我们把 Bean 的创建交给容器，而不是我们在调用时候传递一个实例化好的 Bean 对象，另外还需要考虑单例对象，在对象的二次获取时是可以从内存中获取对象的。此外不仅要实现功能还需要完善基础容器框架的类结构体，否则将来就很难扩容进去其他的功能了。

1.创建Bean定义信息

public class BeanDefinition {
    private Class beanClass;
    public BeanDefinition(Class bean){
        this.beanClass = bean;
    }
    public Class getBeanClass() {
        return beanClass;
    }
    public void setBeanClass(Class beanClass) {
        this.beanClass = beanClass;
    }
}

2.单实例获取接口

public interface SingletonBeanRegistry {
    Object getSingleton(String beanName);
}

3.单实例获取接口实现类

public class DefaultSingletonBeanRegistry implements SingletonBeanRegistry {
    private Map<String, Object> singletonObjects = new HashMap<>();
    @Override
    public Object getSingleton(String beanName) {
        return singletonObjects.get(beanName);
    }
    protected void addSingleton(String beanName, Object singletonObject) {
        singletonObjects.put(beanName, singletonObject);
    }
}

4.Bean工厂获取bean

public interface BeanFactory {
    public Object getBean(String name);
}

5.Bean信息注册

public interface BeanDefinitionRegistry {
    /**
     * 向注册表中注册 BeanDefinition
     *
     * @param beanName
     * @param beanDefinition
     */
    void registerBeanDefinition(String beanName, BeanDefinition beanDefinition);
}

5.抽象获取Bean工厂

public abstract class AbstractBeanFactory extends DefaultSingletonBeanRegistry implements BeanFactory {
    @Override
    public Object getBean(String name) {
        Object singleton = getSingleton(name);
        if (singleton == null){
            BeanDefinition beanDefinition = getBeanDefinition(name);
            return createBean(name, beanDefinition);
        }
        return singleton;
    }
    protected abstract Object createBean(String name, BeanDefinition beanDefinition);
    protected abstract BeanDefinition getBeanDefinition(String name);
}

6.抽象创建Bean工厂

public abstract class AbstractAutowireCapableBeanFactory extends AbstractBeanFactory{
    @Override
    protected Object createBean(String name, BeanDefinition beanDefinition) {
        Object bean = null;
        try {
            bean = beanDefinition.getBeanClass().newInstance();
            addSingleton(name,bean);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return bean;
    }
}

7.默认Bean注册实现类

public class DefaultSingletonBeanRegistry implements SingletonBeanRegistry {
    private Map<String, Object> singletonObjects = new HashMap<>();
    @Override
    public Object getSingleton(String beanName) {
        return singletonObjects.get(beanName);
    }
    protected void addSingleton(String beanName, Object singletonObject) {
        singletonObjects.put(beanName, singletonObject);
    }
}

8.默认Bean工厂实现类

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements BeanDefinitionRegistry{
    private Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap();
    @Override
    protected BeanDefinition getBeanDefinition(String beanName) {
        BeanDefinition beanDefinition = beanDefinitionMap.get(beanName);
        if (beanDefinition == null) {
            System.out.println("No bean named '" + beanName + "' is defined");
        }
        return beanDefinition;
    }
    @Override
    public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition) {
        beanDefinitionMap.put(beanName,beanDefinition);
    }
}

9.定义异常信息

public class BeansException extends RuntimeException {
    public BeansException(String msg) {
        super(msg);
    }
    public BeansException(String msg, Throwable cause) {
        super(msg, cause);
    }
}