Spring 中 BeanFactory 和 FactoryBean

两者区别

• 共同点：BeanFactory 和 FactoryBean 两者都是接口

• 
  package org.springframework.beans.factory;
  public interface FactoryBean {
  
    String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";
  
    /**
     * Return an instance (possibly shared or independent) of the object
     * managed by this factory.
     */
    @Nullable
    T getObject() throws Exception;
  
    /**
     * Return the type of object that this FactoryBean creates,
     * or {@code null} if not known in advance.
     */
    @Nullable
    Class getObjectType();
  
    /**
     * Is the object managed by this factory a singleton? That is,
     * will {@link #getObject()} always return the same object
     * (a reference that can be cached)?
     */
    default boolean isSingleton() {
      return true;
    }
  }

• package org.springframework.beans.factory;
  public interface BeanFactory {
  
    String FACTORY_BEAN_PREFIX = "&";
  
    // Return an instance, which may be shared or independent, of the specified bean.
    Object getBean(String name) throws BeansException;
  
    //  Return an instance, which may be shared or independent, of the specified bean.
     T getBean(String name, Class requiredType) throws BeansException;
  
    // Return an instance, which may be shared or independent, of the specified bean.
    Object getBean(String name, Object... args) throws BeansException;
  
    // Return the bean instance that uniquely matches the given object type, if any.
     T getBean(Class requiredType) throws BeansException;
  
    // Return an instance, which may be shared or independent, of the specified bean.
     T getBean(Class requiredType, Object... args) throws BeansException;
  
    /**
     * Return a provider for the specified bean, allowing for lazy on-demand retrieval
     * of instances, including availability and uniqueness options.
     */
     ObjectProvider getBeanProvider(Class requiredType);
  
    /**
     * Return a provider for the specified bean, allowing for lazy on-demand retrieval
     * of instances, including availability and uniqueness options.
     */
     ObjectProvider getBeanProvider(ResolvableType requiredType);
  
    boolean containsBean(String name);
  
    boolean isSingleton(String name) throws NoSuchBeanDefinitionException;
  
    boolean isPrototype(String name) throws NoSuchBeanDefinitionException;
    /**
     * Check whether the bean with the given name matches the specified type.
     * More specifically, check whether a {@link #getBean} call for the given name
     * would return an object that is assignable to the specified target type.
     */
    boolean isTypeMatch(String name, ResolvableType typeToMatch) throws NoSuchBeanDefinitionException;
  
    boolean isTypeMatch(String name, Class typeToMatch) throws NoSuchBeanDefinitionException;
  
    /**
     * Determine the type of the bean with the given name. More specifically,
     * determine the type of object that {@link #getBean} would return for the given name.
     */
    @Nullable
    Class getType(String name) throws NoSuchBeanDefinitionException;
  
    /**
     * Determine the type of the bean with the given name. More specifically,
     * determine the type of object that {@link #getBean} would return for the given name.
     */
    @Nullable
    Class getType(String name, boolean allowFactoryBeanInit) throws NoSuchBeanDefinitionException;
  
    // Return the aliases for the given bean name, if any.
    String[] getAliases(String name);
  }

• 不同点：
  • BeanFactory：表示一个 Bean 的工厂，用来管理 Bean 的工厂，Spring中，所有的 Bean 都是由 BeanFactory 进行管理的，也就是 Ioc 容器。通过 getBean() 方法可以得到一个对象的 Bean。

  • BeanFactory beanFactory = new ClassPathXmlApplicationContext("xxx");
    beanFactory.getBean(xxx.class);

  • FactoryBean：它是一个 Bean，可以用于产生对象或者用于修饰对象的工厂 Bean。也就是说，一个类如果实现了 FactoryBean，那么这个类中存在两个对象，一个是 getObject() 方法返回的 Object(TempDaoFactoryBean)，另一个是当前对象 (DaoFactoryBean)。

  • 
    public class TempDaoFactoryBean {
        public void test() {
            System.out.println("FactoryBean");
        }
    }
    
    //----------------------------------------------
    
    @Component("daoFactoryBean")
    public class DaoFactoryBean implements FactoryBean {
    
        @Override
        public Object getObject() throws Exception {
            return new TempDaoFactoryBean();
        }
    
        @Override
        public Class getObjectType() {
            return TempDaoFactoryBean.class;
        }
    
        @Override
        public boolean isSingleton() {
            return false;
        }
    
        public void testBean() {
            System.out.println("testBean");
        }
    }

  • 注意：getObject() 得到的对象存储的是当前类指定的名字，如果在调用 getBean() 时，在当前类 (DaoFactoryBean) 的前边加上字符 &，那么可以调用当前对象。

  • DaoFactoryBean daoFactoryBean = (DaoFactoryBean) context.getBean("&daoFactoryBean");
    daoFactoryBean.testBean();
    // 不加字符 &，只能调用指定的类
    TempDaoFactoryBean tempDaoFactoryBean = (TempDaoFactoryBean) context.getBean("daoFactoryBean");
    tempDaoFactoryBean.test();

使用场景

• BeanFactory：大部分情况我们只会用到 BeanFactory，直接调用 getBean() 方法就可以得到这个对象的实例，从而调用这个对象中的方法。

• FactoryBean：当一个类的依赖关系很复杂的时候,我们又想对外提供简单的配置关系(外部可以配置这个类)，可以使用 FactoryBean。

  • 例如：sqlSessionFactoryBean，这个类实现了 FactoryBean，我们不需要去维护 sqlSessionFactoryBean 类的依赖关系，把它交给 FactoryBean，我们只需要调用实现 FactoryBean 的这个类就能实现简单配置，从而可以使用这个类，平时我们配置 MyBatis 最简单的，就是只需要添加 MySQL 的 user、password、driverClass、jdbcUrl，其他的则不需要我们关心。

  • 

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