Spring IoC--BeanFactory、FactoryBean、ObjectFactory

BeanFactory

Spring Ioc容器顶层接口。

FactoryBean

以Bean结尾，表示它是一个bean,但它不是普通的的bean，而是一个能生产对象的工厂bean，它的实现与设计模式中的工厂模式和修饰器模式类似。

根据Bean的id从BeanFactory中获取的实际上是FactoryBean的getObject()方法返回的对象，而不是FactoryBean本身，如果要获取FactoryBean本身，需要在id前面加一个“&”符号。

if (isFactoryBean(beanName)) {
    Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
    if (bean instanceof FactoryBean) {
        final FactoryBean<?> factory = (FactoryBean<?>) bean;
        boolean isEagerInit;
        if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
            isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                            ((SmartFactoryBean<?>) factory)::isEagerInit,
                    getAccessControlContext());
        }
        else {
            isEagerInit = (factory instanceof SmartFactoryBean &&
                    ((SmartFactoryBean<?>) factory).isEagerInit());
        }
        if (isEagerInit) {
            getBean(beanName);
        }
    }
}

ObjectFactory

一个普通的对象工厂接口，它可以在调用时返回一个对象实例(可能是共享的或独立的)。

通过查看Spring源码，可以发现SpringObjectFactory的应用之一就是:将创建对象的步骤封装到ObjectFactory中，交给自定义的Scope来选择是否需要创建对象来灵活的实现Scope。

• singleton

  sharedInstance = getSingleton(beanName, () -> {
      try {
          return createBean(beanName, mbd, args);
      }
      catch (BeansException ex) {
          // Explicitly remove instance from singleton cache: It might have been put there
          // eagerly by the creation process, to allow for circular reference resolution.
          // Also remove any beans that received a temporary reference to the bean.
          destroySingleton(beanName);
          throw ex;
      }
  });

• scope

  Object scopedInstance = scope.get(beanName, () -> {
      beforePrototypeCreation(beanName);
      try {
          return createBean(beanName, mbd, args);
      }
      finally {
          afterPrototypeCreation(beanName);
      }
  });
  
  public class ServletContextScope implements Scope, DisposableBean {
      ...
  
      public Object get(String name, ObjectFactory<?> objectFactory) {
          Object scopedObject = this.servletContext.getAttribute(name);
          if (scopedObject == null) {
              scopedObject = objectFactory.getObject();
              this.servletContext.setAttribute(name, scopedObject);
          }
  
          return scopedObject;
      }
  
      ...
  }