bean是如何创建的实例化bean的三种方式

构造方法

bean本质上就是对象，创建bean使用构造方法完成
提供可访问的构造方法

public class BookDaoimpl implements BookDao {
    private BookDaoimpl() {
        System.out.println("book dao constructor is running...");
    }

    public void save(){
        System.out.println("user dao save...");
    }
}

配置

<bean id="bookDao" class="com.dao.impl.BookDaoimpl"/>

无参构造方法如果不存在，将抛出异常BeanCreationException
例

静态工厂（了解）

静态工厂

public class OrderDaoFactory {
    public static OrderDao getOrderDao(){
        return new OrderDaoimpl();
    }
}

配置注意配置的工厂名及方法名

  <bean id="orderDao" class="com.dao.factory.OrderDaoFactory" factory-method="getOrderDao"/>

实例工厂与FactoryBean

实例工厂

public class UserDaoFactory {
    public UserDao getUserDao() {
        return new UserDaoImpl();
    }
}

配置（先配置工厂的bean并使用factory-bean指定工厂bean）配合使用实际无意义方法名不固定每次需要配置

		<bean id="userFactory" class="com.dao.factory.UserDaoFactory"/>
        <bean id="userDao" factory-method="getUserDao" factory-bean="userFactory"/>

FactoryBean

FactoryBean(务必掌握)

public class UserDaoFactoryBean implements FactoryBean<UserDao> {
    @Override
    public UserDao getObject() throws Exception {
        return new UserDaoImpl();
    }

    @Override
    public Class<?> getObjectType() {
        return UserDao.class;
    }

}

配置

 <bean id="userDao" class="com.dao.factory.UserDaoFactoryBean"/>

FactoryBean默认为单例如果要改非单例需要加上isSingleton返回值true为单例false为非单例

public class UserDaoFactoryBean implements FactoryBean<UserDao> {
    //代替原始实例工厂中创建对象的方法
    @Override
    public UserDao getObject() throws Exception {
        return new UserDaoImpl();
    }

    @Override
    public Class<?> getObjectType() {
        return UserDao.class;
    }

    @Override
    public boolean isSingleton() {
        return false;
    }
}