Bean是如何创建的

bean本质上就是对象，创建bean使用构造方法完成。

实例化Bean的几种方式

构造方法方式

BookDaoImpl实现类:

public class BookDaoImpl implements BookDao {
    public BookDaoImpl() {
        System.out.println("book dao constructor is running ....");
    }
    public void save() {
        System.out.println("book dao save ...");
    }
}

applicationContext.xml配置:

1 2	<!--方式一：构造方法实例化bean--> <bean id="bookDao" class="edu.heuet.dao.impl.BookDaoImpl"/>

AppForInstanceBook测试类:

public class AppForInstanceBook {
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");

        BookDao bookDao = (BookDao) ctx.getBean("bookDao");

        bookDao.save();
    }
}

运行结果:

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

静态工厂方式

OrderDao接口和OrderDaoImpl实现类:

public interface OrderDao {
    public void save();
}
public class OrderDaoImpl implements OrderDao {
    public void save() {
        System.out.println("order dao save ...");
    }
}

OrderDaoFatory工厂类:

//静态工厂创建对象
public class OrderDaoFactory {
    public static OrderDao getOrderDao(){
        System.out.println("factory setup....");
        return new OrderDaoImpl();
    }
}

applicationContext.xml配置:

1 2	<!--方式二：使用静态工厂实例化bean--> <bean id="orderDao" class="edu.heuet.factory.OrderDaoFactory" factory-method="getOrderDao"/>

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AppForInstanceOrder测试类:

public class AppForInstanceOrder {
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");

        OrderDao orderDao = (OrderDao) ctx.getBean("orderDao");

        orderDao.save();
    }
}

运行结果

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实例工厂方式

UserDao接口和UserDaoImpl实现类:

public interface UserDao {
    public void save();
}
public class UserDaoImpl implements UserDao {
    public void save() {
        System.out.println("user dao save ...");
    }
}

UserDaoFactory工厂类

//实例工厂创建对象
public class UserDaoFactory {
    public UserDao getUserDao(){
        return new UserDaoImpl();
    }
}

applicationContext.xml配置

<!--方式三：使用实例工厂实例化bean-->
<bean id="userFactory" class="edu.heuet.factory.UserDaoFactory"/>

<bean id="userDao" factory-method="getUserDao" factory-bean="userFactory"/>

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AppForInstanceUser测试类:

public class AppForInstanceUser {
    public static void main(String[] args) {
        //        //创建实例工厂对象
        //        UserDaoFactory userDaoFactory = new UserDaoFactory();
        //        //通过实例工厂对象创建对象
        //        UserDao userDao = userDaoFactory.getUserDao();
        //        userDao.save();
        ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
        UserDao userDao = (UserDao) ctx.getBean("userDao");
        userDao.save();
    }
}

运行结果:

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实现FactoryBean<T>方式

定义UserDaoFactoryBean实现FactoryBean<UserDao>

UserDaoFactoryBean中实例化什么类型的对象泛型就是该类型。

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

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