanDefinition getOriginatingBeanDefinition();
}